Ultrafast Pulsed Irradiation Facility - TU Delft

Latest news: new picosecond accelerator operational - 23th of December 2009

2009-12-23T22:45:00.003+01:00

Our new picosecond pulsed high-energy electron accelerator has delivered its first pulses today (23 December 2009). Pulses are being produced for pump and probe experiments with a world-wide unprecedented high repetition frequency of 100 Hz.

This terrific achievement is the outcome of intense collaboration and commitment of technicians and scientists at the Department of Chemical Engineering and the Reactor Institute Delft.

In particular Martien Vermeulen and Walter Knulst have shown enormous persistence since they started the design and construction of the accelerator about four years ago.

The electron pulses will be combined with optical and terahertz probe pulses for studies on new materials for application in opto-electronics (e.g. solar cells) and nanomedicine.

Movie of the electron beam - 23rd of December 2009

2009-12-23T15:07:00.003+01:00

1 - The Sample Chamber has an extra port through which we can see the sample in the Sample Chamber from the front side. This makes is possible to place a CCD Camera behind this window and monitor the beam position and shape by using a phorphor screen at the sample position.

2 - The CCD image. In the center you can see the phosphor screen positioned in the center of the Sample Chamber.

Here you can see the movie with the electron beam visible on the phospor screen.

UV generated electron pulse - 23rd of December 2009

2009-12-23T14:57:00.000+01:00

A view scope images. In purple the forward RF pulse, in green the reflected RF pulse, in light blue the beam dump signal measured over 50 Ohm.

1 - Clearly visible the large light blue spike which is the UV generated electron pulse measured in the beam dump.

2 - Same situation only now with the background of dark current visible. This can be achieved by changing the strength of the main magnet around the cavity.

3 - Now, we made the RF pulse shorter and changed the timing of the laser so that it arives at the end the RF pulse when the electric field in the cavity reaches it maximum.

4 - Zoom in at the end of RF pulse.

The Great Day - 23rd of December 2009

2009-12-23T09:50:00.000+01:00

On the 23rd of December we prepared the UV laser beam and synchronization to excite the first electron pulses.

Walter Knulst and Martien Vermeulen who have contructed the picosecond-pulsed 4.5-MeV electron accelerator. This project could not have been successfull without the support and help of many people: John Suijkerbuijk, Paul Rijkers, Juleon Schins, Laurens Siebbeles, Rene den Oudsten, Raymon Bresser, Rene Gommers, Ruud van Tol, Eindhoven University of Technology, Pulsar Physics, and many others.

2 - On the scope screen in blue the UV excited electron pulse is visible. To celebrate this event a bottle of champagne is donated by Laurens Siebbeles, the initiator of the accelerator project.

3 - We invited a lot of contribitors to the construction of the electron accerlator to see the first electron pulses on screen and take part of the celibration.

4 - Taking a look at the setup.

5 - Taking a view at the setup.

Synchronization of laser system with accelerator - December 2009

2009-12-02T21:17:00.021+01:00

In this post, I will present some details about the synchronization of the laser system with the RF electron accelerator.

1 - A photodiode monitors the 75-MHz output of the Ti:S oscillator. This signal is transported to a Phase Lock Loop than synchronizes a 3-GHz Voltage Controlled Oscillator to this laser, with ability to control the phase. This system has been published by Kiewiet et al. in 2001. The laser oscillator is the main clock of the system. To match the derived 3-GHz frequency to the resonance frequency of the RF cavity, the laser cavity length can be changed by displacing one of the end mirrors.

2 - These two boxes allow us to trigger the laser system by one external trigger from the main Trigger and Delay Generator. One trigger to the laser system leads to a pulse from the high power pump laser of the amplifier and the pulse selection by the two Pockel Cells in the regenerative amplifier cavity. This system operates at 1-kHz frequency.

3 - Because the accelerator operates at 100-Hz or lower frequency the laser frequency is down shifted by the use of a chopper and shutter combination. The chopper decreases the frequency to 100-Hz from which a fast shutter can select only a single pulse or it sub harmonic. We perform this operation to prevent the cathode of the RF cavity of being exposed to too many laser pulses that can cause damage.

6 - In front you can see the shutter and in the back the vacuum laser transport line to the accelerator room.

7 - The chopper and shutter controllers. The chopper is synchronized to the 1-kHz trigger of the laser system. Then only the right phase has to be selected to pass the right laser pulse from the 1-kHz train.

Details of Laser Beam - December 2009

2009-12-02T21:14:00.000+01:00

In this post, I will present some details of the optical layout.

1 - This is the side optical table. In the right corner in the front the laser beam transport line can be seen. Through this vacuum line the 800-nm fundamental laser beam from the Ti:S amplifier is transported. Using vacuum reduces instabilities in the laser beam caused by turbulence of air flow.

2 - We choose to transport the stretched amplified laser pulse directly output of the regenerative amplifier. At the output of the laser system a telescope is positioned to shape the laser beam to a smaller size. On this picture you see the compressor that is positioned on the side table.

3 - After the compressor the laser beam passes through the second and third harmonic unit. The third harmonic is used either to generate the electron pulse at the cathode surface of the RF cavity or excite the sample optically. The left over fundamental beam will be used to probe the sample. If necessary this fundamental wavelength can be converted into other wavelengths by using for instance white light generation.

4 - Around the Sample Chamber a frame is mounted to hold the opto-mechanical components. The laser beams (pump and probe) have to be deflected upwards to reach the windows of the vacuum system.

5 - Detail of the UV in coupling. One can see the prism's in the cross.

6 - Detail of the probe laser beam transport line to the window of the Sample Chamber.

Details of Sample Chamber - December 2009

2009-12-01T20:37:00.000+01:00

In this post, I will introduce some details of the Sample Chamber and explain their function.

1 - At the side of the Sample Chamber one can view through a big window into the chamber and see the carousel holding multiple samples. We have a standard piece mounted with diaphragm’s, a phosphor screen and test samples such as ZnS and ZnSe.

2 - The rotational feed through mounted on a x,y,z stage. We can rotate the right sample into the electron beam, retract the complete carousel out of the electron beam line and displace the sample through the electron beam.

3 - Behind the sample chamber but before the electron beam dump a beam viewer can be inserted into the electron beam. Using a Gigabit Ethernet CCD Camera we are able to follow the electron beam at a high frame rate.

4 - Here you see the beam dump at the deflected output port of the sample chamber. Using this beam dump, we can optimize the electron beam without irradiating the sample and causing unwanted degradation.

5 - Input window of the laser beam to the sample. We are not only able to excite the samples with the electron pulses, but in the same sample chamber we can excite them also with the laser pulse. In this way we can compare laser flash photolysis with electron beam photolysis without exchanging the sample to different experimental setups.

Details of Electron Beamline - December 2009

2009-11-30T20:02:00.000+01:00

In this post I will introduce the components of the beamline and explain their function.

1- Directly behind the RF cavity and RF input coupler a valve is positioned to separate the vacuum compartment of the cavity from the rest of the setup.

2- Then around the bellows a steering magnet is positioned. With this magnet we are able to position the electron beam on the sample and find the overlap with the probe beam. The probe beam is a time delayed ultrashort laser pulse than analyses the status of the sample after being excited by the electron pulse.

Additionally, we are able to displace the electron beam over the full sample surface. This is necessary, because the electron pulse is causing irreversible damage to the sample. By taking each time a fresh piece of sample, the measurement time can be extended before replacing the sample.

3 - Next, we have a cross-piece which is used to couple the UV pulse into the vacuum system to illuminate the cathode surface and generate the electron pulse. We use 266-nm laser light, which is the third harmonic of the 800-nm fundamental wavelength coming from a Ti:S amplifier.

In this cross-piece two aluminum coated prim's are mounted on a moving base. This base is constructed in such a way that when rotating the feed through below the gap between the prima's can be varied. One has to realize that the UV beam has to pass through the inner conductor in the coaxline upstream to the cavity. This has a narrow diameter. On the other hand, the prism's have to be placed to the side, because the electron beam is passing through the center. Due to the control of the gap, we can find the optimum position of the prisma's. Finally, the reflection of the UV pulse on the cathode surface can be seen at the output window. This is essential to find with the UV laser beam the center of the cathode surface. One can see all the diamond turning rings in the reflection of the UV laser beam.

4 - Sample chamber. It is a very complex piece. The essential function is that a carousel with multiple samples can be positioned with respect to the electron beam using a rotational feed through with x,y,z linear stages. The chamber has four laser windows (two input and two output) mounted at small angles with respect to the electron beam to probe the sample. We like to setup a visible probe line and a infrared probe line.

5 -In front of the sample chamber a bending magnet is mounted. This enables us to deflect the beam away from the sample into a side beam dump. This function serves multiple purposes: We can scan the electron beam energy; We can optimize the phase injection of the UV laser beam into the RF cavity; We can measure the background level to correct the measurement; By rapid switching of this magnet we prevent the sample from degrading and exposure to excessive electron pulses.

Installation of beamline including the Sample Chamber - December 2009

2009-11-28T16:35:00.007+01:00

In a previous post, I have introduced the concept of the beamline and Sample Chamber. Due to the failure of the orginal RF Window, the accelerator was out of order for at least two month. Due to this long break, we decided to upgrade the beamline and started to install the Sample Chamber into the beamline.

Below, you will find some pictures of the Sample Chamber. Beginning of December the complete chamber was installed.

Sample Chamber after the vacuum beaking out procedure.

Viewed from the other side. In the slot of the optical table you can see a large valve in the connection piece to the turbo pump.

Sample Chamber installed on the optical table. The RF cavity is situated at the left side.

RF Window problems - September 2009

2009-09-30T13:08:00.021+01:00

Shortly after we have achieved very good RF processing results in September 2009, we had a serious RF breakdown event at the RF window. This RF window separates the RF waveguide section at SF6 overpressure from the RF cavity at high vacuum.

In short, while running the klystron at about 7.5 MW output power a breakdown event occurred at the vacuum site of the RF window. We installed photodiodes to monitor breakdown light flashes from the RF cavity and from the RF window. At this specific event the light output from the window was evident. After this event, we had to turn the RF power level down to 1 MW and even then small discharges were appearing each pulse. RF processing did not improve this situation and lead to the conclusion that the RF window has been damages and had to be replaced.

We decided to replace the current CPI RF window of type VWX-1053 with a pillbox type RF window. After consulting some manufactures, we have ordered a RF window from CML Engineering of type 3020-02. This window has higher specifications in terms of peak and average RF power level and therefore will be more reliable. At the end of November 2009 this RF Window and adapter piece has been delivered.

The RF window that was installed originally.
The new RF window of CML Engineering installed in the RF section. The connection at the bottom is to the adapter piece for the connection to the RF input coupler of the RF cavity.
Side view of the RF window.

Milestones achieved during RF processing - 10th of September 2009

2009-09-17T09:13:00.003+01:00

While continuing RF processing of the RF cavity, the level of RF input power is gradually increasing. For instance, on the 10th of September we could ramp up very fast up to 3.9-MW input power and 3.0-MeV electron energy at 50 Hz without experiencing any breakdown.

By the end of the day the level of 7.1-MW input power and 3.8-MeV electron energy has been reached in case of a 1.0-us short pulse and 40-Hz repetition frequency. The measured peak dark current was 0.17 mA and the charge per pulse was 0.079 nC.

7.0 MW and 3.8 MeV

7.1 MW and 3.8 MeV

First accelerated electrons - 3th of September 2009

2009-09-17T08:30:00.004+01:00

During the first week of September we continued the RF processing of the RF cavity.

The second day we also turned on the main solenoid magnet around the RF cavity. As a result we had to repeat the training session of the day before, because the magnet influences all the processes inside the cavity. For instance, field emitted electrons follow different paths when the magnet is turned on.

On the 2^nd of September we soon reached 3.0-MW input power and 2.4-MeV electron energy. We continued the RF processing by replacing the 10-dB attenuator at the input of the preamplifier to 6 dB. Now we sometimes observe breakdown events with a measurable electron signal on the beam dump at the exit of the cavity and at the same time observe a light pulse on the photodiode that looks inside the RF cavity. Surprisingly, not all breakdown events are accompanied by electrons and light. Possibly, these events are taking place outside the RF cavity. By the end of this day, we have reached 3.6-MW input power and 2.9-MeV electron energy operating at 50 Hz.

The 3^th of September was a very exciting day, because we observed for the first time dark current coming out the RF cavity. The power level at that moment was 3.7-MW input power and 3.0-MeV electron energy. We were able to optimize the setting of the main solenoid magnet around the cavity by maximizing the dark current on the beam dump. We also observed single-side electron multipacting at the photocathode in the RF gun (see Han et al. in Phys. Rev. ST AB 11, 013501 (2008)). For instance, we have measured at the setting of 3.9-MW input power and 3.0-MeV electron energy a peak dark current of 52 nA and a total charge of 0.11 nC per pulse.

Below you can find two oscilloscope images displaying with the following signals: Light Blue - Modulator current; Purple - Forward RF Pulse; Green - Backward RF pulse; Dark Blue - Beam dump signal.

The first observation of dark current out of the RF cavity measured at the beam dump at the exit.

Dark current at a higher setting of the acceleration field.

FIrst RF Processing session of RF cavity - 31th of August 2009

2009-09-16T15:29:00.006+01:00

On Monday 31^th of August 2009 it was the day of starting the RF Processing procedure.

RF Processing is a neccesary step during the installation of the RF accelerator in which gradually increased RF power is applied to the cavity in order to clean and smoothen the inner surface. As a result the cavity can hold increasingly higher electric field levels without resulting in a breakdown event.

In order to perform a successful RF processing procedure, we have equiped the setup with several detection systems:
1- Breakdown detector: A piece of analog electronics monitors the backward reflected RF waves from the cavity and senses very abrupt changes in RF power that will occur upon breakdown inside the cavity. This is due to the change of impedance of the cavity when a breakdown occurs. When a breakdown is detected the RF pulse is immediantely interrupted.
2- Vacuum pressure monitor with switch: When the vacuum pressure inside the cavity exceeds a certain level the RF power to the cavity will be interupted untill the pressure has droped below the level again.
3- Faraday cup or electron beam dump at the exit of the cavity to monitor the electrons coming out of the cavity. The signal is presented on an oscilloscope.
4- Photodiode that monitors the light output of the cavity to detect the intensity of the breakdown event. The signal is presented on an oscilloscope.
5- Photodiode that monitors the light output near the RF window to detect the intensity of the breakdown event.

We have a few parameters that we can set in the system:
1- RF peak power by changing the RF input power to the Klystron tube by changing a variable discrete attenuator between 0.0 and 15.5 dB in steps of 0.5 dB.
2- RF peak power by changing the high voltage setting of the modulator. This can be seen as a fine adjustment of the RF power as compared to option 1.
3- RF pulse length by changing the modulator pulse length. This can be changed between 0.50 and 2.80 us flat top.
4- RF pulse repetition frequency can be set between 0 - 100 Hz as long as the average power of the modulator does not exceeds 6.5 kW. In general, we do not exceed 50 Hz.

The method that we apply is to start with the setting of the shortest pulse and increase in RF power. After a certain level has been achieved, the pulse length is increased, while keeping the maximum field strength in the cavity constant. Do to this the RF filling time of the cavity has to be taken into account. Once dark current is observed from the cavity, this is easily achieved by keeping the dark current constant. The last step is to increase the pulse repetition frequency. This procedure can be repeated untill the maximum RF input power is reached.

At 18:00 that day we started under supervison of the radiation safety department of TU Delft the RF processing procedure. We replaced the 20 dB attentuator at the input of the preamplifier to a 10 dB. Soon after the start we reached the level of 1 MeV electron energy. By midnight we haved reached 2.9 MW input power and 2.5 MeV electron energy. The estimated electric field inside the cavity is about 60 MV/m.

The accelerator is control from the control room. Left to right: Juleon Schins, Laurens Siebbeles, Koos van Kammen and Walter Knulst.	Walter Knulst is controling RF processing from the accelerator GUI build in LabView.
Walter Knulst together with Martien Vermeulen who is the allround electron accelerator engineer.	Another look at the control room.
A breakdown event is recorded by the oscilloscoop. In green you can see the abrupt change in RF reflection from the cavity and 400 ns later the breakdown detector has switched of the RF power.	Another breakdown event, which has unfortunately not been detected by the breakdown detector due to the fact that the rising edge is less steap than the event to the left.

Low power tests of RF Cavity - 27th of August 2009

2009-09-16T13:59:00.005+01:00

On Thursday 27^th of August 2009 the system was ready to be switched on. All subsystems have been tested before. We connected on this day the RF waveguide for the first time to the accelerator cavity.

The first test is to use very low power RF and measure the resonance frequency as a function of cavity temperature. This test is very exciting, because all RF parts have never been tested as one assembly.

The RF cavity has been checked by measuring the resonance frequency using a network analyzer after manufacturing. You can read more about electrical validation in a post from 2006. It has been observed that the resonance frequency is 2997.802 MHz at 20 ^oC in air.

In between the RF waveguide section and the RF cavity, a RF window and input coupler is placed. You can read more about the RF waveguide section in a post from 2006. The input coupler has been tuned in such a way that maximal RF power is coupled from the rectangular waveguide into the coaxial waveguide feeding the RF cavity. However, this has never been tested in combination with the actual cavity.

For the lower power test of the RF cavity, we have modified the RF system so that the maximum output power is reduced by 20 dB, i.e. 0.1 MW instead of 10 MW. This is done by inserting a 20 dB attenuator at the input of the 100 W preamplifier. You can read more about the RF system in a post form 2007. This safety measure protects the RF cavity from severe breakdown events.

We started by putting the RF cavity at 20 ^oC. We achieved a perfect incoupling at a frequency of 2999.080 MHz. The difference of -1.28 MHz with the measurement shortly after manufacturing is more than can be explained by the difference between air and vacuum of -0.95 MHz.

We continued the test by increasing the temperature by steps of 5 ^oC up to 35 ^oC. The resonance frequency shifts to lower frequencies when increasing the temperature. The resulting coefficient of 48 kHz/K is very close to the theoretical value of 48.5 kHz/K.

The conclusion is that the resonance frequency at 30 ^oC of the RF cavity (2998.600) is very close to the center frequency of the PLL. Therefore, we have decided to operate the RF cavity at this temperature setting.

We finished the low power test of the RF cavity by increasing the RF power.

Below you will find several scoop displays showing the reflection of RF power at the resonance frequency of the RF cavity at different temperatures. Dark blue: high tention of modulator; Light blue: current of modulator; Purple: Forward RF pulse; Green: Backward RF Pulse.

RF Cavity at 20 ^oC and frequency 2999.080 MHz.	RF Cavity at 30 ^oC and frequency 2998.600 MHz.
The temperature dependence of the RF Cavity.	RF Cavity at 30 ^oC and frequency 2998.600 MHz. Modulator pulse setting is 0.90 us and higher RF power.

Temperature Control of RF Cavity - 21th of August 2009

2009-08-21T10:34:00.003+01:00

We have constructed and tested the temperature control system of the RF Cavity.

This system has been designed to serve two purposes: 1- keeping the RF cavity at a constant temperature, so that the drift of the resonance frequency is as small as possible; 2- Cooling when RF pulses are applied and heating during intervening times.

The temperature control system consists of three segments: 1- Cathode section, 2- Cell sections and 3- Input Coupler section. Each section has one cooling water channel, one Thermocoax heating element and one thermocouple temperature sensor. The temperature of the section is controlled by a Eurotherm controller. The flow of the cooling water can be adjusted by remote control of a valve. The heating rate can be controlled by remote control of power supplies connected to the heating elements.

You can see the cooling water channel of the Cathode and Cell section very clearly at the photo's of the RF cavity model. In the same pictures one thermocouple is visible at the outer diameter of the full cell. There is also a thermocouple at the outer diameter of the half cell. So, we can control the temperature of each cell individually. However, due to the fact that the RF cavity is a big peace of high conductance copper, the thermal conductivity is very high and therefore the three sections are well coupled. This is a great factor of complexity.

Nevertheless, we were able to select the settings of the Eurotherm controllers in such a way that we have reached a very good temperature stability of less than 0.1 degrees Celsius.

A special feature that we have applied in the system is a cooling bias, so that there is always a minimal flow of cooling water. Therefore, the heating elements should deliver a minimal power to get the RF cavity at a constant temperature. Now, we have chosen the minimal flow values in such a way that the ratio of the heat input in the cathode section to the cell section is roughly the same as the expected ratio of the heat dissipation of the RF field inside the cavity from full cell to half cell. We hope that in this way the transient effects will be minimized due to the switching on and off of the RF pulses. This can only be tested when the RF pulses are applied.

On top the three Eurotherm temperature controllers. Below the three power supplies feeding the heating elements. To the left all the connections to the RF cavity are visible.

Concept of beamline and sample chamber - 11th of June 2009

2009-06-11T15:50:00.003+01:00

We have designed part of the beam line that will be placed behind the picosecond electron accelerator. It will consist of a view elements that have the function to illuminate the cathode surface with UV pulses, guide the electron beam to the sample and place different samples in the electron beam. All will take place in vacuum.

The samples will be placed on a big wheel so that we can test more that one sample before we have to open the sample chamer. The sample wheel can be moved with respect to the electron beam, so that the total area of the sample can be used to study the properies. The sample chamber has to laser beam lines that cross the samples at exactly the same position as the electron beam. We have the possibility to use to different wavelength regions to probe the sample, that are the VISible and the InfraRed.

Below, you will find to pictures showing the global layout. One is showing only the essential parts of the beam line and sample chamber. The other is showing the total cross section.

So, far a few components have been realized. One of them is the big sample chamber and the sample wheel. Those components are ready to be installed as soon as the RF processing is succeshully finished.

Essential components of the beam line and sample chamber: UV incoupling, steering magnets, filter wheel, beam monitor and faraday cup.

Total cross section of the accelerator, beam line and sample chamber.

Poster presented at PAC 2009 - 11th of June 2009

2009-06-11T15:40:00.004+01:00

Here is a copy of the poster presented at the Particle Conference 2009 (PAC 2009) in May 2009 in Vancouver Canada.

All rights reserved, Copyright (c) 2009, Delft, The Netherlands

Contribution to PAC 2009 - 11th June 2009

2009-06-11T14:52:00.003+01:00

We presented our accelerator setup as part of a shared contribution to the Particle Accelerator Conference 2009 (PAC 2009) held in May in Vancover Canada together with Eindhoven University of Technology and Pulsar Physics from The Netherlands.

Procedings will be published later. Below you can find the abstract. For more information

Design, construction and operation of the Dutch rf-photoguns

S.B. van der Geer, M.J. de Loos, W. op ‘t Root, W. Knulst, W. van Dijk, G.J.H. Brussaard, O.J. Luiten

Three different S-band rf-photoguns have been constructed by Eindhoven University of Technology in the Netherlands: A 1.5-cell, a 100 Hz 1.6-cell, and a 2.6-cell. They share a design concept that differs from the ‘standard’ BNL-gun in many aspects: Individual cells are clamped and not brazed saving valuable manufacturing time and allowing damaged parts to be replaced individually. The inner geometry employs axial incoupling, inspired by DESY, to eliminate any non-cylindrically symmetric modes. Elliptical irises, identical to a 2.6-cell design of Strathclyde University, reduce the maximum field on the irises and thereby reduce electrical breakdown problems. The manufacturing process uses single-point diamond turning based on a micrometer-precise design. The overall precision is such that the clamped cavities are spot-on resonance and have near-perfect field balance without the need for any post-production tuning. Operational performance of the three Dutch rf-photoguns will be presented.

Update on status - 11th of June 2009

2009-06-11T14:25:00.005+01:00

The current status of the system is that we are preparing for the first RF processing procedure to train the copper cavity. A few systems have to be put into operational before we can start, such as cavity temperature control, vacuum control and RF power control.

Last year we have had many setback, such as vacuum leaks to do cracks in the soldering. All have been repared and since the end of the summer 2008 the cavity has been installed into its stainless steel container. After that, a vacuum backout session has been conducted to clean the cavity.

In September 2009 we also finished a demonstration version of the RF cavity made out of brass that has been caoted by gold. I have added a few pictures.

Side view of the RF cavity. On the left is the cathode surface and on the right the coaxial incoupling of RF power that is also the electron beam exit.

On this side you can clearly see the springs that clamp the cavity cells together and press it on the RF in coupler. You can also see the cooling channels and the thermocouples to measure the internal temperature.

On this side you can see the coaxial structure. The RF in coupler or mode transformer is connected to the cavity on this side.

Our Nano Letters article receives attention

2008-05-28T15:52:00.001+01:00

Researchers demonstrate 'avalanche effect' in solar cells from PhysOrg.com

Researchers at TU Delft (Netherlands) and the FOM Foundation for Fundamental Research on Matter have found irrefutable proof that the so-called avalanche effect by electrons occurs in specific, very small semiconducting crystals. This physical effect could pave the way for cheap, high-output solar cells. The findings are to be published in scientific journal Nano Letters this week.

[...]

Major setbacks in RF and vacuum system - April 2008

2008-04-09T15:29:00.005+01:00

In the beginning of 2008 we have had two major setbacks in the RF and vacuum system. Restoring these damages aquires a lot of time.

We broke our RF window during a vacuum backing test, when it was vacuum sealed to the RF flange on the RF input coupler using a gold O-ring.

In the left picture you can see the RF input coupler (part with the copper rectangular waveguide). This part transforms the RF rectangular waveguide mode into a coaxial waveguide mode by a "door knob" mode transformer. At the end of the coaxial line the entrance iris of the cavity is positioned, so that the RF power is coupled into the cavity.

Since, we use all-metal seals also the RF window was supposed to be sealed by a gold O-ring of 1 mm diameter cross section. Therefore, in the second half of 2007 the O-ring groove was adjusted. This was done at the time when the RF flange was being resoldered after the discovery of a vacuum leak in the soldering of this flange.

Now, we continue using a Viton O-ring in stead of a gold one, to reduce the risk of breaking the our spare window as well. In the future, we will install a different type of RF window.
We cracked the soldering of the stainless steel flange connected to the cavity housing on the RF input coupler. This results in a big vacuum leakage.

In the right picture you can the flange in which the soldering has been cracked. This flange is sealed by using a Helicoflex metal O-ring to achieve complete all-metal seals. Probably, during assembling the forces we so high on the fange that it was deforming and thereby cracking the joint between the stainless steel flange and copper part.

At this moment, we are trying the repair the damage. When restored, we will use a Viton O-ring. In the future, we have to redesign the RF input coupler so that it is suitable for using metal seals.

Hopefully, we will get back on track next month by restaring the vacuum system. After a vacuum backing procedure we will start RF conditioning of the accelerator cavity.

RF input coupler installed in vacuum system. The RF window is connected to the upper RF flange of the RF input coupler.

The flange of the RF input coupler at which the inner joint was cracked when trying to assemble it to the stainless steel cavity housing using a metal O-ring.

Cavity Magnet installed and first test - 31th of October 2007

2007-11-02T15:53:00.001+01:00

This week the Cavity Magnet is connected to its power supply and cooling water. Subsequently, the first field measurements are performed.


Xantrex XDC 30-400 HV Power Supply (12 kW)

The on-axis field maximum is reached at 0.481 T at a current of 400 A. The power consumption is about 10 kW.

More measurements are required to get the full magnetic field information. More important is the overlap of the magnetic axis with the geometrical axis of the accelerator cavity.

Items on New-Energy TV [Dutch]

2007-10-23T15:34:00.000+01:00

Er zijn een aantal nieuwe filmpjes op New Energy TV verschenen over onderzoek naar nieuwe materialen voor zonnecellen, waar onze groep Opto-electronic Materials ook aan meewerkt:

14-10-2007
PV-research: Nederland doet weer mee
Joint Solar Programme: het volledige verslag over het fundamenteel onderzoek naar de ontwikkeling van zonnecellen
Bekijk het item »

19-09-2007
Johan Bijleveld onderzoekt plastic zonnecellen [bij TU Eindhoven]
Joint Solar Programme dl 1: plastics zijn goedkoper en prima in staat stroompjes door te geven.
Bekijk het item »

20-09-2007
Linda Aarts sleutelt aan de kleur van het licht [bij Universiteit Utrecht]
Joint Solar Programme dl 2: door met poeders het licht op een zonnecel te breken, haal je er meer stroom uit.
Bekijk het item »

21-09-2007
Joep Pijpers koerst op quantumdots [bij AMOLF in amsterdam]
Joint Solar Programme dl 3: quantumdots ketsen twee keer zoveel elektroden los uit een zonnecel = meer stroom
Bekijk het item »

Making demonstration model of accelerator cavity - 2th of October 2007

2007-10-02T15:20:00.000+01:00

Some pictures of the demonstration model of the cavity which is in preparation.

Picture 1	Picture 2
Picture 3	Picture 4

Installation of Cavity Magnet - 20th of June 2007

2007-06-22T12:10:00.000+01:00

This week we have installed the magnet around the cavity. This magnet will collimate and focus the electron beam exiting the accelerator cavity.

The magnet consists of an iron return joke and of hollow copper windings for water cooling. The windings are separated into nine sections. The current is connected in series through the section, while the water cooling is parallel through the section.

The Magnet is ready for installation. The bore of the magnet is large to be placed around the stainless steel container of the cavity.	For installation the stainless steel container has to be removed. The magnet will be attached to the aluminum 'manipulator'. With this support frame we can adjust the orientation and position of the magnet with respect tot the cavity.
The magnet is installed and the stainless steel container is put back into place.	Side view. You can see the bars with tubes. By turning the tubes the length can be adjusted so that the magnet orientation of position can be alterned.
The space between the magnet and stainless steel container is just a few milimeters. Just big enough manipulate the orientation and position of the magnet.	The weight of the magnet is supported by a spring. This makes the movement of the magnet much easier.

Cross Section of Accelerator Cavity

2007-06-22T11:20:00.001+01:00

This pictures shows a cross section of the electron accelerator that will be installed.

The cavity consists of two cells: one 0.6 cell and one full cell. The design frequency is 2998.5 MHz (about 10 cm wavelength in vacuum). The RF waves are coupled in axial by the coaxial line. The RF waves are converted from rectangular to coaxial waveguide by the RF input coupler. The inner counductor is hollow to pass the UV excitation pulse and electron bunch.

An ultrashort UV pulse illuminates the cathode surface. By the photolectrical effect electrons are emitted from the copper cathode (up to 1 nC). These electrons will experience the very high acceleration field (about 100 MV/m) inside the cavity. Due to the very high field strength the bunch leving the cavity will be still very short, about 1 ps. The final electron energy is little less than 5 MeV. The required RF power is roughly 8.5 MW peak power during a few microseconds.

Special feature of this cavity (designed by the Eindhoven University of Technology) is the fact that the cells are no longer brassed together but pressed by screws. Therefore, a stainless steel container is needed for vacuum purpose. However, due to the fact that the faces of the copper parts are turned by single diamand turning machines a very large compression rate is expected between the inner and outer vacuum sections.

On the outside of the copper cavity cooling and heating facilities are designed. There are in total three seperate cooling channels: 1- integrated in the cathode part, 2- integrated in the center iris part, 3- on the RF input coupler. There are two integrated heating elements (Thermocoax): 1- on the cathode part, 2- on the iris part. Two thermocouples will measure as good as possible the local temperature of the cells. Through the back flange of the stainless steel container several feedthrough can be seen to support all those features.

The back flange of the stainless steel container contains also a bucking coil. This magnet compensates locally on the cathode surface the magnetic field generated by a big magnet around the cavity. For electron beam optical reasons the magnetic field has to be zero at the point where the electrons are generated.

Due to the cilindrical symmetrical design a magnet can be placed around the cavity to focus the electron beam exitting the cavity.

Test installation of vacuumsystem - 4th of June 2007

2007-06-22T10:50:00.000+01:00

We have made the first test assembly of the vacuumsystem of the electron accelerator.

The cavity consists of two vacuum sections:

inner part, which are the cells of the cavity that will be pumped to a pressure below 10^-9 mbar by a Varian iongetter pump of type VacIon 150.
outer part, which is the space between the copper cavity and the stainless steel container that will be poump to a pressure of about 10^-6 mbar by a Varian iongetter pomp of type VacIon 20.

Outer section with RGA unit (MKS e-vision) attached.	Inner section. Directly under RF input coupler the VacIon 150 is placed on the floor. The double cross has two windows to send the UV beam to the cathode surface of the cavity.
Outer section. Via the double cross a pump by-pass is realized to the hollow inner conduction of the coaxial RF line.	Below the table the VacIon 150 is placed supported by springs instead of using a section with belows.

Electron Bunch Length Measurement

2007-06-19T09:19:00.000+01:00

Electro-optic technique for real-time, non-destructive, single-shot measurements of femtosecond electron bunch profiles.

Explanation of measurement technique can be found at the website of Geil Berden at the FOM-institute for Plasma Physics Rijnhuizen in The Netherlands.

Announcement conference PULS'2008

2007-06-19T08:58:00.000+01:00

The 8^th International Conference on Pulse Investigations in Chemistry, Biology and Physics PULS'2008 will be held on September 6 - 12, 2008, at Guest House of the Jagiellonian University "PRZEGORZAŁY" in Kraków, Poland.

Chairmen: Krzysztof Bobrowski (INCT) and Jerzy Lech Gębicki (IARC).

Poster NWO Spectroscopie en Theorie Meeting 2007

2007-06-06T14:40:00.000+01:00

Poster presented at the NWO Spectroscopie en Theorie Meeting 2007:

Abstract and Presentation Miller Conference 2007

2007-06-06T14:26:00.000+01:00

You can find my abstract and presentation of the Miller Conference 2007:

Highest RF peak power - 29th of May

2007-05-29T14:06:00.000+01:00

Today we reached the highest peak power of RF. We measured 6.8 MW of RF. We hope to increase this soon to 10 MW by tuning all the parameters.

RF pulse from 2157A Klystron - 25th of May

2007-05-29T13:28:00.001+01:00

In the figure you see the RF pulse (2998.5 MHz) produced by the 2157A Klystron. The modulator (ScandiNova AB) was operated at 165 kV, 4.9 us FWHM and 63 Hz. This is at full power level of 6.3 kW average power.

We have driven the input of the klystron at a low power (estimated at 2.5 W peak power, which is a tenth of the optimal power).

The output level measured by the RF diode detectors corresponds to 2.5 MW peak power. This corresponds to a small signal gain of +60 dB.

High Power RF Test - 23th of May

2007-05-25T15:16:00.000+01:00

We have performed the first high power RF tests in May.

The RF setup is orginized in the following way:
- The output of the RF source, Phase-Locked Loop (PLL) Synchroniser, is fed to the 100-W continuous RF amplifier, MAL tld..
- Inside the PLL a RF switch is integrated at the output, so that we can produce a low power (13 dBm) RF pulse of tens of microsecond in duration.
- The MAL amplifier (type AM82-3S-45-50R) has a small signal gain of 41 dB and has a output maximum of 50.2 dBm.
- The output of the MAL amplifier is fed to the input of the Klystron Tube. The klystron tube has a small signal gain of 57 dB.
- At the output of the Klystron Tube the waveguide section (WR284) is mounted, which contains a circulator with load (Ferrite SC3-118), bi-directional coupler and RF pressure window (CPI VWX-1053) at the accelerator cavity.
- To the bi-directional coupler RF diode detectors (Agilent 423B) are connected to monitor the forward and reflected RF power.

In this test phase the RF pressure window is replaced by a reflection plate, so that all the RF power is dissipated in the RF load on the circulator.

In the control room we have placed all the measurement equipment: two puls generators to supply the trigger pulse for the modulator and RF switch inside the PLL Synchronizer. The signals are displayed on a oscilloscoop.

For the computer in the control room the modulator can be controlled and all the operating parameters can be monitored.

Upper: PLL Synchronizer, Lower: MAL RF Amplifier

N-type Coax cable connected to RF input of Klystron Tube.

Bi-directional coupler in waveguide section. RF diode detectors connected to forward and backward outputs.

Trigger signal generation and signal analyzing in control room.

Control computer in control room.

Red warning light indicates accelerator operation.

Assembly of Laser Transport System - 23th of Februari 2007

2007-02-23T16:24:00.000+01:00

We have mounted the Laser Transport System to transport the high-power (2.5 W), femtosecond laser pulses (50 fs) from the laser laboratory to the accelerator laboratory. The laser beam is transported through vacuum pipes (10^-4 mbar) to remove all the influences of air (turbulances and dispersion).

The Laser Transport System consists of 4 boxes containing turning mirrors. Between the boxes vacuum tubes are mounted to transport the beam. The entrance and exit tubes are closed by vacuum windows (anti-reflection coated). Two complete laser beam lines can be hosted in the Laser Transport System. For the time being only one line will be used to transport the high-power fundamental beam (800 nm) from the femtosecond laser system.

Entrance of the Laser Transport System on the laser source optical table.	The first box (#1) is placed above the optical table and reflects the laser beams through the wall to the accelerator laboratory.
The three boxes in the accelerator room. The right most box is #2. Between box #3 and #4 the laser beam passes high above the walking area.	Layout of the mirrors in box #3. The laser beam is comming from the right. On the left the ion getter pump is located.
This is box #4.	Layout of the mirrors in box #4. The laser beam is coming from the right and reflected down.
The vacuum pipes from box #4 down to the laser table.	The exit of the Laser Transport System on the accelerator optical table.

Test assembly of Cavity Container with RF input coupler - Half Februari 2007

2007-02-23T16:10:00.000+01:00

Last week we tested the assembly of the stainless steel container for the cavity together with the RF input coupler. Now, we can finish soon the RF waveguide section to prepare the RF test phase.

Secondly, we can also start the assembly of the vacuum components.

(a) Stainless Steel container with flange on one side and RF input coupler (copper) on the other side. The flange will host cooling water feedthroughs and vacuum pump connector.

(b) The RF waveguide (black) is connected to the RF input coupler. Between a RF window is placed to isolate the vacuum side (10-9 mbar) from the pressure side (2 bar SF6).

(c) This is the side on with the electron beam line will be constructed.

Repair of azimuthal orientation - 7th of Februari

2007-02-12T08:49:00.000+01:00

During the assembly of the cavity on 20th of September 2006 in Eindhoven we discovered that there was an azimuthal mismatch between the cavity orientation and the flange of the stainless steel vacuum container (see foto 1 and 2).

The solution was to replace the fitting pin with a new one having an offset. Last week we installed this new pin. The last photo shows that the replacement was successful. Now both the cooling channel and all the bolts fit correctly.

1- Assembly in Eindhoven in September 2006
2- The mismatch between the cavity and SS container.

3- The new fitting pin.
4- Correct assembly.

25th Miller Conference on Radiation Chemistry

2007-01-03T11:15:00.000+01:00

I am planning to participate in the Miller Conference on Radiation Chemistry in April this year.

Current status just before Christmas

2006-12-22T17:23:00.000+01:00

A small overview of current status of the assembly of the electron accelerator setup.

Modulator and klystron assembly. Blue hat is lead shielding of klystron collector.	Cooling water machine to seperately cool the klystron tube, collector and magnet.
19"-inch cabinet with (top-to-bottom) three power supplies for klystron magnet, Ion getter pump power supply, PLL Synchronizer and RF amplifier .

Test assembly of RF waveguide section - 22th of December

2006-12-22T17:10:00.000+01:00

Today I have finished the test assembly of the waveguide section between the klystron tube and the RF input coupler at the cavity side.

Below you see a small impression.

Total waveguide section	Circulator directly behind klystron tube
Bridging section	Section to Cavity with bi-directional coupler
RF input coupler with RF window	4-port Circulator

Modulator test 28th of November 2006

2006-12-20T11:42:00.000+01:00

On this day we tested the modulator for the first time. The test was to check the pulses produced by the modulator without driving the klystron tube with RF power.

From the BNC output connectors the current and voltage pulses were obtained and recorded by an oscilloscope:

Note: The tension pulses are multiplied by a factor 2, because they were recorded using 50Ω termination, which causes the measured tension to be half of the proper value.

Final installation of Modulator and klystron at correct height – 3 November 2006

2006-12-20T11:39:00.000+01:00

Some pictures of the final installation.

Assembly of Modulator and klystron into frame – 30 October 2006

2006-12-20T11:33:00.000+01:00

Some pictures of the installation of the modulator and klystron into the frame.

Modulator	Klystron Magnet
Klystron tube

Note: The output of the klystron tube was to low. Therefore, we had to lift the modulator by almost 100 mm using cilinders.

Installation of waveguide frame – 24 October 2006

2006-12-20T11:30:00.000+01:00

In the picture you can see the frame that will support the RF waveguide section.

Frame for modulator and waveguide section.

Final assembly of Accelerator Cavity – 20 September 2006

2006-12-20T11:24:00.000+01:00

This day we went to Eindhoven University of Technology to do the final assembly of the accelerator cavity and take it to Delft.

Middle part	RF input coupler part
Full assembly of copper parts.	Copper cavity fixed into stainless steel container.

Electrical validation of Accelerator Cavity – 25 August 2006

2006-12-20T11:19:00.000+01:00

This day we went to Eindhoven University of Technology to perform electrical measurements on the accelerator cavity after the final turning. The measurements are summarized in the following report: ElectricalValidationCavity.pdf

Resonance frequency measured by Network Analyzer.

Beat ball measurement setup to measure the on-axis electric field strength.

Installation of optical table for electron accelerator – 8 August 2006

2006-12-20T11:14:00.000+01:00

In August the optical table on which the electron accelerator will be constructed, is installed.

Positioning of optical tables.	Optical table assembly positioned in the accelerator room.
Viewed from the other side.

Shipment of Modulator – 7 Aug 2006

2006-12-20T08:57:00.000+01:00

The shipment of the Modulator and klystron has arrived in Delft.

Modulator	Klystron
Klystron Magnet

Test of soldering at workshop of Eindhoven University of Technology – 11 Jul 2006

2006-12-20T08:46:00.000+01:00

The cooling water channels and Thermocoax Heating elements have been soldered. Before the final turning will start, this soldering is tested on leakages.

Copper cavity parts.	RF-input coupler side with Thermocoax heating element.
Assembly of middle part with cathode part including cooling water channels and Thermocoax heating element.

Visit to ScandiNova in Uppsala Sweden – 7 Jul 2006

2006-12-19T12:26:00.000+01:00

Beginning of July 2006 we have visited ScandiNova in Uppsala for the Factory Acceptance Test of the Modulator.

Left-to-right: W. Knulst (TU Delft), M. Vermeulen (TU Delft), H. Jansson (ScandiNova), M. Lindholm (ScandiNova)

Completion of Accelerator Room in DelftChemTech building – 3 March 2006

2006-12-19T11:51:00.000+01:00

The accelerator room is delivered beginning of March 2006.

Below you see two pictures taken at that time.

Entrance to the accelerator room.

Accelerator Room.

Start of Weblog

2006-12-19T11:43:00.000+01:00

Today I will start the weblog of the picosecond pulsed, 4.5-MeV electron accelerator.

The construction of the electron accelerator started already by the end of the summer 2006. So, the first posts on the weblog are describing event or milestones that have been achieved during the last year.

From now on, my intension is to post a few messages per month on the current status.