RF Window problems - September 2009

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.

First accelerated electrons - 3th of September 2009

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.

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

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.