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.