Superconducting RF Group
First Cool-down in
February 08
In February 08 we cooled the gun down for the first time to liquid helium temperatures (4.2o K), after the gun was cleaned at Jefferson Lab in Newport News, VA. The gun was not jet modified to receive the cathode plug. The above pictures show the cryostat in the crate that prevents it from tipping over and the gun assembly. The gun is mounted upside down, so that no particulate can drop into the gun. On top of the assembly there is an ion pump, which produced a vacuum of 2∙10-10 torr in the warm state. The ion pump is equipped with a pressure gauge. A number of temperature sensors are installed in different locations at the gun and helium liquid level was measured with a probe.
The liquid level is shown in the above graph. After about 15minutes of filling the cryostat has cooled down enough, so that the liquid helium stops evaporating immediately. After another two hours the cryostat is 100%. After 3½ hours the system has reached a thermal equilibrium. From the slope of the curve we calculate a heat load of 5 Watts. This is higher than expected. We will try to reduce the circulation in the baffles to improve the performance.
The second graph shows the temperature at the gun and the vacuum pressure
measured with the pressure gauge at the ion pump about 4 feet away from the
gun. Once the temperature at the gun has reached about 20o K the
ion pump is turned off and all vacuum pumping is done through cryo-pumping
of the gun. The pressure gauge bottoms out at 10-11 torr, but the
slope of the curve indicates that the pressure might have reached 10-12
torr. This confirms that an excellent vacuum can be reached in a
superconducting gun.
Finally, the Q of the gun was measured using a network analyzer. From the width of the resonance we find Q=1.5∙108 at 4.2o K This agrees with the value measured after the cleaning at Jefferson Lab and shows that the gun survived the transport without damage. At 1.8o K, which can be reached by lowering the pressure in the cryostat a Q > 109 will be reached.
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