Superconducting RF Group

The Polarized SRF Gun Experiment

Layout: In the experiment we will operated a ½ cell 1400 MHz SRF gun in a cryostat as shown in the picture on the left. The electron beam will be accelerated in the gun to an energy of 0.8 MeV and exit the cryostat on the top.
The beam pipe has a ceramic break at the exit of the gun, so that the electron current leaving the gun can be measured. The ceramic is protected against damage by a G10 sleeve (light green). The picture shows also (in grey) the canister filled with NEG vacuum pump elements, which are used to maintain ultra high vacuum when the gun is not cold.  A solenoid magnet (yellow) is used to focus the electron beam. This magnet is placed inside of the aluminum baffles that are used as a heat shield. This position is above the liquid helium level, but it is cold enough for high temperature superconducting coils. Outside the cryostat the beam will be bent by 90o using a normal conducting dipole. The beam is then sent into a Faraday cup (not shown in this picture, coming soon). A window in the vacuum pipe above the dipole allows shining a Laser beam onto the cathode which releases the electrons from the cathode.
Below the gun is a valve, which is part of a mechanism allowing replacement of the cathode.
 
Since we expect degradation of the quantum efficiency of the cathode (this is what we want to measure), it is important to be able to move the cathode from the gun to a preparation chamber (shown below). This has to be done without breaking the vacuum or the cathode will immediately be “gunked up” by vapors of H₂O, CO₂, O₂ and others. For that purpose the cathode is mounted on the tip of a “cathode plug”, shown in red in the below picture.
 
For the replacement of the cathode the gun assembly is removed from the cryostat. A transport mechanism is connected to the gun valve. The space between the two valves is pumped down before the valves are opened. A magnetic actuator is then used to grab the cathode plug with a bayonet connector and retract it into the transporter. The valves are closed and the transporter is disconnected and then attached in the same way to a cathode preparation chamber. After regeneration of the cathode the procedure is repeated to move the plug back into the gun. This arrangement is designed and will be built by AES.

Figure 1: Transport mechanism and cathode plug

Figure 2: Gun assembly with attached transporter

Figure 3: Clamp mechanism to hold cathode in place when detached from the transporter

Figure 4: Preparation chamber with attached transporter
 

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