First experience with He conditioning of a superconducting rf photoinjector

The recent achievements in the performance of superconducting rf (SRF) photoinjectors have opened a new era in the development of reliable high-brightness cw electron sources. While the SRF guns become one of the most promising technologies, the compatibility of the SRF environment with the complex photocathodes remains at the forefront of the modern accelerator science. The SRF cavities operate at cryogenic temperatures providing the ultrahigh vacuum environment highly beneficial for the photocathode performance. However, the necessity to keep the photocathodes at room temperature while being surrounded by the cavity walls that are kept at cryogenic temperatures creates an additional complexity for the SRF gun design and operation. The complex and volatile chemical compounds used for photocathodes have a high chance of contaminating the surfaces of the SRF cavity. When deposited, such compounds could create centers for cold electron emission which degrade the performance of the SRF guns. Such a circumstance would require the development of the in-situ processing techniques for restoring the SRF cavity performance. This paper presents the results of the successful implementation and application of the He conditioning method for cavity restoration using the existing SRF photoinjector at Brookhaven National Laboratory (BNL). The method has proven to be extremely effective and resulted in a dramatic improvement in the BNL gun performance.

Figure 1

Schematic of the CeC SRF gun assembly indicating the major system components.

Figure 2

Detailed schematic of the cathode puck and cathode manipulator end effector assembly.

Figure 3

Detailed view of the SRF gun cryostat with the critical vacuum components. The cathode insertion is on the right side of the schematic and the FPC is on the left side. Here $\mathrm{ip}=\text{ion}$ pump; $\mathrm{tc}=\text{thermal}$ conductivity gauge, reads pressure from atmosphere to $1\times {10}^{-3}\mathrm{Torr}$; $\mathrm{cc}=\text{cold}$ cathode gauge, reads $1\times {10}^{-3}$ to $1\times {10}^{-11}\mathrm{Torr}$; $\mathrm{sv}=\text{vacuum}$ valve.

Figure 4

Schematic of the clean He gas delivery system for the conditioning of the CeC SRF cavities. Here $\mathrm{ip}=\text{ion}$ pump; $\mathrm{tc}=\text{thermal}$ conductivity gauge; $\mathrm{cc}=\text{cold}$ cathode gauge; $\mathrm{sv}=\text{vacuum}$ valve; $\mathrm{NEG}=\text{nonevaporable}$ getter pump; $\mathrm{UHP}=\text{ultrahigh}$ purity grade.

Figure 5

(a) Cathode installed at the end of the cathode stalk. An asymmetric circular pattern can be seen here. (b) After the cathode was extracted from the gun, this image shows a gap that had opened up between the puck and the end effector.

Figure 6

(a) Spring finger element. One of the fingers clearly shows a burn mark. (b) Shows the plastic deformation of the spring finger being “pulled forward”.

Figure 7

The highest achieved cw (a) and pulsed (b) cavity voltage after the first case of He conditioning.

Figure 8

Gun voltage, pressure in the cavity, and radiation levels during HeC.