Standard Cs2Te Cathode
Cs2Te is a p-type semiconductor with the work function of 3.5 eV (with a band gap of 3.3 eV and an electron affinity of 0.2 eV). Because of its high efficiency and robustness in rf field, Cs2Te is chosen as the standard photocathode material for the SRF guns at HZDR.
Cs2Te coated on Molybdenum has demonstrated favourable QE and a prolonged operational lifetime in the ELBE SRF gun-I [1]. However, in 2017 the overheating phanomina during Gun operation were observed in the cathodes due to the insufficient thermal conductivity between the Mo plug and Cu cathode body. since 2020, Cu plugs have been adopted instead of Mo, despite the inherent challenges associated with utilising copper as a substrate for Cs2Te. Generally Cs2Te (on Cu) photocathodes in SRF Gun-II provide a QE above 1%, an operation time of several months to one year and a charge life time up to 85 Coulomb (data from Cs2Te #2023.08).
Preparation and transport
The Cs2Te photocathodes were prepared in the HZDR cathode laboratory and subsequently transported to the SRF gun via a vacuum suitcase. More information can be found in [2]. The prepared cathodes are stored in the transport suitcase with a vacuum around 5 ×10-11 mbar. Up to six photocathodes can be moved together in the vacuum transfer system from the photocathode lab to the SRF gun. During cathode exchange, a vacuum in level of 10-10 -10-9 mbar can be maintained. An important notice is, that the manipulation of cathode transfer must be particle free for cathode plug and thus for the gun cavity.
Cathode evolution during operation
Cathode QE and distribution can be tracked during its whole life time. for example cathode #2023.08 as illustrated in the following figure. A pronounced decline in the QE was at first observed when the radio frequency power was initiated although a dedicated multipacting training process was carefully performed before the high gradient CW RF was finally built in the cavity. The coating exhibited homogeneity with a narrow shadow of the mask along the border. During the beam operation, the cathode QE exhibited a tendency to very slow slippery but remain relatively constant. After CW RF field was successfully loaded in the cavity, the cathode showed a homogeneous platform with a narrow border. However, the cathode exhibited uniform degradation, manifesting as a "basin" in the center of the cathode. As the operation continued, the right side of the film exhibited a more rapid degradation trend than the left side. The center platform underwent a continuous decline in elevation, yet ultimately returned to a homogeneous state.
QE evolution of Cs2Te
Source: Dr. Xiang, Rong
Considering the laser illumination, beam production and extracted charge versus serving time, there is no obvious relation between the charge extraction and the QE change. It is evident that the vacuum was not the primary factor responsible for this phenomenon. SRF cavity should provide a good vacuum environment around the cathode. Vacuum measurements at the gun exit demonstrated a pressure of 4 - 5 × 10-10 mbar, indicating that no visible accidents had occurred.
It is postulated that the photoelectrons and dark current impact the cavity wall, resulting in the release of absorbed gases, which subsequently contaminate the Cs2Te surface. However, if the remaining gas molecules were ionized by accelerated photoelectrons and dark current, these ions could also be accelerated by the RF field and the cathode DC field (-5 kV on cathode), respectively, and subsequently back bombard the cathode surface [3]. A further related topic of investigation is whether the presence of X-rays within the gun cavity exerts any influence on the cathode film.
Reference:
[1] R. Xiang, et al., Cs2Te normal conducting photocathodes in the superconducting rf gun, Phys. Rev. ST Accel. Beams 13, 043501 (2010)
[2] R. Xiang et al., Photocathode study in SRF Gun-II at HZDR, in proceedings of 32nd Linear Accelerator Conference (LINAC2024), 25.-30.08.2024, Chicago, USA. DOI: 10.18429/JACoW-LINAC2024-TUPB034
[3] R. Xiang et al., Study on QE Evolution of Cs₂Te Photocathodes in ELBE SRF Gun-II, in Proc. IPAC'22, Bangkok, Thailand, Jun. 2022, DOI:10.18429/JACoW-IPAC2022-THPOPT022