High-power experiment of a C-band photocathode gun

The C-band electron gun is an attractive option for lower emittance with compactness. In this paper, a new C-band photocathode gun has been developed. The electron gun experienced high-power test, and has preliminary reached the designed gradient on the cathode, 150MV/m with a repetition rate of 10Hz and pulse length of 2.5μs. The gun shows the potential for better performance with a higher gradient during high-power operation. Under extreme conditions, the gradient on the cathode is about 180MV/m. These high-power experimental results are the basis of the beam dynamics iteration and beam testing.


Introduction
The photocathode gun plays a critical role in an accelerator.In the 2000s, many mature designs of the pulsed gun came out, such as the design of BNL [1], Tsinghua, and PITZ [2], and have been applied in facilities like LCLS, SXFEL [3], and FLASH.The growing demand for low-emittance beams in advanced light sources has highlighted the need for better photoinjector designs.Moreover, the initial beam extracted by a shorter laser pulse based on stacking technology is a good option to improve the microstructures which become more visible in compression.Additionally, modern accelerator facilities require more compact designs.With the current RF gun designs, there is only limited scope or potential to further improve beam quality.
Beam dynamics studies of the photoinjector indicate that a higher gradient on the cathode can contribute to reducing emittance and maintaining electron beam brightness.Therefore, the C-band electron gun is an attractive option for achieving better beam quality with a short initial beam and compact photoinjector.A 3.6-cell gun [4,5] has been designed and optimized based on the research experience of C-band technology, which includes mode separation, field on the surface, and RF coupler.In addition to the standard RF design, the gun incorporates several other design features such as the removable cathode, the additional pumping holes on the cathode, and the separated cold head with adjustable water temperature attached to the cathode plate which can maintain the operating environment of the copper cathode.
This paper presents the high-power experiment of the 3.6-cell C-band photocathode gun, including the layout of the high-power experiment platform, the high-power RF conditioning results, and the discussions based on the experimental results.

High-power set-up of the gun
The optimized parameters of the C-band electron gun are shown in Table 1  The C-band electron gun has been processed and manufactured (Figure 1).As shown in Figure 2, the field balance is in general agreement with the ideal result at simulation through the low-power test which includes the bead-ball pulling method and tuning.And the frequency was increased from 5710.3MHz to 5712.4MHz.The RF frequency will be further compensated by proper water temperature.The low-power test results basically match the simulation results, with little deviations in coupling and quality factor.
The test platform for high-power is based on the SXFEL and is depicted in Figure 3.The adjustable power splitter is utilized to transmit the RF power generated by a 50 MW C-band klystron to the gun cavity.To prevent any possible reflected power, a circulator filled with SF6 is placed between the adjustable power splitter and the electron gun cavity.Ceramic RF windows isolate the vacuum at both ends of the circulator.Ion pumps are installed on both sides of the circulator to prevent vacuum deterioration in the waveguide due to SF6 leakage.Furthermore, a directional coupler is positioned downstream of the circulator and ion pump to measure the forward and reverse RF signal.Additionally, a Faraday cup is installed downstream of the electron gun to measure the dark current.
The online RF properties such as the frequency, the coupling coefficient, and the quality factor were tuned on the high-power test platform because of the vacuum environment and the  water-cooling environment.As shown in Figure 4, the resonant frequency of the operating mode with the compensation of the vacuum and the 58℃ water cooling is 5712.055MHz,which is very close to the design value.
By calibrating the klystron, the power level within the electron gun can be determined by recording the modulator panel voltage during the high-power experiment.The charge voltage is positively correlated with the output power, with 29MW of power output at 33kV.

High-power process and results
To reduce the BreakDown Rate (BDR) in high-power operation, the high-power test begins by RF conditioning the gun cavity from short to long pulses.Once the goal power of 14MW, corresponding to the 150MV/m gradient on the cathode, is reached, the input power is maintained for several tens of hours.Following this, higher power is fed into the cavity to test the peak gradient of the gun cavity.The RF conditioning process is facilitated by the C-band LLRF system and an auto-conditioning algorithm [6] in SXFEL, which includes an advanced breakdown identification and interlock system.According to the simulation results, the gradient on the cathode is 150MV/m at the forward power of 14MW.However, in order to obtain the actual gradient, beam testing is required.RF conditioning was not conducted continuously due to equipment maintenance and the use of TDS.The conditioning lasted more than 300 hours.At approximately 60 hours, the forward RF power increased to 20MW, which corresponds to a gradient of 180 MV/m on the cathode.To ensure stable operation of the electron gun, the breakdown threshold which is related to the reverse RF power in the auto-conditioning algorithm was lowered and the RF conditioning was restarted.The RF conditioning process is presented in Figure 6, which shows a gradual increase  At 270h of the commissioning, the 3.6-cell gun can reach the goal of 14MW under the pulse length of 2.5µs, and can operate stably at 17MW, corresponding to the gradient of 165MV/m.

Discussion
During the high-power experiment, repeatable gradient fluctuations were observed, with the gradient reaching 180 MV/m before the power level in the cavity dropped to below 10 MW.Upon rebooting the platform, the phenomenon occurred again.The conditioning process was stopped at 300h, and the rubber gasket at the pumping waveguide was found not assembled correctly, resulting in serious contamination in the gas area due to charring of the rubber After the replacement of the circulator and RF windows, the gun underwent another high-power test lasting two months, during which the phenomenon reoccurred, and an abnormal breakdown rate was observed.By analyzing the RF signal waveform, it was confirmed that the breakdown occurred before the gun cavity, and it was discovered that the new circulator cannot operate at very high-power levels for a prolonged period.At present, a third circulator is ready for another high-power experiment.The design of the internal load of the third circulator has been improved by replacing the small load patch design with an integral welding design.To date, the 3.6-cell gun has experienced the high-power test for more than 1700 hours, and there was no indication that the electron gun itself cannot work steadily at the designed gradient.
Even so, the high-power experimental results were applied to the beam dynamic simulations of the photoinjector.As shown in Figure 7, the beam emittance can be below 0.5 mm-mrad with the 500pC, 5ps beam, which is better than the current beam quality of SXFEL in simulation.

Conclusion
The 3.6-cell C-band photocathode gun has been developed in SXFEL in order to improve beam quality and compactness.The new gun can achieve a gradient of 150MV/m when operating at 2.5µs pulse length and 10 Hz repetition rate.Breakdown due to the component of the RF system has been observed.The electron gun demonstrates potential as an upgrade scheme for current XFEL facilities and will be further tested on the beam testing platform in subsequent studies.

Figure 2 .
Figure 2. The low-power test of the electron gun.The field distribution by bead-ball pulling(left), and the S11 of the gun(right).

Figure 3 .
Figure 3. Layout of the high-power test platform(left) and the C-band gun test area in the SXFEL beamline(right).

Figure 4 .
Figure 4. Online measurement of electron guns on high-power platforms.

Figure 5 .
Figure 5. Scope traces of the RF signals; blue: forward power; red: reverse power; yellow: gun probe.

14th
International Particle Accelerator Conference in the power the gun can withstand.

Table 1 .
. RF performance of the 3.6-cell electron gun