Development of an X-band RF gun with four-feed coupler

With the high accelerating gradient, radiofrequency (rf) gun has a significant feature of suppressing the growth of transverse emittance caused by space charge. Field emission cathodes were first used in vac-uum electronic devices, which do not require the high electron beam intensity, but the compact geometry size of the cathode. A new X-band (11.424 GHz) rf electron gun has been proposed with the highlight of four-feed coupler, which can eliminate the quadrupole field component observed and analysed from the imagine experiment carried on, which have affected the resolu-tion of the imaging system to some content.


Introduction
In the past studies, field emission cathode rf gun have been confirmed to be competent for the research of rf breakdown.Combined with the diagnostic imaging system, it brings a possibility to fulfill the insitu mesoscopic-scale diagnosis of cathode with different appearance.Argonne National Laboratory (ANL) has developed an L-band rf gun as a test bed for study in Argonne Wakefield Accelerator facility (AWA).Faya Wang, Jiahang Shao, et al. have demonstrated a dark current imaging system based on this L-band (1.3GHz) rf gun, and confirmed two important conclusions: 1.There's a very strong correlation of the field-emission current with the stored energy; 2. rf breakdown and field emission current are closely related [1,2].Tsinghua University accelerator laboratory has also con-ducted related experiments on Tsinghua X-band high-power test stand (TpoT-X).Liuyuan Zhou have built an X-band (11.424GHz) RF gun combining with an imaging beamline to observe the evolution of copper cathode [3,4].Scheme of the two test stands above is illustrated in Figure 1.

Cavity
Relied on X-band high gradient acceleration technology, X-band rf electron gun has the advantages of a more compact structure and higher acceleration gradient, which can bring huge improvement of beam quality [5].In order to reduce the surface electric field intensity on the iris, so that the field emission current comes from the cathode surface as much as possible, the surface optimization scheme for S-band photo-cathode electron gun at Tsinghua University has been utilized in this design [6].The iris shape of the beam aperture is adjusted from a circular arc to an elliptical arc, which set the long-to-short axis ratio to 1.8.Since the first cell of the field emission electron gun is thinner than that of the photocathode electron gun, the long-to-short axis ratio of the elliptical arc of the iris of the first cavity is further optimized to 2.5 after parametric scanning in order to reduce the surface electric field intensity on the beam aperture.According to previous calculations, the maximum energy gain of the electron beam will be obtained at the exit when the first cavity length is set to 0.37-cell [3].We took the 0.5cell singlecavity L-band rf gun in AWA as a precedent and modified our gun from 2.37-cell to 0.37-cell singlecavity structure [8].The final geometry and the electromagnetic field map of the vacuum part of the rf gun simulated are plotted in Figure 2. The advantage of single-cell design is obvious, due to the extreme short cavity length of the first cavity (2.85mm in this design) in X-band rf gun, turning with the traditional tuner is a challenging and meticulous job.It seems to be an available way to tuning by adjusting the depth (distance that the cathode surface away from the cavity wall) and we verified the possibility when inserting the cathode in Ansys Electronics Desktop (HFSS).The simulation result is plotted in Figure 3.The 1.9 MHz frequency shift appeared when the depth moved 0.01 mm, with little deviation of the S-parameter and band-width, and the range of variation of Q value is acceptable.Therefore, such a structure can lower the requirement of tuning to a certain extent.Table 1 gives the parameter.

Coupler and Multipole Field
The scheme of T-type power splitter has been consulted in the design of the new four-feed coupler [7].
Matching structure has been added into WR90 wave-guide to realize the division and combination of microwave power.Moreover, low-loss transmission is achieved as a result of reduction of stand-wave in power coupler.Figure 4 illustrates the final geometry and the electric field map of the vacuum part of the coupler.Thanks to the compact characteristic of the X-band acceleration structure, the coupler can be machined directly on the copper block by using a CNC milling machine.Such process ensures the maximum circumferential symmetry of the coupler and prevents it from destroying the symmetry by machining errors.
In the present design of rf gun, field distribution on the circle with the radius of 1mm, which is located at the center of the cavity, is illustrated in Figure 5(a).Compared with the dual-feed structure in previous design shown in Figure 5(b), the octupole field takes the leading place among the multipole field components, and the four-feed structure design is effective.

Leakage of RF Power
To investigate the effects of different cathode materials (such as copper, silver-copper solder and other materials commonly used in accelerator processing), patterns and fabrication techniques on rf breakdown and other topics, the design of an X-band rf gun with replaceable cathodes was proposed.It means the cathode must be separated from the wall of the cavity, and the rf power leakage from the gap needs to be avoided.Liuyuan Zhou has indicated that the power wastage level of the coaxial-linelike gap should be pay more attention in his earlier dissertation.Since the power loss is proportional to the amplitude of magnetic field, the gap has been set at the place with a minimum magnetic field in TM02 mode.The leakage level in the gap of the new designed rf gun is plotted in Figure 6.An attenuation of -20 dB at the depth of 1mm inside the gap is achieved when comparing with the power level at the entrance of the gap.According to our machining accuracy, it keeps working effectively within the ±0.02 mm offset of the centre of the gap from A1 to A2 shown in Figure 1.And the 2.37cell X-band rf gun with replaceable cathodes has been fabricated and tested on TpoT-X in Tsinghua University [3], further illustrating the feasibility of this replaceable cathode structure.However, the prediction about the long-time exposure of electron gun and cathode to the air during the replacing period is absence in our scheme.It further led to a change in the electron emission capacity of the cathode and a decrease in the Q value of the electron gun.In the future experiment, a new cathode replacement system is under consideration in this plan, which will be equipped with the capability of changing cathodes in vacuum.Such cathode load-lock system has been verified on SwissFEL injector Test Facility [8].The blue line shows the result when the cathode is just split at the minimal magnetic field place, while the other lines represent the results of ± 0.02 mm offset at this place.

Conclusion
In this paper, we complete the rf design and thermal simulation of an X-band rf gun with four-feed coupler.The new gun inherits the advantages of the previous type such as high gradient, good quality factor.The suppression of the quadrupole field and the reduction of tuning difficulties have been achieved preliminarily.
In the future work, the simulation of beam dynamic will be finished to optimize the structure and an upgrade of beamline will be put on the schedule.Load-lock system is also under consideration.

Figure 1 :
Figure 1: rf gun test stand in (a) Argonne Wakefield Accelerator facility (b) Tsinghua University

Figure 2 :
Figure 2: Electromagnetic field map of the RF gun, simulated by HFSS.(a) Electric field (|E|) map of the pi-mode.(b) Magnetic field map (|H|) of the pi-mode.The white part in the red dash rectangle is the replace-able cathode.The cathode is separated from the cavity wall at the magnetic field minimal place but not the electric field minimal place.The upper block diagram shows an enlarged image of the separation location.(c) Electric field and magnetic field in transverse view.(d) The pi-mode longitudinal electric field value Ez(z) along the axis under an input power of 2.55 MW.

Figure 3 :
Figure 3: (a) Variation of scattering parameter with depth (distance that the cathode surface away from the cavity wall).(b) Variation of Frequency and Q value with depth.Blue line represents frequency; red line represents Q value.Table 1: Parameters of the gun

Figure 4 :
Figure 4: Electric field map of the four-feed coupler, simulated by HFSS.

Figure 5 :
Figure 5: Electric field distribution on the circle with the radius of 1mm along z-axis in rf gun.(a) With four-feed coupler.(b) Without dual-feed coupler.

Figure 6 :
Figure 6: Attenuation of rf power along the line from point A1 to point A2 in Figure 2. The blue line shows the result when the cathode is just split at the minimal magnetic field place, while the other lines represent the results of ± 0.02 mm offset at this place.

Table 1 :
Parameters of the gun