A C-band Test Platform for the Development of RF Photo Cathode and High Gradient Accelerating Structures

In order to promote the studies of low emittance RF photo cathode and high gradient accelerating structures, a C-band test platform has been initialized since late 2021. In this paper, an overview of the present status and future plans of this platform is given. A 3.6-cell C-band RF photo cathode has been manufactured and now is under test. In addition, studies such as the design for a C-band RF traveling-wave accelerating structure and the design of a C-band high gradient parallel-coupled accelerating structure utilizing short pulse are introduced.


Introduction
The Southern Advanced Photon Source (SAPS) [1] is a 3.5 GeV 4th generation storage ring which is proposed to be built by the side of the China Spallation Neutron Source (CSNS), which is located at the Dongguan City, Guangdong Province, China.The baseline design has been finished in 2022.At the same time, several studies to verify new technologies and try prototypes of key devices have been started recently.We proposed a full energy linac as the SAPS injector, featuring a low emittance C-band RF photo gun and C-band high gradient accelerating structures (HGAS) [2].Based on the practical needs of SAPS and some potential applications utilizing the electron beam in compact systems, a C-band test platform has been initialized in late 2021 at the Dongguan Branch of the Institute of High Energy Physics, China Academy of Science.In addition, the C-band cryo-copper technology in HGAS attracted wide interests recently in the collider [3] and FEL communities [4].Such a platform can be used to test the cryogenic C-band HGAS with the addition of a cryogenic environment.
In this paper, the latest status of the platform will be given.Specifically, the progress of the development of a C-band RF photo gun, the design for a C-band travelling wave structure and a program to test high gradient parallel-coupled accelerating structure driven by short RF pulse, will be introduced.

Test platform overview
The test platform shares the space with other existing programs testing superconducting cavities inside a concrete bunker in the D2 Test Hall of the SAPS Research and Test Platform, which is a new research site built side-by-side to the CSNS Campus.For the first phase (Phase-I), the primary goal is to develop a low emittance C-band RF photo gun.The second phase (Phase-II) for the platform is to adding a normal conducting C-band accelerating structures to the RF photo gun, while other HGAS can be tested at the same time without beam.
Figure 1 shows the layout for Phase-I.It includes a 266 nm laser system at the beginning.The laser beam will be directed to hit the cathode surface of a 3.6-cell C-band RF photo gun with the help of a reflection lens (RL).The downstream is mainly designed for the beam diagnostics.A current transform (CT) is used to measure the charge of the electron beam.The beam emittance and transverse beam profiles are measured by the emittance monitor and several screens.The energy spread can be measured with the help of the bending magnet (BM).Faraday Cup (FC) is used as a beam dump and to monitor the beam charge and the dark current during the operation.At present, all systems for Phase-I are ready and being installed.Figure 2 shows a schematic layout of Phase-II.For this phase, a 2m-long C-band travelling wave structure (TWS) will be added to boost the energy to several tens of MeV.The downstream of Phase-I will be replaced to accommodate the beam diagnostics at this energy range.For example, quadruple triplet will be used to transport the beam and do the emittance measurement.Deflection cavity will be needed for the longitudinal beam shape measurement.Details of Phase-II are still under design.

C-band RF photo gun
In order to suppress the beam emittance further, especially for the modern Free Electron Laser (FEL), development of the C-band RF gun has made some solid progresses in recent years.For example, CompactLight Project [5], which is a hard x-ray design project, adopts the C-band RF photo gun in its baseline design and some prototyping program came along [6].The electric field on the cathode surface of a C-band RF photo gun can reach 150-180 MV/m in the high power test [8].
A 3.6-cell C-band RF photo cathode has been chosen mostly following the design in ref. [7] with some modifications for 100 Hz operation, targeting a normalized emittance goal of 0.2 µm at 100 pC.The fabrication has been finished recently and the cold test without tuning have been done.Figure 3 shows a photo of the prototype with the coupler mounted and the phase signal with the "bead-pull" method before tuning is shown in Figure 4. High power test and the beam test are planned within this year after tuning.A 266 nm laser system for the RF photo gun has been just installed in March 2023.Its parameters have been measured on site, as shown in Table 1.
Table 1.Measured parameters of the laser system for the gun(All are RMS values if not specified, measured at 100 Hz).

Summary and outlook
In this paper, a new C-band test form focusing on the development of C-band RF photo gun and high gradient accelerating structures have been introduced.While the gun will be tested soon to generate the low emittance electron beam, the development of a 40 MV/m C-band TWS and a 80 MV/m parallel-coupled accelerating structure will span over the next three years.In the near future, we expect to extend our research into some potential applications such as the terahertz (THz) radiation and ultrafast electron diffraction (UED) based on the technologies being developed.

Figure 1 .
Figure 1.Layout for the C-band test platform (Phase-I).

Figure 2 .
Figure 2. Schematic layout for the C-band test platform (Phase-II).

Figure 3 .
Figure 3. Photo of the 3.6cell C-band RF photo gun prototype with the coupler mounted.

Figure 4 .
Figure 4. Phase signal of the 3.6 cell C-band RF photo gun before tuning.

Figure 5 .
Figure 5. Two stage pulse compression scheme for the C-band test platform.
S-band X-band C-band Unit

Table 4 .
Parameters for the proposed parallel-coupled accelerating structure.