Positron sources are the key elements for the future and current lepton collider projects such as ILC, CLIC, SuperKEKB, FCC-ee, Muon Collider/LEMMA, etc., introducing challenging critical requirements for high intensity and low emittance beams in order to achieve high luminosity. In fact, due to their large production emittance and constraints given by the target thermal load, the main collider parameters such as the peak and average current, the emittances, the damping time, the repetition frequency and consequently the luminosity are determined by the positron beam characteristics. In this paper, the conventional positron sources and their main properties are explored for giving an indication to the challenges that apply during the design of the advanced accelerator concepts. The photon-driven positron sources as the novel approach proposed, primarily for the future linear colliders, are described highlighting their variety and problematic. 

Positron sources: from conventional to advanced accelerator concepts-based colliders

Positron sources are the key elements for the future and current lepton collider projects such as ILC, CLIC, SuperKEKB, FCC-ee, Muon Collider/LEMMA, etc., introducing challenging critical requirements for high intensity and low emittance beams in order to achieve high luminosity. In fact, due to their large production emittance and constraints given by the target thermal load, the main collider parameters such as the peak and average current, the emittances, the damping time, the repetition frequency and consequently the luminosity are determined by the positron beam characteristics. In this paper, the conventional positron sources and their main properties are explored for giving an indication to the challenges that apply during the design of the advanced accelerator concepts. The photon-driven positron sources as the novel approach proposed, primarily for the future linear colliders, are described highlighting their variety and problematic.

Pulse compressor is a useful and common RF structure for Free Electron Laser and Linear Collider Facilities, which can improve the input power of accelerating structures significantly. In this study, we designed, fabricated and tested a prototype of a compact C-band spherical pulse compressor. This pulse compressor utilizes one high Q0 spherical RF resonant cavity that works with two degenerated TE1,1,3 modes and a dual-mode polarized coupler. The cavity and coupler were designed and machined separately and then brazed together. The prototype was tested carefully using vector network analyzer, and a cold-test system was set up to check the working conditions of the spherical pulse compressor with inversed input power Associated with the C-band high-gradient accelerating structures, the peak power gain and energy multiply factor can be as high as 6.1 and 1.85, respectively, in theory. The average power gain is 3.8 in the first 0.372 μs after the phase reverse, which is the accelerator filling time. During the cold test, a peak power multiply factor of 6.04 and an energy multiply factor of 1.83 were achieved.
 The test results agreed with the design well. The prototype was successfully made as a pre-research for the future energy multiplying scheme of SXFEL.

Design, fabrication, and cold-test results of C-band spherical RF pulse compressor prototype

A multipurpose S-band high-power test stand (HPTS) developed at the Institute of High Energy Physics (IHEP) of China is routinely used for radio-frequency (RF) conditioning of couplers and resonant structures, as well as other high-power experiments in the S-band frequency regime. The HPTS with a peak power of 230 MW was developed to meet requirements of various applications with multiple frequencies (2856 MHz/2998 MHz) and power levels (45 MW/50 MW/65 MW/80 MW) for different accelerator projects at the IHEP, e.g. the High Energy Photon Source (HEPS), Beijing Electron Positron Collider (BEPCII), Circular Electron Positron Collider (CEPC). This paper describes the design and implementation of the system in detail. The highlight of the HPTS is the impedance-matched design to accommodate different impedances, allowing a single pulse modulator to supply different types of klystron loads. The control system, developed under the Experimental Physics and Industrial Control System (EPICS), adopts a combination of Micro Telecommunications Computing Architecture (MTCA) based low-level RF system (LLRF) and programmable logic controller (PLC) based interlock unit controllers, and enables a real-time data acquisition and visualization. To ensure a high reliability and high-gradient RF power conditioning, the interlock unit integrates various protection features, like the short circuit load, high standing wave ratio, low vacuum, arcing detector, and abnormal dark current. It is worth mentioning that the entire conditioning procedure can be performed unattended automatically controlled. The test stand has been operated stably for more than 7,000 hours for many rounds of high-power RF conditioning with different power/frequency requirements, which indicates that the HPTS can provide a stable, reliable, and automatic high-power testing for the development of S-band related devices.

Design and development of the S-band high power test stand for multi-purpose applications at IHEP

A compact $X$-band two-stage rf pulse compression system has been successfully designed, fabricated, and tested at Tsinghua $X$-band high-power test stand. The pulse compression system consists of a correction cavity chain, a first-stage, and a second-stage storage cavity. The correction cavity chain adopts a new design whose transmission loss and length are reduced by half compared with the old one. A detuning device is applied to the second-stage storage cavity so that the system can work in one-stage or alternatively two-stage compression mode. In the one-stage compression mode, a 150-ns, 70-MW flattop output, with a standard deviation of 1.5% in amplitude and 1\ifmmode^\circ\else\textdegree\fi{} in phase, was generated with a gain factor of 3. In the two-stage compression mode, a first two-stage pulse compression experiment with correction cavities in the $X$ band was performed. A peak power of 320 MW was achieved with a gain factor of 9.7 and full-width at half-maximum pulse durations of 53 ns. 

X
-band two-stage rf pulse compression system with correction cavity chain

The Circular Electron–Positron Collider (CEPC) is a 100-km ring e+e− collider for a Higgs factory. The injector of CEPC is composed of Linac and Booster. The Linac is a normal conducting S-band Lin...

CEPC Linac design

Circular Electron Positron Collider (CEPC) is a 100-km-ring e+ e− collider for a Higgs factory. The injector is composed of a 10-GeV Linac and a 120-GeV Booster. The CEPC Linac is a normal conducting S-band Linac with frequency of 2860 MHz providing electron and positron beams at an energy up to 10 GeV with 100 Hz repetition frequency. The positron source is the most important part of the CEPC Linac, and the bunch charge of positron beam within some cut-off condition should be larger than 3 nC. The positron source of CEPC is composed of a target, a flux concentrator, a capture section, a pre-accelerating section and a beam separation system. Higher positron yield and capture efficiency are the design goal. Based on simulation, the design will be presented and discussed in detail. The positron yield at the PSPAS exit is larger than 0.55, and the normalized rms emittance is 2370 mm-mrad with a strict cut-off condition. The detailed simulation results will be presented. The design of the positron source including positron production and capture will be presented and can meet the requirements.

CEPC positron source design

The pulse compressor plays an important role in accelerators by enhancing the RF peak power. An S-band (2856 MHz) spherical cavity pulse compressor consisting of a special 3 dB coupler and a single spherical energy storage cavity has been developed in Institute of High Energy Physics. The special 3 dB coupler converts the $$\hbox {TE}_{10}$$
 mode in rectangular waveguide to two polarization degenerated $$\hbox {TE}_{11}$$
 modes in cylindrical waveguide. It is also a circular polarizer and its general scattering matrix is given for the first time. The magnitude and the phase difference of the two $$\hbox {TE}_{11}$$
 modes are −3.016, −3.005 dB and 90.02
 $$^\circ $$
 in HFSS, while it is −3.01, −3.01 dB and 89.79
 $$^\circ $$
 in CST. The simulated S parameters show agreement with the general scattering matrix. The $$\hbox {TE}_{114}$$
 mode with an unloaded Q factor of 190,000 has been chosen to resonate in the spherical cavity. An energy multiplication factor of 1.75 and a peak power gain of 5.75 were obtained for the whole pulse compressor, which agrees with the design requirements. A good pulse compression performance is also proved by the CST time-domain simulation. In addition, the scheme for tuning and detuning is designed and given in detail. Compared to the traditional SLED type, the spherical cavity pulse compressor uses only one spherical cavity to realize the pulse compression, which makes it compact as a whole.

RF design of an S -band spherical cavity pulse compressor

C. Meng, Y. Chi, X. Li, G. Pei, S. Pei, D. Wang, J. Zhang Institute of High Energy Physics, Beijing, China Abstract Circular Electron-Positron Collider (CEPC) is a 100 km ring e e collider for a Higgs factory, which is organized and led by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) in collaboration with a number of institutions from various countries. The linac of CEPC is a normal conducting S-band linac with frequency in 2856.75 MHz and provide electron and positron beam at an energy up to 10 GeV with bunch charge in 1.0 nC and repetition frequency in 100 Hz. The linac scheme will be detailed discussed in this paper, including electron bunching system, positron source design, and main linac. Positrons are generated using a 4 GeV electron beam with bunch charge 10 nC hit tungsten target and the positron source design are presented. The beam dynamic results with longitudinal short Wakefield, transverse Wakefield and errors are presented. INTRODUCTION With the discovery of the Higgs particle at the Large Hadron Collider at CERN in July 2012, further research and measurement in Higgs is very important for particle physics. Because of the energy of Higgs is much lower than expected before, it is big possibility to build a circular collider as a Higgs factory. In September 2012, Chinese scientists proposed a Circular Electron Positron Collider (CEPC) in China at 240 GeV centre of mass for Higgs studies [1]. It could later be used to host a Super Proton Proton Collider (SppC) in the future as a machine for new physics and discovery. After that a great effort have been made in parameter choice and physics design [2][3]. With the deep study and more consideration in CEPC the design scheme has several versions compare to the Pre-CDR [4]. The latest scheme has some updates: the circumference is 100 km, baseline design of main ring is double ring, the linac energy is 10 GeV and also some more detailed optimizations. Table 1: Main Parameters of CEPC Linac Parameter Unit Value e /e beam energy GeV 10 Repetition rate Hz 100 e /e bunch population nC 1.0 Energy spread (e /e ) <2×10 Emittance (e /e) mm-mrad <0.3 e beam energy on Target GeV 4 e bunch charge on Target nC 10 The injector is composed of linac and booster. The Booster provides 120 GeV electron and positron beams to the CEPC collider and is in the same tunnel as the collider. The first part of the injector is a normal conducting Sband linac with frequency in 2856.75 MHz and provide electron and positron beam at an energy up to 10 GeV. The main parameters are shown in Table 1. The repetition rate is increased to 100 Hz from 50 Hz at Pre-CDR and one-bunch-per-pulse is considered, the linac mode maybe will be changed according to the injection scheme study. With the study of CEPC main ring and injection scheme, the bunch charge is decreased to 1.0 nC from 3.2 nC at Pre-CDR, however we also keep the ability to provide a 3.2 nC bunch beam by now. To obtain a 3.2 nC positron beam, one need a 4 GeV primary electron beam with bunch charge in 10 nC hit a tungsten target. Based on a lot of discussion of linac scheme, we choose the linear scheme as the baseline design by now, which is

CEPC Linac Design and Beam Dynamics

The CEPC includes a main ring and an injector. The injector consists of a booster and a linac. In order to meet the requirements of the booster, the baseline design of the linac is a 10 GeV electro...

R&D of CEPC injector linac

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