Shengwen Quan

Peking university superconducting accelerator facility for free electron laser

Peking University Superconducting Accelerator Facility (PKU-SCAF) is to generate high-quality electron beams with high average current. It is mainly composed of a DC-SC photocathode injector and a superconducting accelerator. It will operate in CW mode. The energy of the electrons is 20–35 MeV and the emittance is o15p mm mrad. PKUSCAF will be used for Free Electron Lasers. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.85.Ar; 41.60.Cr

Ultrafast electron diffraction with megahertz MeV electron pulses from a superconducting radio-frequency photoinjector

We report ultrafast relativistic electron diffraction operating at the megahertz repetition rate where the electron beam is produced in a superconducting radio-frequency (rf) photoinjector. We show that the beam quality is sufficiently high to provide clear diffraction patterns from gold and aluminium samples. With the number of electrons, several orders of magnitude higher than that from a normal conducting photocathode rf gun, such high repetition rate ultrafast MeV electron diffraction may open up many new opportunities in ultrafast science.

Emittance measurement by using Duo image pattern of Cherenkov radiation

The emittance of the electron beam is to be measured by using Cherenkov radiation in a novel way. The image patterns of the radiation are formed both in the focal plane and image plane of an achromatic lens with long focus. Both the angular spread and radial distribution of the e-beam are obtained by processing both patterns and in this way the beam RMS emittance is directly resulted. Combining this technology with computerized tomography (CT), the actual particle density distribution in transverse phase space can be determined without any preassumptions. The emittance of the electron beam from the DC-SC photoinjector of Peking University is simulated and a He-Ne laser experiment is presented and discussed in this paper.

Research and experiments on niobium-sputtered superconducting QWR in Peking University

R&D on SC quarter wave resonators (QWRs) for heavy ion linacs were initiated in China several years ago. A DC bias-voltage sputtering technology was developed in Peking University for niobium-coated copper QWR. A niobium-sputtered copper QWR with good niobium films was successfully made in October 1999. Low temperature experiments were carried out from November 1999 to September 2000. The accelerating gradient of 5/spl sim/6 MV/m can be obtained with no beam load at 4.2 K. The QWR is installed after the 2/spl times/6 MV tandem and is used to accelerate proton beams. The accelerating gradient of 3 MV/m was obtained with a proton beam load.

An alternative way to increase the power gain of resonant rings

Resonant rings which can amplify RF power through the coupling of waves are used for high power breakdown tests, unidirectional filters, or pulse-shaping techniques. Usually, the RF output terminal of a resonant ring is connected to a matched load. For the resonant ring at Peking University, the matched load has been replaced by a waveguide shorting plate to obtain higher conditioning power for the 1.3 GHz capacitive type power couplers. The power gain is increased significantly with this short termination with the same input RF power. Working mechanism analysis, experiments, and results of this modified resonant ring will be presented.

Electron Bunch Train Excited Higher-Order Modes in a Superconducting RF Cavity

Higher-order mode (HOM) based intra-cavity beam diagnostics has been proved effectively and conveniently in superconducting radio-frequency (SRF) accelerators. Our recent research shows that the beam harmonics in the bunch train excited HOM spectrum, which have much higher signal-to-noise ratio than the intrinsic HOM peaks, may also be useful for beam diagnostics. In this paper, we will present our study on bunch train excited HOMs, including the theoretic model and recent experiments carried out based on the DC-SRF photoinjector and SRF linac at Peking University.

Drive laser system for the DC-SRF photoinjector at Peking University*

Photoinjectors are widely used for linear accelerators as electron sources to generate high-brightness electron beams. The drive laser, which determines the timing structure and quality of the electron beam, is a crucial component of a photoinjector. A new drive laser system has been designed and constructed for the upgraded 3.5-cell DC-SRF photoinjector at Peking University. The drive laser system consists of a 1064 nm laser oscillator, a four-stage amplifier, second and fourth harmonic generators, an optical system to transfer the UV pulses to the photocathode,and a synchronization system. The drive laser system has been successfully applied during stable operation of the DC-SRF photoinjector and its performance meets requirements. A 266 nm laser with an average power close to 1 W can be delivered to illuminate the Cs2 Te photocathode and the instability is less than 5% for long time operation.The design considerations for improving the UV laser quality, a detailed description of the laser system, and its performance are presented in this paper.Photoinjectors are widely used for linear accelerators as electron sources to generate high-brightness electron beam. Drive laser, which determines the timing structure and quality of the electron beam, is a crucial device of photoinjector. A new drive laser system has been designed and constructed for the upgraded 3.5-cell DC-SRF photoinjector at Peking University. The drive laser system consists of a 1064 nm laser oscillator, a four- stage amplifier, the second and fourth harmonic generators, the optical system to transfer the UV pulses to the photocathode, and the synchronization system. The drive laser system has been successfully applied in the stable operation of DC-SRF photoinjector and its performance meets the requirements. 266 nm laser with an average power close to 1W can be delivered to illuminate the Cs2Te photocathode and the instability is less than 5% for long time operation. The design consideration for improving the UV laser quality, a detailed description of laser system, and its performance are presented in this paper.

A 1.3 GHz Cryomodule with 2x9-Cell Cavity for SETF at Peking University

The straight beam line of SETF at Peking University is under construction, which consists of a DC-SRF photoinjector and a superconducting linac with two 9-cell cavities. Stable operation of the DC-SRF photoinjector has been realized and the design, manufacture and assembly of the cryomodule with two 9-cell cavities have been completed. Improved capacitive coupling RF power coupler and fast tuner with piezo are adopted.

Improved Capacitive Coupling Type RF Power Couplers for a Cryomodule With Two 9-Cell Cavities

A capacitive coupling [1] RF power coupler has been used for the DC-SRF [2] photoinjector at Peking University. Recently, improved capacitive coupling type power couplers, which will be used for a new cryomodule with two 9-cell cavities have been designed and fabricated. The main modifications include enlarging the supporting rods of inner conductors in order to increase heat conduction, moving the bellows from the quarterwave transformer to the 50 Ohm coaxial line to avoid the mismatch during Qext adjusting. RF conditioning of two modified power couplers has been carried out and 10kW RF power passed the couplers with a duty factor 30%. In this paper, detailed design based on multi-physics analysis and the conditioning of this improved capacitive coupling type RF Power coupler are presented. INTRODUCTION A cryomodule with two 1.3GHz 9-cell cavities [2] is under construction at Peking University. The capacitive coupling type RF power coupler, which has been used for the DC-SRF photo injector, will still be used for this cryomodule will still use the kind of capacitive coupling type RF power coupler. Figure 1: Structure of capacitive coupling type power coupler for the DC-SRF photoinjector at Peking University. The capacitive coupling structure as shown in Figure 1 has the following advantages [3]: 1) the ceramic window is a whole disc, which makes it easy for fabrication and process. 2) As the inner conductor is divided into two parts, it is convenient to install the coupler into the cryomodule. 3) There is no axial stress due to the separation of inner conductor and window. However, problems of this kind of coupler have been found during the beam loading experiment of the DC-SRF photoinjector. An obvious temperature increase around supporting rods was observed when the power was over 8 kW. With the original design, the bellows is within the quarter-wave transformer. During Qext adjusting, the variation of the bellows’ length will cause RF mismatching. Therefore, modified couplers have been designed and fabricated. In this paper, the design, fabrication and conditioning of this improved capacitive coupling type RF Power coupler are presented. ENLARGING SUPPORTING RODS FOR INNER CONDUCTOR For solving the problem of temperature increase, we try to enlarge the supporting rods for inner conductor and the thermal analysis has been carried out. We compared the temperature distribution of couplers with original design and modified design by using ANSYS code. Figure 2 shows the temperature distribution of the original PKU coupler with the input RF power of 10kW and the supporting rod diameter of 4mm. The highest temperature is 452.5K and it occurs at the end of the inner conductor. Figure 2: Temperature distribution of original power coupler at 10kW, CW. Figure 3 shows the temperature distribution of the modified coupler with the input RF power of 10 kW and the supporting rod diameter of 24.1mm in warm section and of 18 mm in cold section. The highest temperature is 388.1K and it occurs at the end of inner conductors of the warm section. Figure 3: Temperature distribution of modified power coupler at 10kW, CW. ____________________________________________ *Work supported by Major State Basic Research Development Program of China (Grant No. 2011CB808303 and 2011CB808304) Email: kxliu@pku.edu.cn Proceedings of SRF2015, Whistler, BC, Canada THPB079 SRF Technology Ancillaries G03-Power Coupler ISBN 978-3-95450-178-6 1313 C op yr ig ht

DC-SC Photoinjector with Low Emittance at Peking Univeristy

High average power Free Electron Lasers require the high average current electron beams with the low emittance. The DC-SC photoinjector developed at Peking University, directly combining a DC Pierce gun with an SRF cavity, can produce the high brightness beam with moderate bunch charge and high duty factor. In order to optimize the transverse emittance which sharply increases at the beginning of the DC gun, a carefully design for the DC gap and the cavity shape is needed. In this paper, the simulation of an upgraded design has been performed. This photoinjector, consisting of a DC gap and a 2+1/2 cell SRF cavity, can produce 4.2 MeV electron beams with 100 pC per bunch and 1.1 μm normalized emittance.