Study of the heavy ion bunch compression in CSRm

The feasibility of attaining nanosecond pulse length heavy ion beam is studied in the main ring （CSRm） of the Heavy Ion Research Facility in Lanzhou. Such heavy ion beam can be produced by non-adiabatic compression, and it is implemented by a fast rotation in the longitudinal phase space. In this paper, the possible beam parameters during longitudinal bunch compression are studied with the envelope model and Particle in Cell simulation, and the results are compared. The result shows that the short bunch ²³⁸U²⁸⁺ with the pulse duration of about 50 ns at the energy of 200 MeV/u can be obtained which can satisfy the research of high density plasma physics experiment.     

Bunch compression study using the envelope model in CSRm

The feasibility of attaining a short-pulse-duration heavy ion beam with a nanosecond pulse length is studied in the main ring of the Heavy Ion Research Facility in Lanzhou (HIRFL). Such a heavy ion beam can be produced by non-adiabatic compression, and it is implemented by fast rotation in the longitudinal phase space. In this paper, the possible beam parameters during longitudinal bunch compression are studied by using the envelope model. The result shows that a shortest heavy ion bunch ²³⁸U²⁸⁺ of 29 ns with energy of 200 MeV/u can be obtained, which can satisfy high energy density physics research.     

用于HIRFL-CSRm纵向压缩高频腔的磁合金环测试

在HIRFL-CSRm上进行高能量密度物理和等离子体物理的研究需要一个能提供高电压的高频腔来对束团进行纵向压缩, 而磁合金加载腔获得较高的加速场梯度可以满足这一要求, 且腔体不需要调谐, 从而简化了高频控制系统。为了选择磁导率和阻抗较大、Q值(品质因数)小于1的磁合金材料来加载压缩高频腔, 对型号为V和A的两种磁合金材料进行了测试。测试结果表明: 型号为V的材料具有较大的磁导率、 阻抗和小于1的Q值, 将被用来加载腔体以获得足够高的加速电场梯度。A high voltage RF cavity is required to compress the beam bunch in the longitudinal for high energy density physics and plasma physics research at HIRFL CSRm. A magnetic alloy (MA) loaded cavity which has high acceleration gradient and without tuning loop (can simplify the RF control system), can meet the requirement. In order to select a proper MA material with higher permeability and shunt impe dance, Q< 1 value to load the RF compression cavity, two different MA materials V and A have been tes ted. The results indicate that the material V will be better for the construction of the RF cavity than material A because of its higher permeability and shunt impedance, Q< 1 value.     

Study of a magnetic alloy-loaded RF cavity for bunch compression at the CSR

The Heavy Ion Research Facility and Cooling Storage Ring（HIRFL-CSR）accelerator in Lanzhou offers a unique possibility for the generation of high density and short pulse heavy ion beams by non-adiabatic bunch compression longitudinally,which is implemented by a fast jump of the RF-voltage amplitude.For this purpose,an RF cavity with high electric field gradient loaded with Magnetic Alloy cores has been developed.The results show that the resonant frequency range of the single-gap RF cavity is from 1.13 MHz to 1.42 MHz,and a maximum RF voltage of 40 kV with a total length of 100 cm can be obtained,which can be used to compress heavy ion beams of 238U72＋ with 250 MeV/u from the initial bunch length of 200 ns to 50 ns with the coaction of the two single-gap RF cavity mentioned above.     

A waveguide overloaded cavity kicker for the HLS Ⅱlongitudinal feedback system

In the upgrade project of Hefei Light Source(HLSⅡ),a new digital longitudinal bunch-by-bunch feedback system will be developed to suppress the coupled bunch instabilities in the storage ring effectively.We design a new waveguide overloaded cavity longitudinal feedback kicker as the feedback actuator.The beam pipe of the kicker is a racetrack shape so as to avoid a transition part to the octagonal vacuum chamber.The central frequency and the bandwidth of the kicker have been simulated and optimized to achieve design goals by the HFSS code.A higher shunt impedance can be obtained by using a nose cone to reduce the feedback power requirement.Before the kicker cavity was installed in the storage ring,a variety of measurements were carried out to check its performance.All these results of simulation and measurement are presented.     

Tracking simulations for the HLS-Ⅱ with a passive harmonic cavity in the symmetricand asymmetric fill patterns

A simulation code that executes the tracking of longitudinal oscillations of the bunches for the double rf system of the Hefei Light Source Ⅱ Project (HLS-Ⅱ) is presented to estimate the mean beam lifetime and the Robinson instabilities. The tracking results show that the mean beam lifetime is in agreement with the analytical results and the system is stable when we tune the harmonic cavity in the optimum lengthening conditions. Moreover, the simulated results of the asymmetric fill pattern show that some bunches are compressed only with a 7% gap (3 gaps), which will lead to the reduction in the mean bunch lengthening and potential beam lifetime. It is demonstrated that HLS-Ⅱ with a passive higher harmonic cavity is not suitable for operating in an asymmetric fill pattern.     

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.     

RF deffecting cavity for bunch length measurement in Tsinghua Thomson scattering X-ray source

An RF deffecting cavity used for bunch length measurement has been designed and fabricated at Tsinghua University for the Thomson Scattering X-Ray Source. The cavity is a 2856 MHz, π-mode, 3-cell standing-wave cavity, to diagnose the 3.5 MeV beam produced by photocathode electron gun. With a larger power source, the same cavity will again be used to measure the accelerated beam with energy of 50 MeV before colliding with the laser pulse. The RF design using MAFIA for both the cavity shape and the power coupler is reviewed, followed by presenting the fabrication procedure and bench measurement results of two cavities.     

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.     

A new method to generate relativistic comb bunches with tunable subpicosecond spacing

We propose and analyze a scheme to produce comb bunches, i.e. a bunch consisting of micro-bunch trains, with tunable subpicosecond spacing. In the scheme, the electron beam is first deflected by a deflecting cavity which introduces a longitudinal-dependent linear transverse kick to the particles. After passing through a drift space, the transverse beam size is linearly coupled to the longitudinal position of the particle inside the beam, and a mask is placed there to tailor the beam, then the mask distribution is imprinted on the beam's longitudinal distribution. A quadrupole magnet and another deflecting cavity are used in the beam line to compensate the transverse angle due to the first deflecting cavity. Analysis shows that the number, length, and spacing of the trains can be controlled through the parameters of the deflecting cavity and the mask. Such electron bunch trains can be applied to an infrared free electron laser, a plasma-wakefield accelerator and a supper-radiance THz source.     

Study of a magnetic alloy-loaded RF cavity for bunch compression at the CSR

The Heavy Ion Research Facility and Cooling Storage Ring(HIRFL-CSR)accelerator in Lanzhou offers a unique possibility for the generation of high density and short pulse heavy ion beams by non-adiabatic bunch compression longitudinally,which is implemented by a fast jump of the RF-voltage amplitude.For this purpose,an RF cavity with high electric field gradient loaded with Magnetic Alloy cores has been developed.The results show that the resonant frequency range of the single-gap RF cavity is from 1.13 MHz to 1.42 MHz,and a maximum RF voltage of 40 kV with a total length of 100 cm can be obtained,which can be used to compress heavy ion beams of 238U72+ with 250 MeV/u from the initial bunch length of 200 ns to 50 ns with the coaction of the two single-gap RF cavity mentioned above.     

SSC新高频腔设计与分析

简要分析了HIRFL SFC与SSC之间的束流匹配关系, 给出了新高频腔的频率范围。 利用经典三维电磁场数值模拟软件MAFIA对SSC新高频腔体进行了模拟计算, 得出了SSC新高频腔体的相关物理参数, 并对频率范围、Q值、 并联阻抗和电压分布等参数进行了分析。 高频腔体的模拟计算结果完全符合SSC回旋加速器改造的物理设计及空间结构要求。     

合肥电子储存环上新旧高频腔的尾场及耦合阻抗的计算

 合肥800MeV电子储存环的二期升级工程正在进行，其中高频腔的改造和加工将要完成，由于原有高频腔存在较多不合理因数，将要被新腔所替换。为了充分估计在将来运行时，高频腔的耦合阻抗对储存高流强的影响，需要对新旧高频腔宽带耦合阻抗进行比较计算。用电磁场计算软件MAFIA对高频腔的耦合阻抗及电子束在此结构中产生的尾场进行了计算，结果说明旧高频腔的耦合阻抗比新高频腔小。     

HIAF-CRing 中的俘获加速效率研究(英文)

以HIAF-CRing上典型离子238U34+为研究对象,对其纵向俘获和加速的动力学过程进行了研究。累积后的粒子能量为800 MeV/u, 经过绝热俘获和加速后,粒子被加速至1 130 MeV/u。研究结果表明,通过选择适当的俘获时间、绝热参数以及相空间面积因子等参数,应用优化后的高频俘获加速曲线,可以获得更高的俘获和加速效率。通过粒子纵向动力学追踪软件ESME 上进行模拟,得到了优化后的高频相位、高频电压曲线,使得俘获效率达到99.3%,加速效率近乎100%。同时确定出了CRing 高频腔加速U34+ 所需满足的特性参数,即电压需达到40 kV,频率范围是0:31s0:34 MHz。To reduce the beam loss during the capture and acceleration processes of CRing in HIAF project, the longitudinal beam motion is investigated using the typical ion of 238U34+during the two processes mentioned above. The ions will be captured adiabatically firstly and then will be accelerated from 800 to 1130 MeV/u with a high efficiency using optimized RF voltage and RF phase program. After that the bunched beam will be debunched for the later beam compression. Simulation of these processes by tracking appropriate distributions with the longitudinal beam dynamics code ESME has been used tofind optimum parameters such as RF phase, RF voltage. The variation of the parameter during the RF cycle and the character parameters of the RF cavity are presented.      

RF deflecting cavity for bunch length measurement in Tsinghua Thomson scattering X-ray source

An RF deflecting cavity used for bunch length measurement has been designed and fabricated at Tsinghua University for the Thomson Scattering X-Ray Source.The cavity is a 2856 MHz,π-mode,3-cell standing-wave cavity,to diagnose the 3.5 MeV beam produced by photocathode electron gun.With a larger power source,the same cavity will again be used to measure the accelerated beam with energy of 50 MeV before colliding with the laser pulse.The RF design using MAFIA for both the cavity shape and the power coupler is reviewed,followed by presenting the fabrication procedure and bench measurement results of two cavities.     

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10° to 30°, and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.     

四轩型RFQ加速腔高频特性的数值模拟与实验研究

为设计一个高负载因子、高流强的RFQ加速器，使用三维电磁场计算程序MAFIA对加速腔的高频特性进行了模拟计算. 本文主要研究RFQ支撑板高度、宽度、间距、电极形状等结构参数对高频腔体高频特性的影响，根据RFQ加速器的比分路阻抗以及电磁场分布的要求，对RFQ的结构参数进行优化设计，然后根据模拟计算结果加工冷模，进行冷模测量并将测量结果与计算结果加以比较.     

Emittance-dominated long bunches in dual harmonic RF system

The storage of long bunches for long time intervals needs flattened stationary buckets with a large bucket height. The longitudinal motion of the initially mismatched beam has been studied for both the single and dual harmonic RF systems. The RF amplitude is determined to be r.m.s wise matched. The bucket height of the single harmonic system is too small even for shorter bunch with only 20% increased energy spread. The Halo formation and even debunching can be seen after a few synchrotron periods for single particles with large amplitude. In the case of small energy spread for a cooled beam, Coulomb interaction cannot be ignored. The external voltage has to be increased to keep the r.m.s bunch length unchanged. The new voltage ratio R(N) simplifies physics for the emittance-dominated bunches with modest particle number N. For the single harmonic system, substantial amount of debunching occurs without increasing the external voltage, but very little if the RF amplitude is doubled. Results from the ORBIT tracking code are presented for the 1 GeV bunch in the HESR synchrotron, part of the GSI FAIR project.