Beam dynamics studies of the photo-injector in low-charge operation mode for the ERL test facility at IHEP

Abstract

The energy recovery linac test facility (ERL-TF), a compact ERL-FEL (free electron laser) two-purpose machine, was proposed at the Institute of High Energy Physics, Beijing. As one important component of the TF, the photo-injector started with a photocathode direct-current gun has been designed. In this paper optimization of the injector beam dynamics in low-charge operation mode is performed with iterative scans using Impact-T. In addition, the dependencies between the optimized beam quality and the initial offset at cathode and element parameters are investigated. The tolerance of alignment and rotation errors is also analyzed. PACs: 29.20.Ej, 29.27.Bd, 41.85.Ja IntroductionThe energy recovery linac (ERL) and free electron laser (FEL) are considered to be candidates of the fourth generation light sources, and have received much attention worldwide. Since both of them are based on linac technologies, it is possible to combine FEL into an ERL facility, resulting in a compact two-purpose light source. A test facility, named energy recovery linac test facility (ERL-TF), was proposed at the Institute of High Energy Physics, Beijing, to verify this principle [1]. Physical design of the ERL-TF started a few years ago and is well in progress [2][3][4]. The layout and main parameters of the facility are presented in Fig. 1 and Table 1, respectively. Among the components of the facility, one extremely important device dominating the machine performance is the photo-injector. The injector, including a 500-kV photocathode direct-current (DC) gun equipped with a GaAs cathode, a 1.3 GHz normal conducting RF buncher, two solenoids, and two 2-cell superconducting RF cavities, was designed for the ERL-TF [2], with the layout shown in Fig. 2. With the initial parameters listed in Table 2, beam simulation of the designed injector was made for the high-charge operation mode (bunch charge 77 pC, rep. rate 130 MHz) with the ASTRA program [5], and finally an electron beam, with kinetic energy E k of 5 MeV, normalized emittance  n,x(y) of 1.49 mm.mrad, rms bunch length  z of 0.67 mm and rms energy spread   of 0.72%, was achieved at the end of the injector. In this paper, we optimize the beam dynamics of low-charge operation mode (bunch charge 7.7 pC, rep. rate 1.3 GHz) with iterative scans using the Impact-T program [6]. Thanks to the relatively weak space charge force, an electron beam with  n,x(y) of 0.4 mm.mrad,  z of 0.74 mm and   of 0.33% is obtained in the case of 0.5-mm incident laser rms transverse size. Moreover, the dependency of the beam quality on various variables, such as initial offset at cathode and element parameters, and the sensitivity of beam dynamics to element alignment and rotation errors are also investigated in this paper.

Evolutionary genetic optimization of the injector beam dynamics for the ERL test facility at IHEP

“…Based on the beam dynamics study for the ERL-TF injector in low-charge operation mode which is presented in Ref. [5], in this paper an evolutionary genetic method, non-dominated sorting genetic algorithm II, is applied to optimize the injector beam dynamics, especially in the high-charge operation mode. It appears feasible to achieve an electron beam with kinetic energy of 5 MeV, bunch length of 2 ~ 4 ps, and emittance below 1 mm.mard, at the end of the injector by using an incident laser with rms transverse size of 1 ~ 1.2 mm.…”

“…In the optimization for the high-charge operation mode, the population converges relatively slowly to the Pareto front. Taking the case with σ laser of 0.75 mm as The result obtained previously [5] with the iterative scan method is also plotted as a star.…”

“…A test facility, named energy recovery linac test facility (ERL-TF), was proposed at the Institute of High Energy Physics, Beijing, to verify this principle [1]. Physical design of the ERL-TF started a few years ago and is well in progress [2][3][4][5]. The layout and main parameters of the facility are presented in Fig.…”

“…In addition, it was found that the optimized result had rather high tolerance to the parameter fluctuation, magnetic and alignment errors (For more detail, see Ref. [5]).…”

Overview of CW electron guns and LCLS-II RF gun performance

“…CW gun R&D has been pursued worldwide since the 1980s. The gun technologies include DC [5][6][7][8], Superconducting RF (SRF) [9][10][11][12][13][14][15][16][17][18], hybrid DC-SRF [19] and normalconducting (NC) RF [20,21]. This paper first surveys these technologies and then discusses the design and performance of the NC RF gun and injector at the SLAC Linac Coherent Light Source II (LCLS-II).…”