Beam Diagnostic of the LINAC for the Compact High-Performance THz-FEL-猫眼课题宝

DOI：

10.18429/jacow-ipac2015-mopty023

发表时间：

2015-06

期刊：

影响因子：

通讯作者：

中科院分区：

其他

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作者：

T. Hu;Qushan Chen;Wei Chen;Jie Liu;Kaifeng Liu;Y. Pei;B. Qin;P. Tan;Zhenxing Tang;Zhongming Wang;Y. Xiong;Jun Yang

With the aim to obtain short-pulse bunches with high peak current for a terahertz radiation source, an FELbased LINAC is employed in HUST THz-FEL, and the LINAC consists of an EC-ITC RF gun, a disk-loaed waveguide structure with a constant gradient and collinear absorbing loads with focusing coils surrounded and so on. To achieve a balance between compactness and high performance, beam diagnostic system should be simple and high-precision. So that a cost-effective measurement scheme for the high-brightness beam extracted by the LINAC is needed. This paper will describe the beam line and beam diagnostic system of the LINAC in the HUST THz-FEL in detail and give corresponding assembly scheme. In addition, online monitor system is introduced. BEAM LINE OF THE FEL-BASED LINAC To achieve a balance of high performance and compact layout for a THz-FEL facility, HUST and USTC are cooperating to construct such a machine and perform corresponding experiment researches. And the facility is under commissioning right now. This facility is mainly composed of a novel EC-ITC RF gun, constant gradient travelling wave structure with a collinear absorbing load and an input coupler which makes the electric field be symmetry, and its focusing coil, beam diagnostics system, microwave power system, vacuum system, control system and so on[1]. The layout and main parameters of the LINAC are given by Fig. 1 and Table 1 respectively, and beam diagnostic equipments are sketched by Fig. 1 either. Table 1: Main Specifications Parameter Unit Value Energy MeV 4-15 Current A 0.571(Macro pulse) Width us ps 1-5(Macro pulse) 1-10(Micro pulse) Energy spread % 0.2-0.5 Nor. emittance mm mrad <15 RF frequency MHz 2856 Input power MW 20 Figure 1: Layout of the HUST THz-FEL LINAC. As the most important element, the EC-ITC RF gun plays a role of transfer DC beam to short bunches, and determines beam properties of the LINAC such as energy spread, transverse emittance, bunch length. In addition, by using velocity bunching of the standing-wave cavity, this type of gun can realize energy spread self-compensation, so that it helps to achieve the purpose of compressing the facility sizes. Previous researches shown that, high quality bunches with ~0.3% energy spread, ~10mm mrad emittance, 201pC charge and ~2ps length can be generated by the EC-ITC RF gun[2,3]. However, in the process of dynamic calculations, a 5A DC beam are adopted as the input beam of the EC-ITC RF gun by means of parallel injection. In the actual situation, the DC electron gun has been designed to generate 4A DC beam, and it would be injected to the EC-ITC RF gun by negative angle. To press close to the actual situation, a 4A beam with negative angle is imitated, which can be shown by Fig. 2. By adjusting power and phase parameters of two cells of the ITC cavities, the optimal results can be obtained. Dynamic results calculated by Parmela are shown in Fig. 3, meanwhile, detailed specifications of the bunches generated by EC-ITC RF gun with negative angle injection are listed in Table 2. Figure 2: Longitudinal profile at different time of the DC beam with negative angle injection. Table 2: Main Specifications of the Bunch Generated by the EC-ITC RF Gun with Negative Angle Injection Parameter Unit Value Energy MeV 2.6 Width ps 1.0 (Micro pulse) Energy spread % 0.14 Nor. emittance mm mrad 8.0 Effective charge pC 245 (Micro pulse) Figure 3: Phase spectrum, energy spectrum and transverse phase space of the bunch extracted from the EC-ITC RF gun. Obviously, by using negative angle injection, the most important parameter, effective charge is improved by 20%, while other targets such as energy spread and emittance are not deteriorated. At a certain content, the dynamic results are encouraged us to perform experiment researches for the HUST THz-FEL LINAC. BEAM DIAGNOSTIC SYSTEM For the sake of compactness, the beam diagnostic system should use the elements which already exist in the facility as far as possible. As Fig. 1 shows, the online beam testing system contains two Torrids, one Flag with a fluorescent screen and a OTR screen, energy analysis system, two fluorescent targets, and three CCD(ChargeCoupled Device) cameras. Correspondingly, beam parameters we need measure and corresponding testing methods are simply described as the following, Macro pulse current: TOR1, TOR2. Beam energy: Analysis magnet, Target1, CCD2. Energy spread: Flag, CCD1, Analysis magnet, Target1, CCD2. Normalized emittance: Quadrupoles (Q1, Q2 or Q3). Micro pulse width: Linac, Flag, CCD1, Analysis magnet, Target1, CCD2 and phase shifter. The online monitor system is showed by Fig. 4 clearly. Figure 4: online monitor system Micro Pulse width Measurement In order to measure the beam length, we will adopt a method which is by means of a relation of electron energy and its phase in the Linac. When beam located at the "0" phase, the energy spread will change less. If the beam phase located in which is different from the "0", their energy spread will change and larger than initial energy spread. This change depend on beam length, so measuring these energy, energy spread and their phase, we can get pulse width[4]. Normal Emittance Measurement As mentioned above, we will use Quadrupoles in the beam line to perform normalized emittance measurement. And the well-known quadrupole scanning technique is widely used in the accelerator physics community[5]. However, the common method of thin focusing lens approximation is not suitable for the HUST THz-FEL LINAC. Because our case cannot satisfy 0 Q k L by considering facility compactness during previous designing process. Where, k and LQ are the focusing constant and the effective length of the Quadrupole adopted to measure the emittance, respectively. Therefore, we will use transmission matrix to solve equation set directly. Apparently, the beam matrix at Target2 can be written as 1. And the beam matrix at the entrance of the Quadrupole chosen to be scanned is 0. So that the two beam matrixes can be connected by the transmission matrix of the Quadrupole M, 1 0 t M M , which can be expressed in detail as the following, 11 12 11 12 11 12 11 21 12 22 21 22 12 22 12 22 (1) (1) (0) (0) (1) (1) (0) (0) m m m m m m m m (1) If we change the current of the Quadrupole three times, three different beam spot sizes 11 (1) , 11 (2) , 11 (3) will be obtained on Target2 by CCD3, then we can obtain the following equation set,

Beam Diagnostic of the LINAC for the Compact High-Performance THz-FEL

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