Cherenkov ring to observe longitudinal phase space of a low energy electron beam extracted from RF gun
Cherenkov ring to observe longitudinal phase space of a low energy electron beam extracted from RF gun
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切伦科夫环用于观察从射频枪提取的低能电子束的纵向相空间
DOI：
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发表时间：
2010
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通讯作者：
Y. Tanaka
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作者：
H. Hama;K. Nanbu;M. Kawai;S. Kashiwagi;F. Hinode;T. Muto;F. Miyahara;Y. Tanaka
从热电子射频枪中提取的电子束在纵向相空间的粒子分布对于电子束团压缩至关重要。由于射频枪中的空间电荷效应尚未完全被理解，采用磁压缩器或α磁体的高效束团压缩方案不容易设计。为了测量相对较低能量电子（低于2MeV）在纵向相空间的分布，我们研究了一种利用切伦科夫辐射张角的速度依赖性直接观测电子能量的新方法。通过数值光线追迹模拟讨论了固有能量和时间分辨率。
用于飞秒电子脉冲的ITC - 射频枪
日本东北大学开发了一种基于电子加速器的强太赫兹辐射源，称为t - ACTS项目[1]。来自等时环的相干太赫兹辐射将作为宽带短脉冲源同时提供给多个用户。此外，还研究了由更短电子脉冲（小于共振波长）驱动的太赫兹区域的自由电子激光（FEL）[2]，其中理论模拟表明，当腔长完全调谐时，FEL相互作用可以连续放大辐射脉冲的头部。极短电子束团的稳定产生是t - ACTS项目的一个关键问题。光注入器已经成功产生了具有相当束团电荷量的飞秒脉冲。然而，由于稳定性、多束团运行和成本较低，我们为射频枪选择了热电子阴极。尽管束团电荷量会较小（几十皮库仑），因此辐射的相干增强不是很强，但空间电荷效应可能不是一个非常严重的问题，并且热电子阴极应该具有良好的稳定性，所以有望获得优异的束流质量。此外，多束团模式下的高重复频率运行将开启应用实验的另一个方面。
热电子射频枪由两个独立的腔室组成，用于操控纵向束流相空间（见图1），因此它被命名为ITC（独立可调腔室）射频枪[3]。纵向相空间中的粒子分布对于包括空间电荷效应在内的束团压缩非常重要。ITC - 射频枪的设计是通过改变相对射频相位和场强来产生合适的纵向粒子分布，如图2所示。为了优化枪的参数，例如两个腔室之间的场强和相位差，我们对ITC - 射频枪的束流产生进行了一些数值模拟。由于相当一部分电荷集中在提取束的头部，空间电荷效应会使电子分布偏离简单的平滑线。这种现象很难理解，因为不同的*本工作得到日本文部科学省科学研究资助（S）的支持，项目编号20226003。hama@lns.tohoku.ac.jp
图1：ITC - 射频枪。采用了直径为1.85mm的单晶LaB6小尺寸阴极，它可以提供超过50A/cm²的束流电流密度。
图2：由FDTD代码[4]计算的纵向相空间中的粒子分布。相位差为π + 0°（普通射频枪）的情况能量较高。同时，通过相位调谐进行相空间操控会产生线性粒子分布。两种计算的阴极电流均为1.34A。
美国新墨西哥州圣达菲BIW10会议论文集 TUCNB03
Particle distribution of the electron beam extracted from a thermionic RF gun in longitudinal phase space is crucial for electron bunch compression. Because space charge effects in the RF gun are not fully understood, an efficient bunch compression scheme employing magnetic chicane or alpha (α-) magnet is not easily designed. In order to measure the distribution in the longitudinal phase space of relatively lower energy electrons (below 2 MeV), we have studied a novel method for direct observation of electron energy employing velocity dependence of opening angle of Cherenkov radiation. Intrinsic energy and temporal resolution are discussed by showing a numerical ray-trace simulation. ITC-RF GUN FOR FEMTO-SECOND ELECTRON PULSE An intense terahertz radiation source based on an electron accelerator has been developed at Tohoku University, and is called the t-ACTS project [1]. Coherent THz radiation from an isochronous ring will be provided to multiple users simultaneously as a wide-band short-pulse source. Furthermore, a free electron laser (FEL) in the THz region driven by shorter electron pulses (less than the resonant wavelength) has been studied [2], in which a theoretical simulation suggests that FEL interaction can continuously amplifies the head part of radiation pulse when the cavity length is completely tuned. Stable production of very short electron bunches is a key issue for the t-ACTS project. Photoinjectors have already successfully produced femtosecond pulses with considerable bunch charge. However, we have chosen thermionic cathode for the RF gun because of stability, multi-bunch operation, and cheaper cost. Although the bunch charge will be small (a couple of tens of pC), and then coherent enhancement of the radiation is not so strong, the space charge effect may not be a very serious concern, and the thermionic cathode should have good stability, so excellent beam quality would be expected. Moreover, high repetition operation in multi-bunch mode will open another aspect of application experiments. The thermionic RF gun consists of two independent cavity cells to manipulate the longitudinal beam phase space (see Fig. 1), so it is named the ITC (IndependentlyTunable Cells) RF gun [3]. Particle distribution in longitudinal phase space is very important for the bunch compression including the space charge effect. The ITCRF gun has been designed so as to produce appropriate longitudinal particle distribution by changing the relative RF phase and field strengths as shown in Fig. 2. In order to optimize the parameters for the gun, such as field strengths and phase difference between two cells, we have done some numerical simulations of the beam production for the ITC-RF gun. Because a considerable part of the charge is concentrated into the head of the extracted beam, the space charge effect acts to deviate the electron distribution from a simple smooth line. This phenomenon is difficult to understand because different *Work supported by Grant-in-Aid for Scientific Research (S), the Ministry of Education, Science, Technology, Sports and Culture, Japan, Ccontact No. 20226003. hama@lns.tohoku.ac.jp Figure 1: ITC-RF gun. A small size (φ = 1.85 mm) cathode of single crystal LaB6 is employed, which can provide a beam current density of more than 50 A/cm. Figure 2: Particle distribution in the longitudinal phase space calculated by an FDTD code [4]. One for the phase difference of π + 0° (usual RF gun) leads higher energy. Meanwhile linear particle distribution results from phase space manipulation performed by phase tuning. Cathode current is 1.34 A for both calculations. Proceedings of BIW10, Santa Fe, New Mexico, US TUCNB03