Cherenkov Ring to Observe Longitudinal Phase Space of a Low Energy Electron Beam Extracted from Rf Gun *

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. [email protected] 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. codes give different results as shown in Fig. 3, though almost the same results have been obtained when the particle charge is set to be zero. The phase space distribution calculated by PARMELA looks like that from the FDTD code. However, one can see a significant difference in the energy spectra, and some structure can be seen in that of PAMELA. The energy spectrum of GPT is much broader than that of FDTD, and there is a small bend in the phase space distribution. In this study we are not going to pursue details of the simulation codes and correctness of the space charge models. We have, however, considered that direct measurement of the particle distribution in the longitudinal phase space is crucial for designing of the bunch compression scheme as well as understanding of the space charge effect. CHERENKOV RING RADIATED FROM MeV ELECTRONS As shown in Figs. 2 and 3, the beam energy from the ITC-RF gun is not so high (kinetic energy T ~ 1.7 MeV) and velocity is still slower than light, which means the particle distribution in the phase space may vary even in drift spaces due to the space charge force. Consequently a conventional analyzer magnet is not suitable for measurement of the particle energy because a considerable path length is inevitably required. Cherenkov light is widely used for beam diagnostics and particle counters. It is well known that the Cherenkov angle θ c is inversely proportional to the particle velocity β as cosθ c = 1/n ω ( )β , (1) where n ω ( ) is the refractive index of the Cherenkov radiator medium at a radiation frequency ω [6]. The Frank-Tamm formula gives the energy dissipation of Cherenkov radiation with a frequency region from ω1 to ω2 as, dE