Design of RF chopper system for improving beam quality in FEL injector with thermionic gun

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Abstract

For a linac-based Free Electron Laser (FEL), good beam quality largely contributes to the success of the final radiation. An imperfection confronted with the HUST THz-FEL facility is the long beam tail that emerges in the electron gun and exists through the whole beam line. This paper proposes to deploy a chopper system after the electron gun to truncate the beam tails before they enter into the linac. Physical dimensions of the chopper cavity are discussed in detail and we have developed and derived new analytical expressions applying to all frequencies for the optimal design. Also, technical issues of the cavity are considered. Beam dynamic simulation is performed to examine the truncation effect and the results show that more than 78% of the beam tail can be removed effectively, while preserving the emittance and energy spread in acceptable level.

Introduction

As free electron laser (FEL) is the most prominent candidate for producing high average power and continuously tunable terahertz radiation, a linac-based THz-FEL facility has been planned to be built in Huazhong University of Science and Technology (HUST). Preliminary design of the whole system is finished and engineering parameters of some key components have been obtained [1], [2], [3]. The HUST FEL facility mainly consists of an independently tunable cell (ITC) RF gun, a normal temperature S-band linac, a planar undulator and an optical cavity. The FEL injector includes the ITC-RF gun and the S-band linac, both of which are designed to operate at 2856 MHz. For successful operation, the injector must provide the oscillator with high quality beams delivered by the transport line. Beam requirements at the exit of the injector are listed in Table 1.

A common drawback of a thermal RF gun is its imperfect performance in longitudinal bunching. According to the analysis in Ref. [4], with present scheme the linac would undergo undesirably intensive beam current, which might result in difficulties or failure in low energy operation. This is due to the compromise between performance and compactness demanded of the electron gun. The charge of one micro-bunch accelerated by the linac is around 500 pC while the useful portion for radiation, about 15 ps in the head, is only 200 pC. The major portion, located in the tail, will increase the feeded RF power due to beam loading effect, and may cause radiation in the waveguide of the optical cavity. A simple and economical solution is to fix a slit after the first bend magnet, because particles with different energies will follow different trajectories. Then by reasonably designing the transverse position and width of the slit, it is possible to cut off the tail portion of one bunch. However, the solution just eliminates the risk in the waveguide while doing nothing for the linac. In low energy operation, e.g. 6 MeV, the undesirably intensive beams will drain out RF power before they come to the end of the linac. If this is the case, the beams will induce electromagnetic field (wake field) in the vacant power accelerating cells and the energy spread of latter coming bunches will largely increase [5]. So, it is very meaningful to develop a chopper system located before the linac to select only the head portion without changing the initial injector layout much. An attractive proposal, on which our work is based, suggests replacing the short lens with a chopper cavity as shown in Fig. 1, for the self-bunching effect of electrons generated by the ITC-RF gun indicates dispensability in short lens usage. In the rest context, we will refer the initial structure as Scheme 1 and the new structure as Scheme 2.

A chopper is usually a magnetic deflector, which can be used for measurement of beam dose [6] and suppression or selection of charged particle beams [7], [8], [9], [10]. In Ref. [11], Haimson developed systematic and detailed optimization methods for designing chopper cavities and some specific designs have been reported [12], [13], [14], [15]. Recently choppers have found extensive applications in proton linacs, which need choppers to produce precise gaps in bunched linac beams [16], [17], [18]. In our project, the chopper is utilized to generate pulses with predefined width, cutting off the tail portion after the electron gun and improving power efficiency in the linac. Nevertheless, the input beams of the chopper are not continuous as the previous cases, which makes it possible to develop some new characteristics of the chopper cavity. For example, in our design the chopper cavity shares power source with the gun and the linac. Besides, the centroid particle will undergo zero deflection force and no extra bias field is required, so the whole structure will be compact.

Section snippets

Optimization of RF chopper for pulsed electron beams

In heavy ion cases, it is popular to use electrostatic fields to create gaps between beam pulses. In order to make the whole system compact and the timing scheme easy to operate, we prefer a RF chopper cavity excited by the same power source of the gun and the linac. The RF chopper system is mainly composed of a resonator and a beam scraper (Fig. 1). Electrons in a micro-bunch will get transverse deflection determined by their longitudinal position and energy when traveling through the

Beam dynamic simulation and analysis

To checkout the effect of the chopper system on electron beams, beam dynamic simulation is carried out from the exit of the ITC-RF gun down to the entrance of the lianc. The initial distribution of electrons comes from the output file related to the simulation of the ITC-RF gun. The bunch charge and the bunch length at the entrance of the linac between the two schemes are the key points to judge the new design system.

Parmela code [24] is used to complete the beam dynamic simulation. And

Discussion and conclusion

The major issue of the beam tails is that it will increase the feed-in RF power significantly due to the beam loading effect, furthermore it might bring instability of the FEL injector. For this consideration in HUST FEL injector, a RF chopper system, which consists of a resonator and a scraper, is proposed to remove the useless portion of the bunched beam before the linac. Originated from the microwave theory, the resonant field is analyzed and the physical dimensions are optimized accordingly

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    This work was supported by National Natural Science Foundation of China (11375068), and 2011 project – Hubei collaborative Innovation Center of Non-power Nuclear Technology.

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