Electron beam characterization of a combined diode rf electron gun

Abstract

Experimental and simulation results of an electron gun test facility, based on pulsed diode acceleration followed by a two-cell rf cavity at 1.5 GHz, are presented here. The main features of this diode-rf combination are: a high peak gradient in the diode (up to 100 MV=m) obtained without breakdown conditioning, a cathode shape providing an electrostatic focusing, and an in-vacuum pulsed solenoid to focus the electron beam between the diode and the rf cavity. Although the test stand was initially developed for testing field emitter arrays cathodes, it became also interesting to explore the limits of this electron gun with metallic photocathodes illuminated by laser pulses. The ultimate goal of this test facility is to fulfill the requirements of the SwissFEL project of Paul Scherrer Institute [B. D. Patterson et al., New J. Phys. 12, 035012 (2010)]; a projected normalized emittance below 0:4 m for a charge of 200 pC and a bunch length of less than 10 ps (rms). A normalized projected emittance of 0:23 m with 13 pC has been measured at 5 MeV using a Gaussian laser longitudinal intensity distribution on the photocathode. Good agreements with simulations have been obtained for different electron bunch charge and diode geometries. Emittance measurements at a bunch charge below 1 pC were performed for different laser spot sizes in agreement with intrinsic emittance theory [e.g. 0:54 m=mm of laser spot size (rms) for Cu at 274 nm]. Finally, a projected emittance of 1:25 þ = À 0:2 m was measured with 200 pC and 100 MV=m diode gradient.

Towards high brightness electron beams from multifilamentaryNb3Snwire photocathode

“…From the presented measurements of charge and emittance, one can estimate the transversal brightness, B n , as B n ¼ I p 4 2 2 n ¼ 10 13 A=ðrad 2 m 2 Þ; (15) where I p ¼ Q= ffiffiffiffiffiffi ffi 2 p t is the peak current, n is the normalized emittance and under assumption of the electron bunch length being equal to the laser pulse duration. This assumption is valid only in the vicinity of the cathode, since space-charge forces expand the electron bunch significantly upon propagation (Fig.…”

“…12. The 500 kV pulser with its elliptical diode electrodes and the diagnostic beam line have been described in detail in [15,17]. Recently, low emittance electron beams at low charge ($ pC) have been demonstrated in this combined diode-rf electron gun using a flat metal cathode [5].…”

“…Section III presents the experimental results from a tabletop 50 kV diode-based accelerator, followed by Sec. IV where the results of the multifilamentary-wire cathode in a 500 kV combined diode-rf electron gun [15] are discussed.…”

A high voltage pulse generator using avalanche‐mode thyristors

“…High pulse voltage sources using thyratron switches generate pulses of several microseconds; discharge threshold decreases with shorter pulses, which makes possible generation of high electric fields 3 . There are attempts at generating high‐quality beams in DC‐RF electron guns and other applications using high electric field produced by short high‐voltage pulses 4 . Besides, electron guns require large ceramic insulators to prevent discharge, but these insulators, as well as electron guns themselves, can be made compact in case of nanosecond pulses.…”

A numerical approach to designing a versatile pepper-pot mask for emittance measurement

“…The pepper-pot method is a well-known technique for phase space characterisation at low energies, before energy boosting, where the space charge force is significant for high brightness beams [1,2]. It is widely used in radio-frequency photoinjectors which produce high brightness electron beams for light sources [3,4] and test facilities for other large scale applications [5][6][7]. Today, this method continues to be popular to measure the phase space of electron beams generated by conventional and advanced accelerators [8][9][10][11].…”