Abstract
We propose a novel gridless continuous-wave radiofrequency (rf) thermionic gun capable of generating nC ns electron bunches with a rms normalized slice emittance close to the thermal level of 0.3 mm mrad. In order to gate the electron emission, an externally heated thermionic cathode is installed into a stripline-loop conductor. Two high-voltage pulses propagating towards each other in the stripline-loop overlap in the cathode region and create a quasielectrostatic field gating the electron emission. The repetition rate of pulses is variable and can reach up to one MHz with modern solid-state pulsers. The stripline attached to a rf gun cavity wall has with the wall a common aperture that allows the electrons to be injected into the rf cavity for further acceleration. Thanks to this innovative gridless design, simulations suggest that the bunch emittance is approximately at the thermal level after the bunch injection into the cavity provided that the geometry of the cathode and aperture are properly designed. Specifically, a concave cathode is adopted to imprint an Ƨ-shaped distribution onto the beam transverse phase-space to compensate for an S-shaped beam distribution created by the spherical aberration of the aperture-cavity region. In order to compensate for the energy spread caused by rf fields of the rf gun cavity, a 3rd harmonic cavity is used. A detailed study of the electrodynamics of the stripline and rf gun cavity as well as the beam optics and bunch dynamics are presented.
Highlights
During the last decade a number of projects and proposals on accelerator-based x-ray Compton sources and other compact sources have been put forward [1]
The proposed design occupies a distinguished niche among the thermionic and photo rf guns and has the following features: (i) nanoCoulomb bunch charges; (ii) very low slice emittance thanks to a small-size cathode and careful design of the beam optics; (iii) variable repetition rate of electron bunches up to one MHz in the present design with the possibility of the repetition rate at rf frequencies; (iv) long-term stable operation of thermionic cathodes; (v) compatibility with magnetic fields for emittance compensation; (vi) high accelerating gradients of 20 MV=m in a rf gun cavity
In order to avoid the emittance dilution originating from the magnetic field on the cathode, we propose to gate the electron emission by two transverse electromagnetic (TEM) electromagnetic pulses propagating towards each other so that on the cathode the electric fields sum up in phase whereas the magnetic fields of pulses cancel each other
Summary
Uppsala University, Lägerhyddsvägen 1, Uppsala 75120, Sweden (Received 13 August 2016; revised manuscript received 19 January 2017; published 31 May 2017). In order to gate the electron emission, an externally heated thermionic cathode is installed into a stripline-loop conductor. Two high-voltage pulses propagating towards each other in the stripline-loop overlap in the cathode region and create a quasielectrostatic field gating the electron emission. The stripline attached to a rf gun cavity wall has with the wall a common aperture that allows the electrons to be injected into the rf cavity for further acceleration. Thanks to this innovative gridless design, simulations suggest that the bunch emittance is approximately at the thermal level after the bunch injection into the cavity provided that the geometry of the cathode and aperture are properly designed. A detailed study of the electrodynamics of the stripline and rf gun cavity as well as the beam optics and bunch dynamics are presented
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