Demonstration of cathode emittance dominated high bunch charge beams in a DC gun-based photoinjector

Abstract

We present the results of transverse emittance and longitudinal current profile measurements of high bunch charge (≥100 pC) beams produced in the DC gun-based Cornell Energy Recovery Linac Photoinjector. In particular, we show that the cathode thermal and core beam emittances dominate the final 95% and core emittance measured at 9-9.5 MeV. Additionally, we demonstrate excellent agreement between optimized 3D space charge simulations and measurement, and show that the quality of the transverse laser distribution limits the optimal simulated and measured emittances. These results, previously thought achievable only with RF guns, demonstrate that DC gun based photoinjectors are capable of delivering beams with sufficient single bunch charge and beam quality suitable for many current and next generation accelerator projects such as Energy Recovery Linacs (ERLs) and Free Electron Lasers (FELs).Linear electron accelerators boast a wide range of current and planned applications in the physical sciences. Examples include: x-ray sources [1-3], electron-ion coolers [4], Ultra-fast Electron Diffraction (UED) experiments [5][6][7][8], and fixed-target nuclear physics experiments [9]. A key feature of many of these applications is the potential to produce beams where the initial beam quality, set by the source, dominates the final beam quality at the usage point. This has lead to the design of a next generation of machines, such as high energy Energy Recovery Linacs (ERLs) [2], and Free Electron Lasers (FELs) [3] which could provide diffraction limited hard x-rays with orders of magnitude brighter beams than modern storage rings. The successful design and implementation of such machines has the potential to impact an impressively broad range of research in physics, chemistry, biology, and engineering.For next generation high energy x-ray sources like the proposed Linac Coherent Light Source-II (LCLS-II) [10], the creation (at MHz repetition rates) and effective transport of multi-MeV beams with high bunch charges (≥100 pC), picosecond bunch lengths, and sub-micron normalized transverse emittances represents a beam dynamics regime previously thought attainable only with RF gun based photoinjectors [11]. In this letter, we challenge this assumption, and show that the DC gun-based Cornell ERL injector can produce cathode emittance dominated beams which meet the bunch charge, emittance, and peak current specifications of a next generation light source. In doing so, we also demonstrate excellent agreement between measurement and simulation of the injector, and show that ultimate optimization of the emittance in high-brightness photoinjectors may require advanced transverse laser shaping along with the use of low intrinsic emittance photocathodes.Before discussing our experimental results, we review the definitions of the key figures of merit for beam quality in high-brightness accelerators relevant for this work: emittance and brightness. For the beam densities encountered in this work (10 17 -10 18 e/m 3 ), classical relativisti...

Cold electron beams from cryocooled, alkali antimonide photocathodes

“…For a given bunch charge, the one-dimensional maximum electron beam brightness achievable from the photoemission source depends only on the MTE and the electric field at the cathode surface [7][8][9]. The transverse coherence length, which sets the upper limit to the unit cell size of the crystal that can be imaged using UED setups, depends inversely on the intrinsic emittance [5].…”

Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser

“…Electron beam generation with currents of up to 75 mA was demonstrated at the Cornell photoinjector with an alkali antimonide photocathode, and the charge lifetime was measured as greater than 15 kC [9]. In addition, a low-emittance electron beam for high-repetition-rate X-ray FEL has been demonstrated with an alkali antimonide photocathode [10].…”

A non-conventional ERL configuration for high-power EUV FELs

“…An essential enabling technology for any high-power EUV FEL is a high-brightness, high rep-rate injector. Decisive progress has recently been made and more is expected shortly: X-ray FEL quality beams have been demonstrated at Cornell University [21] out of a DC gun injector and the normal-conducting rf gun APEX program at LBNL is on track to do the same by the end of this year [22]. Primarily because of our familiarity with its design, here we adopt the APEX injector in taking a first stab at estimating the performance of our proposed machine configuration and tentatively select Q¼300 pC for the bunch charge, drawing on our experience from NGLS studies [23].…”