Upgrades of beam diagnostics with application to emittance-exchange experiments

Abstract

The possibility of using electron-beam phase-space manipulations to support a free-electron laser accelerator design optimization has motivated our research. An ongoing program demonstrating the exchange of transverse horizontal and longitudinal emittances at the Fermilab A0 photoinjector has benefited recently from the upgrade of several of the key diagnostics stations. Accurate measurements of these properties upstream and downstream of the exchanger beam line are needed. Improvements in the screen resolution term and reduced impact of the optical system's depth of focus by using YAG:Ce single crystals normal to the beam direction will be described. The requirement to measure small energy spreads (< 10 keV) in the spectrometer and the exchange process which resulted in bunch lengths less than 800 fs led to other diagnostics performance adjustments and upgrades as well. Application of these diagnostics in demonstrating the exchange of transverse (x) and longitudinal phase spaces is also reported.

Formation and acceleration of uniformly filled ellipsoidal electron bunches obtained via space-charge-driven expansion from a cesium-telluride photocathode

“…2). A multislit mask insertable at X3 can be used to measure the transverse emittance by analyzing the resulting beamlet's distribution at the X6 screen [25]. The multislit mask consists of 50 m-wide slits made out of a 3-mm-thick tungsten plate.…”

Evidence for anomalous optical transition radiation linear polarization effects in beam-profile monitors

“…We evaluate this as a small effect and note the lack of any detectable polarization effect with YAG : Ce scintillator light taken with the same system as OTR [19], and the clear increase of the beam size with the parallel component in the JLAB data with the initially roundish beam spot. Three different digitizers were used at the three setups.…”

“…Micropulse charges from 25 to 500 pC were used for beam sigma sizes of 45 to 250 microns. Initial experiments were performed at station X5 comparing OTR with a powder scintillator [19,20]. Subsequently, the prototype station was installed in the user beam line section beyond the horizontal spectrometer in the straight ahead line as indicated in Fig.…”

“…Besides that issue, theoretical modeling has identified only minor polarization effects in the point-spread function (PSF) [6] or single-particle function [7]. However, we examine empirical evidence [15][16][17][18][19] that the utilization of the polarization component orthogonal to the dimension of interest results in a noticeably smaller observed projected image profile than theoretically predicted [5][6][7][8][9]. In those calculations the OTR point-spread-function width was in the few-micron range for 0.10 rad collection angles so only experiments that involve beam sizes with sigma <20 microns might be concerned.…”

Synchroscan streak camera imaging at a 15-MeV photoinjector with emittance exchange

“…In analogy to upgrades performed two decades ago in support of rf linac driven free-electron lasers [1], the opportunity for a new series of streak camera experiments at the Fermilab A0 photoinjector (A0PI) on the [14][15][16] MeV electron beams and the UV component of the drive laser has been identified and now realized. These bunch-length and phase-stability measurements were driven by the need to diagnose accurately the beam's longitudinal emittance for the ongoing emittance exchange (EEX) studies at the facility [2][3][4]. The enabling upgrade was adding the synchroscan plug-in option to the existing C5680 Hamamatsu streak camera mainframe.…”