Electron beam charge diagnostics for laser plasma accelerators

Abstract

A comprehensive study of charge diagnostics is conducted to verify their validity for measuring electron beams produced by laser plasma accelerators (LPAs). First, a scintillating screen (Lanex) was extensively studied using subnanosecond electron beams from the Advanced Light Source booster synchrotron, at the Lawrence Berkeley National Laboratory. The Lanex was cross calibrated with an integrating current transformer (ICT) for up to the electron energy of 1.5 GeV, and the linear response of the screen was confirmed for charge density and intensity up to 160 pC=mm 2 and 0:4 pC=ðps mm 2 Þ, respectively. After the radio-frequency accelerator based cross calibration, a series of measurements was conducted using electron beams from an LPA. Cross calibrations were carried out using an activationbased measurement that is immune to electromagnetic pulse noise, ICT, and Lanex. The diagnostics agreed within AE8%, showing that they all can provide accurate charge measurements for LPAs.

Pico-coulomb charge measured at BELLA to percent-level precision using a Turbo-ICT

“…"# was low, the signal-to-noise ratios for both diagnostics were high, and bunch charge below 1 pC could be measured. The charge independent offset of approximately 0.5 pC between Lanex screen and Turbo-ICT has been observed previously in [7] and is due to the detection threshold of the camera used to capture the Lanex screen images. As described in the previous section, the stray field from the magnetic spectrometer can deflect low energy electrons out of the ICT acceptance and can cause discrepancy between ICTs and Lanex screens.…”

“…Electron beams generated at the laser-plasma interaction propagated in vacuum more than 10 meters without any magnetic transport and were aligned to pass through a fused silica wedge mrad vertically. The light from the Lanex was imaged to a CCD camera (Flea3-GE-14S3M, Pointgrey, Canada), whose signal was calibrated to the beam charge in a previous study [7].…”

“…In LPA environments, ICTs can overestimate the charge by more than an order of magnitude [4,5] if not used under the proper conditions. However, a previous study using a gas-jet based LPA showed that, with special care to reduce the detrimental influence of the EMP and other noise sources described above, an ICT can provide accurate charge information for 5 pC and above [7]. The limitation on minimum charge was due to noise on the ICT signal.…”

Measured bremsstrahlung photonuclear production of 99Mo (99mTc) with 34 MeV to 1.7 GeV electrons

“…These accelerators are primarily under development for high energy physics applications, but have the potential for future broader applications in radioisotope production (Leemans 2001, Reed 2007, Clarke 2013. High power, short pulse lasers interact with gas-sourced plasma targets, from either a gas jet, a preformed capillary plasma channel (Gonsalves, 2007) to guide the laser pulse and extend the accelerating length of the electrons, or a gas jet coupled to a capillary to control both the electron injection and laser guiding (Gonsalves 2011). The 99 Mo activation experiments were performed in parasitic mode to the ongoing accelerator development, and were used to determine yields at low average electron beam current vs. incident energy rather than optimizing for production.…”

“…The dipole spectrometers used have been mapped for magnetic field and referenced to built in Hall probes to allow for day to day variations in field. At the spectrometer exit, the electrons pass through a 0.14 mm Lanex Fast Front scintillating screen (Kodak, Rochester, NY, USA) calibrated against known electron currents to give the absolute integrated charge (Nakamura 2011). The beam energy and angle are determined from the images at these exit screens.…”