Ilan Ben-Zvi

Inverse Cerenkov acceleration and inverse free-electron laser experimental results for staged electron laser acceleration

The goal of the staged electron laser acceleration (STELLA) experiment is to demonstrate staging of the laser acceleration process whereby an inverse free electron laser (IFEL) will be used to prebunch the electrons, which are then accelerated in an inverse Cerenkov accelerator (ICA). As preparation for this experiment, a new permanent magnet wiggler for the IFEL was constructed and the ICA system was modified. Both systems have been tested on a new beamline specifically built for STELLA. The improved electron beam (e-beam) with its very low emittance (0.8 mm-mrad normalized) enabled focusing the e-beam to an average radius (1/spl sigma/) of 65 /spl mu/m, within the ICA interaction region. This small e-beam focus greatly enhanced the ICA process and resulted in electron energy spectra that have demonstrated the best agreement to date in both overall shape and magnitude with the model predictions. The electron energy spectrum using the new wiggler in the IFEL was also measured. These results will be described as well as future improvements to the STELLA experiment.

Ultrashort microbunch electron source

A new type of electron beam source is proposed that combines a DC field and AC field from a laser beam in between the anode-cathode gap. Prior researchers have demonstrated the ability for this configuration to produce ultrashort electron microbunches from field emission when the AC field adds constructively with the DC field. Our new source improves upon this basic scheme by creating mono-energetic electrons with low emittance that are suitable for feeding into dielectric and vacuum laser accelerators, photonic bandgap accelerators, and dielectric wakefield accelerators. The microbunches will have a length that is a fraction of the laser wavelength, corresponding to fs pulse durations. They are also produced as a train of microbunches separated by the period of the laser length and lasting for the duration of the laser pulse.

Pseudoresonant laser Wakefield acceleration driven by 10.6-/spl mu/m laser light

This work describes an experiment to demonstrate, for the first time, laser wakefield acceleration (LWFA), driven by 10.6-/spl mu/m light from a CO/sub 2/ laser. This experiment is also noteworthy because it will operate in a pseudoresonant LWFA regime, in which the laser-pulse-length is too long for resonant LWFA, but too short for self-modulated LWFA. Nonetheless, high acceleration gradients are still possible. This experiment builds upon an earlier experiment called staged electron laser acceleration (STELLA), where efficient trapping and monoenergetic laser acceleration of electrons were demonstrated using inverse free electron lasers. The aim is to apply the STELLA approach of laser-driven microbunch formation followed by laser-driven trapping and acceleration to LWFA. These capabilities are important for a practical electron linear accelerator based upon LWFA.

Backward Compton scattering of picosecond CO/sub 2/ laser pulses using relativistic electron beam for the bright X-ray generation

Laser-Compton scattering can generate X-rays or γ-rays with high brightness by applying high-peak-power laser pulses to relativistic electron bunches. We have proposed laser-Compton scattering to generate polarized γ-rays which can induce pair-creation to produce polarized positrons for JLC (Japan Linear Collider) project. As a photon source of the Compton scattering, a picosecond CO/sub 2/ laser system is going to be applied for our experiment because of the higher scattering cross section than near-infrared solid-state lasers, laser power scalability, the laser efficiency and the possibility of high-repetition rated operation. In our experiment, CO/sub 2/ laser pulses with a pulsewidth of about 180 ps are generated by the optical semiconductor slicing method. The sliced CO/sub 2/ pulses are amplified up to 6 GW by a regenerative amplifier with a high-pressure TE CO/sub 2/ laser system.

ER@CEBAF, a 7 GeV, 5-Pass, Energy Recovery Experiment

A multiple-pass, high-energy ERL experiment at the JLab CEBAF will be instrumental in providing necessary information and technology testing for a number of possible future applications and facilities such as Linac-Ring based colliders, which have been designed at BNL (eRHIC) and CERN (LHeC), and also drivers for high-energy FELs and 4th GLS. ER@CEBAF is aimed at investigating 6D optics and beam dynamics issues in ERLs, such as synchrotron radiation effects, emittance preservation, stability, beam losses, multiple-pass orbit control/correction, multiple-pass beam dynamics in the presence of cavity HOMs, BBU and other halo studies, handling of large (SR induced) momentum spread bunches, and development of multiple-beam diagnostics instrumentation. Figure 1: 12 GeV CEBAF recirculating linac. Location of chicane and dump line for ER@CEBAF. Since it was launched 2+ years ago, the project has progressed in defining the necessary modifications to CEBAF (Fig. 1, Tab. 1, 2), including a 4-dipole phase chicane in recirculation Arc A, beam extraction and a dump line at the end of the south linac, and additional dedicated multiplebeam diagnostics. This equipment can remain in place to Work supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy, † and by Jefferson Science Associates, LLC under Contract No. DEAC05-06OR23177 with the U.S. Department of Energy. ‡ Spokesperson. fmeot@bnl.gov; satogata@jlab.org Table 1: Machine/Lattice Parameters of ER@CEBAF fRF 1497 MHz RF frequency Elinac 700 MeV Gain per linac (baseline) Einj 79 MeV = Elinac × 123/1090 φFODO 60 deg Per cell, at first NL pass and last SL pass M56 <90 cm Compression, Arc A Extraction 8 deg Angle to dump line

Progress of 650 MHz SRF Cavity for eRHIC SRF Linac

A high-current, well-damped 5-cell 647 MHz cavity was designed for ERL-Ring based eRHIC. Two prototype cavities were contracted to RI Research Instruments GmbH: one copper cavity with detachable beampipes for HOM damping study, and one niobium cavity for performance study. The performance study includes high-Q study for ERL-Ring eRHIC design and high gradient study for Ring-Ring eRHIC design. This paper will present the preliminary results of the HOM study, progress on Nb cavity fabrication and preparation for vertical test.

Crab Cavities for eRHIC - A Preliminary Design

The proposed eRHIC electron ion collider at BNL must use a relatively large crossing angle between the ion and electron beams for various reasons, including the reduction of the long-range beam-beam effects and minimization of synchrotron radiation noise in the detector. To prevent significant loss of the luminosity due to this large crossing angle, the design of the collider requires the use of groups of crab cavities to provide local crabbing for both proton/ion and electron beams. We will base our design for eRHIC crab cavities based on our experience in the design of the 400 MHz double quarter wave crab cavity (DQWCC) for the high luminosity. eRHIC CRAB CAVITY PROJECT The high current electron-ion collider (EIC) requires quick separation of the electron and ion beams at the interaction region to prevent beam-beam instabilities. A careful study looked into the possibility of achieving this by a crossing angle of the beams or a separation dipole. The conclusion from the study favoured the crossing angle, because the separation dipole is incompatible with the physics and detector constraints [1]. To prevent significant loss of the luminosity due to large crossing angle, 10mrad or 15mrad for linac-ring or ring-ring scheme respectively, in the future EIC at BNL (eRHIC), there is a demand for crab cavities that deliver high deflecting voltages. Table 1: Preliminary Basic Beam and Crab Cavity Parameters for Linac-ring Scheme of eRHIC. Parameters Baseline Design Ultimate Design Electron Proton Electron Proton Crossing angle (Full, mrad) 10 10 Beam energy (GeV) 20 250 20 250 Beta function at IP (, cm) 12.5 12.5 5 5 Transverse beam size at IP (μm) 15.3 15.3 7.1 7.1 Bunch length (cm) 0.3 15 0.3 5 Piwinski angle (rad) 0.98 49.0 2.14 35.7 Beta function at crab cavity (m) 115 ~1000 27

HOM Absorber Study by Photon Diffraction Model

Photon diffraction model (PDM) is one of the most promising candidates to study High Order Mode (HOM) power absorption on absorbing materials for high current SRF cavities. Because at very high frequency (>10GHz), the wavelengths of HOMs are much smaller compared with accelerators dimension, the phase of those HOM will be negligible. Meanwhile, Finite Element Method (FEM) cannot lend a high resolution on evaluation the HOM field patterns due to limited meshing capability. This PDM model utilizes Monte Carlo simulation to trace the ray diffusive reflection in a cavity. This method can directly estimate the power absorption on the cavity and absorber wall. This method will help design the HOM damper setup for eRHIC HOM damper. In this report, we evaluate HOM absorption on the cavity wall with different absorber setup and give a possible solution for power damping scheme for high frequency HOMs.

Record Performance of SRF Gun with CsK2Sb Photocathode

High-gradient CW photo-injectors operating at high accelerating gradients promise to revolutionize many sciences and applications. They can establish the basis for super-bright monochromatic X-ray and gamma-ray sources, high luminosity hadron colliders, nuclearwaste transmutation or a new generation of microchip production. In this paper we report on our operation of a superconducting RF electron gun with a record-high accelerating gradient at the CsK2Sb photocathode (i.e. ~ 20 MV/m) generating a record-high bunch charge (i.e., 2 nC). We briefly describe the system and then detail our experimental results. INTRODUCTION The coherent electron cooling experiment (CeC PoP) [1, 2] is expected to demonstrate cooling of a single hadron bunch in RHIC. A superconducting RF gun operating at 112 MHz frequencies generates the electron beam. 500MHz normal conducting cavities provide energy chirp for ballistic compression of the beam. 704-MHz superconducting cavity will accelerate beam to the final energy. The electron beam merges with the hadron beam and after cooling process is steered to a dump. The FEL-like structure enhances the electron-hadron interaction. The electron beam parameters are shown in the Table 1. Table 1: Parameters of the Electron Beam

Alkali Antimonide Photocathodes in a Can

The next generation of x-ray light sources will need reliable, high quantum efficiency photocathodes. These cathodes will likely be from the alkali antimonide family, which currently holds the record for highest average current achieved from a photoinjector. In this work, we explore a new option for delivering these cathodes to a machine which requires them: use of sealed commercial vacuum tubes. Several sealed tubes have been introduced into a vacuum system and separated from their housing, exposing the active photocathode on a transport arm suitable for insertion into an injector. The separation was achieved without large loss of QE. These cathodes have been compared to those grown via traditional methods, both in terms of QE and in terms of crystalline structure, and found to be similar.