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Title: Experimental Investigation of Multibunch, Multipass Beam Breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver

Abstract

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW Free Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed.

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
877957
Report Number(s):
JLAB-ACP-06-479; DOE/ER/40150-3825
TRN: US0601421
DOE Contract Number:
AC05-84ER40150
Resource Type:
Journal Article
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ACCELERATORS; CEBAF ACCELERATOR; DESIGN; ELECTRON BEAMS; ENERGY RECOVERY; FREE ELECTRON LASERS; INSTABILITY; LINEAR ACCELERATORS; SIMULATION; THRESHOLD CURRENT

Citation Formats

Christopher Tennant, David Douglas, Kevin Jordan, Nikolitsa Merminga, Eduard Pozdeyev, Haipeng Wang, Todd I. Smith, Stefan Simrock, Ivan Bazarov, and Georg Hoffstaetter. Experimental Investigation of Multibunch, Multipass Beam Breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver. United States: N. p., 2006. Web.
Christopher Tennant, David Douglas, Kevin Jordan, Nikolitsa Merminga, Eduard Pozdeyev, Haipeng Wang, Todd I. Smith, Stefan Simrock, Ivan Bazarov, & Georg Hoffstaetter. Experimental Investigation of Multibunch, Multipass Beam Breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver. United States.
Christopher Tennant, David Douglas, Kevin Jordan, Nikolitsa Merminga, Eduard Pozdeyev, Haipeng Wang, Todd I. Smith, Stefan Simrock, Ivan Bazarov, and Georg Hoffstaetter. Fri . "Experimental Investigation of Multibunch, Multipass Beam Breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver". United States. doi:. https://www.osti.gov/servlets/purl/877957.
@article{osti_877957,
title = {Experimental Investigation of Multibunch, Multipass Beam Breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver},
author = {Christopher Tennant and David Douglas and Kevin Jordan and Nikolitsa Merminga and Eduard Pozdeyev and Haipeng Wang and Todd I. Smith and Stefan Simrock and Ivan Bazarov and Georg Hoffstaetter},
abstractNote = {In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW Free Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Mar 24 00:00:00 EST 2006},
month = {Fri Mar 24 00:00:00 EST 2006}
}
  • It is well known that the multipass, multibunch beam breakup (BBU) instability imposes a potentially severe limitation to the average current that can be accelerated in an energy recovery linac (ERL). Simulation results for Jefferson Lab's FEL Upgrade Driver are presented which predict the occurrence of BBU below the nominal operating current of the machine. In agreement with simulation, BBU was observed and preliminary measurements to identify the higher-order mode (HOM) causing the instability are shown. In addition, measurements performed to experimentally determine the threshold current are described. Using a newly developed two-dimensional BBU simulation code, we study the effectmore » of optical suppression techniques, first proposed by Rand and Smith in 1980 [1], on the threshold current of the FEL. Specifically we consider the effect of (1) reflecting the betatron planes about 45 degrees and (2) rotating the betatron planes by 90 degrees. In two pass recirculators, a 90 degrees rotation significantly increases the threshold current of BBU. The successful installation of a five skew-quadrupole reflector in the backleg of the FEL has been shown to be effective at suppressing the instability and comments on preliminary operational experience will be given.« less
  • An energy recovering linac (ERL) offers an attractive alternative for generating intense beams of charged particles by approaching the operational efficiency of a storage ring while maintaining the superior beam quality typical of a linear accelerator. Two primary physics challenges exist in pushing the frontier of ERL performance. The first is energy recovering a high energy beam while demonstrating operational control of two coupled beams in a common transport channel. The second is controlling the high average current effects in ERLs, specifically a type of beam instability called multipass beam breakup (BBU). This work addresses both of these issues. Amore » successful 1 GeV energy recovery demonstration with a maximum-to-injection energy ratio of 51:1 was carried out on the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory in an effort to address issues related to beam quality preservation in a large scale system. With a 1.3 km recirculation length and containing 312 superconducting radio frequency (SRF) cavities, this experiment has demonstrated energy recovery on the largest scale, and through the largest SRF environment, to date. The BBU instability imposes a potentially severe limitation to the average current that can be accelerated in an ERL. Simulation results for Jefferson Laboratory's 10 kW free electron laser (FEL) Upgrade Driver predict the occurrence of BBU below the nominal operating current. Measurements of the threshold current are described and shown to agree to within 10% of predictions from BBU simulation codes. This represents the first time the codes have been benchmarked with experimental data. With BBU limiting the beam current, several suppression schemes were developed. These include direct damping of the higher-order mode using two different cavity-based feedbacks and modifying the electron beam optics to reduce the coupling between the beam and mode. Specifically the effect of implementing (1) point-to-point focusing (2) a reflection of the betatron planes about 45± and (3) a rotation of the betatron planes by 90± is measured. Each method increased the threshold current for stability. Beam optical control methods proved to be so effective that they are routinely used in the operation of the 10 kW FEL Upgrade.« less
  • In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current has been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to themore » predictions of several BBU simulation codes. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.« less
  • The driver for Jefferson Lab's infrared free-electron laser is a superconducting, recirculating accelerator that recovers about 75% of the electron-beam energy and converts it to radiofrequency power. It is designed to lase continuous-wave at 3--6 {mu}m at kW-level power. In achieving first light, the accelerator operated straight ahead to deliver 38 MeV, 1.1 mA cw current through the wiggler for lasing at wavelengths in the vicinity of 5 {mu}m. The waste beam was then sent directly to a dump, bypassing the recirculation loop. Stable operation at power levels up to 311 W cw have thus far been achieved in thismore » mode. The accelerator has recently recirculated up to 0.6 mA cw current with energy recovery. In this mode it has lased pulsed and cw at low-power. It remains to clean up the transport for high-power cw lasing.« less
  • We report the observation and investigation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating intense pulsed CO/sub 2/ laser beams, operating at different frequencies in a stimulated Compton-scattering scheme. The interaction occurred in the nonlinear (trapping) regime, the physics of which is the same as that which occurs in laser accelerators and efficiency-enhanced free-electron lasers (FEL's) with long wigglers. The experiment is a first demonstration of the principle of inverse FEL acceleration and electromagnetic pump FEL operation in the nonlinear (trapping) regime. It can also be described as a demonstration of a ''travelingmore » beat-wave'' Kapitza-Dirac effect in the nonlinear regime. Two different mechanisms of enhanced energy transfer were observed: electron trapping and phase-area displacement. Experimental results and computer simulations of both mechanisms are presented.« less