Observation of Energy Gain at the BNL Inverse Free-Electron-Laser Accelerator

Abstract

A 40 MeV electron beam, using the inverse free-electron-laser interaction, has been accelerated by DE͞E 2.5% over a distance of 0.47 m. The electrons interact with a 1 -2 GW CO 2 laser beam bounded by a 2.8 mm i.d. sapphire circular waveguide in the presence of a tapered wiggler with B max ഠ 1 T, and a period 2.89 # l w # 3.14 cm. The experimental results of DE͞E as a function of electron energy E, peak magnetic field B w , and laser power W l compare well with analytical and 1D numerical simulations and permit scaling to higher laser power and electron energy.[S0031-9007(96)01273-2] PACS numbers: 41.75.Lx, 41.60.Cr, 41.75.Ht, 52.75.Ms Since the inception of practicable lasers, laser-driven acceleration concepts have been intensively studied. The interest resides in the use of the very large electric fields achievable by focusing the laser to very small spots. These fields are well in excess of the accelerating gradients currently obtained in rf cavities. The study of the inverse free-electron laser (IFEL) as a potential mode of electron acceleration has been pursued at Brookhaven National Laboratory (BNL) for a number of years [1][2][3][4]. Although the concept has been studied theoretically in detail, the first experimental verification of the concept (l 1.65 mm) was performed in 1992 [5]. In this Letter further experimental evidence of the IFEL interaction (l 10.6 mm) is presented. The experiment used a 50 MeV electron beam, a 1-5 GW CO 2 laser beam provided by the BNL's Accelerator Test Facility (ATF) and a uniquely designed period length tapered wiggler.This wiggler is a fast excitation electromagnet with stackable, geometrically and magnetically alternating substacks of vanadium permendur (VaP) ferromagnetic laminations, periodically interspersed with conductive (Cu), nonmagnetic laminations, which act as eddy current induced field reflectors [6,7]. Four current conducting rods, parallel to the wiggler axis, are connected at the ends of the assembly, constituting the excitation loop that drives the wiggler. The overall wiggler stack is easily assembled, is compressed by simple tie rods, and readily permits wiggler period length (l w ) variation. Configured as a constant period wiggler, l w 3.75 cm and B max 1 T, the system has shown [7] an rms pole-to-pole field variation of approximately 0.2%.The CO 2 laser beam is brought into the IFEL interaction region by a low loss dielectric (sapphire) circular waveguide. Two different guide configurations were tested, first a 1 4 l dielectric coating (germanium) deposited on the two lateral walls of a rectangular cross-section metallic waveguide [8] and then a sapphire (Al 2 O 3 ) circular waveguide, which showed very good transmission properties [9] of a high power CO 2 laser beam. Extensive studies were carried out to establish optimum coupling into the guide and to measure the transmission loss of long (1.0 m) extruded single crystal sapphire guides. Also, because of the overmoded guide configuration (i.d. 2.8 mm), attempts were made to determine the transverse...

Microwave-induced control of free-electron-laser radiation

“…Another important application of FEL principle is the particle acceleration by the inverse mechanism. Particle acceleration by inverse free-electron-laser principle has been demonstrated both theoretically [12] and experimentally [13,14]. In contrast, much less seems to be known for the complementary geometry, in which the electron bunch interacts with a counterpropagating electromagnetic wave, perhaps because of the absence of acceleration schemes for this case.…”

Brookhaven Accelerator Test Facility energy upgrade

“…ATF users have experimentally demonstrated two laser acceleration techniques, i.e. Inverse Cerenkov Acceleration (ICA) [6] and Inverse Free Electron Laser (IFEL) [7] acceleration; Self Amplified Spontaneous Emission (SASE) was _______________________ * Work supported by U.S D.O.E contract DE-AC02-98CH10886. # Email: xwang@bnl.gov observed at both 1 and 5 µm [8,9].…”

Enhanced IFEL performance using a novel wiggler

“…The first EEL experiment was reponed in 1991 using a 750kV electron beam that was prebunched by, and powered by, a millimeter-wavelength FEL [4]. Recently, acceleration of a 40MeV beam to 42MeV was reported [5] using a GW C02 laser beam; the bunching of this beam is being studied [6] preparatory to injecting the electron output from the IFEL into another laser accelerator. These experiments used wigglers that provided a sinusoidal field variation, either as a circularly polarized-or dipole-type field, as did previous theory [3].…”