Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO2 laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into approximately 3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

An electron beam microbunched on the optical wavelength scale of ഠ2.5 mm by an inverse free electron laser accelerator was observed. The optimum bunching was achieved for a 1% energy modulation of a 32 MeV electron beam with 0.5 GW CO 2 laser power. The microbunching process was investigated by measuring the coherent transition radiation produced by the energy modulated electron beam. A quadratic dependence of the transition radiation signal on the electron beam charge was observed. The observed shortest wavelength of coherent transition radiation is less than 2.5 mm. The debunching process of the microbunched electron beam was experimentally investigated. [S0031-9007(98)06085-2]

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...

Design and operation of a 50 MeV Electron LinearAccelerator utilizing a low emittance ( 7 c -5 to 10 mm-mrad) radio-frequency gun operating at an output energy of 5 MeV and a charge of 1 nC is described.Design calculations and early radio-frequency measurements and operational experience with the electron gun utilizing a dummy copper cathode in place of the proposed photocathode emitter are given.

Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing ϳ3 fs microbunches to form separated by the laser wavelength at 10.6 mm (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.