Abstract
A new accelerator, LACARA (laser-driven cyclotron autoresonance accelerator), under construction at the Accelerator Test Facility at Brookhaven National Laboratory, is to be powered by a 1 TW ${\mathrm{CO}}_{2}$ laser beam and a 50 MeV injected electron pulse. LACARA will produce inside a 2 m, 6 T solenoid a 100 MeV gyrating electron bunch, with $\ensuremath{\sim}3%$ energy spread, approximately 1 psec in length with particles advancing in phase at the laser frequency, executing one cycle each 35 fsec. A beamstop with a small off axis channel will transmit a short beam pulse every optical cycle, thereby producing a train of about 30, 3.5 fsec long, 1--3 pC microbunches for each laser pulse. We describe here a novel accelerator, a micron-scale dielectric wake field accelerator driven by a 500 MeV LACARA-type injector that takes the output train of microbunches and transforms them into a near-rectangular cross section having a narrow dimension of $\ensuremath{\sim}10\ensuremath{\mu}\mathrm{m}$ and height of $\ensuremath{\sim}150\ensuremath{\mu}\mathrm{m}$ using a magnetic quadrupole; these bunches may be injected into a planar dielectric-lined waveguide (slightly larger than the bunch) where cumulative buildup of wake fields can lead to an accelerating gradient $g1\mathrm{GV}/\mathrm{m}$. This proposed vacuum-based wake field structure is physically rigid and capable of microfabrication accuracy, factors important in staging a large number of accelerator modules. Furthermore, the accelerating gradients it promises are comparable with those for plasma accelerators. A LACARA unit for preparing suitable bunches at 500 MeV is described. Physics issues are discussed, including bunch spreading and transport, bunch shaping, coherent diffraction radiation from the aperture, dielectric breakdown, and bunch stability in the rectangular wake field structure.
Highlights
New techniques for accelerating electrons or positrons using lasers promise very high accelerating gradients.1 GeVm
Plasma microstructures set up by intense laser pulses have offered the promise of large gradients [1], but are troubled by such issues as system efficiency and staging
The system is made up of fixed dielectric structures of small dimension which surround the electron bunch in vacuum; these structures are designed to be excited by fields at optical/IR wavelengths
Summary
New techniques for accelerating electrons or positrons using lasers promise very high accelerating gradients. GeVm. New techniques for accelerating electrons or positrons using lasers promise very high accelerating gradients.. GeVm. Plasma microstructures set up by intense laser pulses have offered the promise of large gradients [1], but are troubled by such issues as system efficiency and staging. We describe a system, a micron-scale dielectric wake field accelerator, which achieves similar gradients as in a plasma accelerator [2], while having better efficiency and possibly easier staging alignment. We outline a technique to prepare a train of bunches of electrons at high energy, each of which is approximately 1 mm (3.5 fsec) in length and carries a few pC of charge, and which can be used to excite wake fields in small dielectric structures to provide GeVm gradients for the acceleration of electron or positron bunches. III describes some of the physics issues of this novel wake field accelerator concept
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