ANL, Argonne
Euclid TechLabs, LLC, Solon, Ohio
Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-06CH11357.
Dielectric loaded wakefield structures are being developed to be used as high gradient accelerator components. The high current electron beam at the Argonne Wakefield Accelerator Facility was used to excite wakefields in cylindrical dielectric loaded wakefield structures in the frequency range of 8 to 14 GHz, with pulse duration of a few nanoseconds. Short electron bunches (13 ps FWHM) of up to 86 nC drove these wakefields, and accelerating fields as high as 100 MV/m were reached. These standing-wave structures have a field probe near the outer edge of the dielectric to sample the RF fields generated by the electron bunches. Monitoring of the field probe signal serves to verify the absence of electric breakdown. Similar structures were used to extract RF power from the electron beam; however, in this case they were travelling-wave structures, driven by electron bunch trains of up to 16 bunches. RF pulses of up to 40 MW were measured at the output coupler of these structures.
UCLA, Los Angeles, California
BNL, Upton, Long Island, New York
SLAC, Menlo Park, California
USC, Los Angeles, California
Funding: Work Supported by US Department of Energy
In the plasma wakefield accelerator (PWFA), high quality accelerated electron bunches can be produced by injecting a witness bunch behind a single drive bunch or a train of N bunches. To operate at large gradient the plasma density must be in the 1017/cc range, corresponding to a typical bunch separation of the order of the plasma wavelength or ≈100μm. We have demonstrated that such a sub-picosecond temporal bunch structure can be produced using a mask to selectively spoil the emittance of temporal slices of the bunch*. The bunches spacing, as well as their length can be tailored by designing the mask and choosing the beam parameters at the mask location. The number of bunches is varied by using an adjustable width energy limiting slit. The bunches spacing is measured with coherent transition radiation interferometry. Experimental results will be presented and compared to simulations of the bunch train formation process with the particle tracking code ELEGANT.
*P. Muggli et al., to appear in Phys. Rev. Lett. (2008).
UCLA, Los Angeles, California
Funding: Work Supported by US Department of Energy
Plasma-based accelerators are one of the emerging technologies that could revolutionize e-/e+ colliders, significantly reducing their size and cost by operating at multi-GeV/m accelerating gradients. Proof-of-principle experiments at SLAC have demonstrated the energy doubling of 42 GeV incoming e- in a plasma only ≈85 cm-long,* corresponding to an unloaded gradient of ≈50 GeV/m. Plasma wakes driven by e+ bunches are different from those driven by e- bunches. The acceleration of e+ in plasmas has been demonstrate,** but the acceleration of high-quality e+ beams is challenging. Measurements show that single e+ bunches suffer halo formation and emittance growth when propagating through dense meter-scale, uniform plasmas.*** Advanced schemes, such as hollow plasma channels, or e+ bunch acceleration on the wake driven by a e bunch, may have to be used in a future plasma-based linear collider. Experimental results obtained with e+ beams in plasmas will be reviewed and compared to those obtained with e- beams. Future experiments including a new scheme to produce a drive e bunch closely followed by a witness e+ bunch appropriate for PWFA experiments will also be discussed.
*I. Blumenfeld et al., Nature 445, 741-744 (15 February 2007).
**B.E. Blue et al., Phys. Rev. Lett. 90, 214801 (2003).
***P. Muggli et al., accepted for publication in Phys. Rev. Lett. (2008).
LBNL, Berkeley, California
Laser driven plasma wakefields have recently accelerated electron beams with quasi-monoenergetic energy distributions and with gradients of ~100 GV/m. Stabilization and optimization of beam quality are now essential. Recent LBNL experiments have demonstrated control of self trapping, resulting in reproducible bunches at 0.5 GeV. Further optimization has been demonstrated using plasma density gradients to control trapping, producing beams with very low absolute momentum spread at low energies. Simulations indicate that use of these beams as an injector greatly improves accelerator performance and experiments are now underway to demonstrate such staging, which will be a crucial technology for laser driven linacs. This talk will cover recent progress in LWFAs to obtain more reproducible, higher quality beams and also cover staging prospects for high energy laser linacs.