MOOBB2
Stanford University, Stanford, California, USA
OHEP/DOE, Germantown, MD, USA
SLAC, Menlo Park, California, USA
UCLA, Los Angeles, USA
We report the first observation of high-gradient acceleration of electrons in a lithographically fabricated micron-scale dielectric optical accelerator driven by a mode-locked Ti:sapphire laser. We have observed acceleration gradients far exceeding those of conventional microwave accelerator structures. Additionally, we have verified the dependence of the observed acceleration gradient on: the laser pulse energy, the laser-electron temporal overlap, the polarization of the laser, and the incidence angle of the laser. In all cases, we have found good agreement between the observed results, the analytical predictions, and the particle simulations.
SLAC, Menlo Park, California, USA
Stanford University, Stanford, California, USA
Purdue University, West Lafayette, Indiana, USA
Technion, Haifa, Israel
Particle acceleration in dielectric laser-driven micro-structures, recently demonstrated at SLAC*, holds the promise of providing low-cost compact accelerators for a wide variety of uses. Laser-driven undulators based upon this concept could attain very short (mm to sub-mm) periods with multi-Tesla field strengths. And since dielectric laser accelerators (DLAs) operate optimally with optical-scale electron bunch formats, radiation production with high repetition rate (10s of MHz) attosecond-scale pulses is a natural combination. We present preliminary analysis of the harmonic field structure for a periodic undulator based on this concept.
SLAC, Menlo Park, California, USA
A photonic band gap (PBG) lattice in dielectric fiber can provide high gradient acceleration in the optical regime, where the accelerating mode is obtained from the presence of a single defect in the lattice. In this paper, we will investigate two aspects of the PBG for acceleration. First, the excitation of the accelerating mode can be achieved by directing high-power lasers from free space. Simulation using ACE3P has demonstrated that, by appropriately shaping the end of the PBG fiber, power can be coupled into the fiber using a simple laser configuration. Second, the wakefield generated by the transit of a beam through a PBG fiber will be simulated using ACE3P. The free-space, outgoing radiation spectrum and distribution of the wakefield will be evaluated and corroborated with measurements from a commercial fiber.
SLAC, Menlo Park, California, USA
Stanford University, Stanford, California, USA
Rapid progress in the development of laser technology and in the sophistication of semiconductor manufacturing has enabled the realization of the first dielectric laser-driven particle accelerator (DLA) on a chip *. Since the accelerating channel in DLA structures typically have dimensions in the 1 micron range, the ability to precisely control particle position within these structures will be critical for operation. A number of beam deflection and focusing schemes have been devised, but without the ability to measure the position of the particle beam to nanometer accuracy, these schemes will be extremely difficult to implement. We present a new concept for a beam position monitor with the unique ability to map particle beam position to a measurable wavelength. Coupled with an optical spectrograph, this beam position monitor is capable of sub-nanometer resolution. We describe one possible design of this device, and present the current status of the structure fabrication and experimental demonstration.
* E. A. Peralta, et al., "High-Gradient Acceleration of Electrons in a Laser-Driven Dielectric Micro-Accelerator." Submitted to PAC'13
SLAC, Menlo Park, California, USA
Stanford University, Stanford, California, USA
Purdue University, West Lafayette, Indiana, USA
Laser driven dielectric accelerators possess advantages in their greatly reduced dimensions and costs, larger breakdown threshold, and higher accelerating gradient *. Several resonant or waveguide based structures have been proposed * ** ***. Electron acceleration with 250 MV/m gradient has been demonstrated using a silica grating structure ***. We describe a new structure of silica rod array for laser driven acceleration. The structure consists of two rows of equally spaced circular or elliptical rods, with a gap between the rows as the particle channel. Similar to the grating, the periodic arrangement of the rods along the channel provides phase reset of the EM fields and thus required phase synchronicity for acceleration. Resonances among rods enhance the near E-field in the channel to achieve high gradient. The structure has been optimized dimensionally in simulations, and exhibited quite uniform net acceleration in the gap region. One advantage of this structure is that the rods are fabricated on one single substrate, therefore positioned and aligned with lithographic level precision. Prototype samples have been fabricated **** for potential laser acceleration experiments.
* T. Plettner, R. Byer., Phys. Rev. ST-AB,11:030407,2008.
** B. Cowan, Phys. Rev. ST-AB,11:011301,2008.
*** E. Peralta et al, manuscript submitted.
**** M. Qi, private communication.
UCLA, Los Angeles, California, USA
SLAC, Menlo Park, California, USA
TUB, Beijing, People's Republic of China
MPI, Muenchen, Germany
Plasma accelerators driven by ultra-relativistic electron beams have demonstrated greater than 50 GeV/m acceleration gradients over a distance of a meter though the accelerated particles typically have had a 100% energy spread when a single drive bunch was used. However, it is known that by locally producing electrons via ionization within the beam-driven plasma wake, they can become trapped and accelerated so that high-energy, mono-energetic electron bunches can be produced. We propose a technique to help identify these bunches of electrons at the 10’s of pC level arising from the ionization injection of Ar electrons that may otherwise be lost or overlooked as part of the discrete betatron-focusing maxima or the maxima inherent the chromaticity of the imaging electron spectrometer.
UCLA, Los Angeles, California, USA
University of Oslo, Oslo, Norway
SLAC, Menlo Park, California, USA
TUB, Beijing, People's Republic of China
MPI, Muenchen, Germany
Evidence of beam loading due to distributed injection in Plasma Wakefield Accelerator experiments carried out at the FACET facility at SLAC during the year 2012 is presented. The source of the injected charge is tunnel ionization of Rb+ inside the wake, which occurs along the length of the interaction at each minima of envelope betatron oscillation. Rb was used specifically to mitigate the problem of head erosion, which limited the energy gain in earlier experiments using Li that were carried out at FFTB in SLAC. In the present experiment however, electrons produced via secondary ionization of Rb were injected in the wake and led to a severe depletion of the accelerating wake, i.e. beam loading, which is observed as a reduction of mean, i.e. measured, transformer ratio. This ‘‘dark current" limitation on the maximum achievable accelerating gradient is also pertinent to other heavier ions that are potential candidates for high-gradient PWFA.
SLAC, Menlo Park, California, USA
University of Oslo, Oslo, Norway
UCLA, Los Angeles, California, USA
MPI, Muenchen, Germany
UCLA, Los Angeles, California, USA
University of Oslo, Oslo, Norway
SLAC, Menlo Park, California, USA
TUB, Beijing, People's Republic of China
MPI, Muenchen, Germany
Plasma wakefield accelerator experiments at FACET at the SLAC National Accelerator Laboratory have shown a correlation between ionization-injected electrons and the betatron x-ray yield. Emittance spoiling foils were inserted into the beam and the x-ray yield, excess charge, and beam energy loss was measured. The excess charge and x-ray yield are attributed to the beam envelope oscillations where at the minima, the field of the beam is strong enough to create secondary ionization, and at the electron oscillation maxima, the beam electrons spontaneously radiate x-rays. Large amplitude beam oscillations are expected to yield more x-rays and create more excess charge, but the results show beam head erosion strongly limits the wakefield excitation.