R. J. England

Laser damage threshold measurements of optical materials for direct laser accelerators

The laser-damage threshold is a fundamental limit for any dielectric laser-driven accelerator and is set by the material of the structure. In this paper, we present a theoretical model of the laser damage mechanism, in comparison with experimental data on the damage threshold of silicon. Additionally, we present damage threshold measurement data of various optical materials, most of which have not been previously characterized in the picosecond-regime.

X-Band Dipole Mode Deflecting Cavity for the UCLA Neptune Beamline

We report progress on the design and construction of a nine-cell cavity operating in a TM110-like dipole mode for use as a temporal diagnostic of the 14 MeV, 300 pC electron bunches generated at the UCLA Neptune Laboratory linear accelerator, with an anticipated temporal resolution of 50 fs at a peak input power of 50 kW. The cavity is a center-fed standing-wave pi-mode structure, operating at 9.6 GHz, and incorporating a knife-edge and gasket assembly which minimizes the need for brazing or welding. Results of initial RF testing of the prototype cavity are discussed and compared with simulation results obtained using the commercial code HFSS.

Grating-based deflecting, focusing, and diagnostic dielectric laser accelerator structures

Recent technological advances has made possible the realization of the first laser-driven particle accelerator structure to be fabricated lithographically. However, a complete particle accelerator requires more than just accelerating elements. In this paper, we present a grating-based design for three other quintessential accelerator elements: the focusing structure, the deflecting structure, and the diagnostic structure.

Design, fabrication, and testing of a fused-silica dual-layer grating structure for direct laser acceleration of electrons

A proof of principle fused-silica grating structure has been designed and fabricated for the purpose of direct laser acceleration of electrons. The optimal structure geometry was determined via 2D-FDTD and 3D-FEFD simulations to maximize the available acceleration gradient. The structure was fabricated with standard nanofabrication techniques, including optical lithography, reactive ion etching, and wafer bonding. Beam tests have been performed with the 60MeV beam at the Next Linear Collider Test Accelerator at SLAC, with successful demonstration of electron transmission through the micron-scale apertures.

Experiment to Detect Accelerating Modes in a Photonic Bandgap Fiber

An experimental effort is currently underway at the E‐163 test beamline at Stanford Linear Accelerator Center to use a hollow‐core photonic bandgap (PBG) fiber as a high‐gradient laser‐based accelerating structure for electron bunches. For the initial stage of this experiment, a 50 pC, 60 MeV electron beam will be coupled into the fiber core and the excited modes will be detected using a spectrograph to resolve their frequency signatures in the wakefield radiation generated by the beam. We will describe the experimental plan and recent simulation studies of candidate fibers.

Investigation of X-Ray Harmonics of the Polarized Inverse Compton Scattering Experiment at UCLA

An Inverse Compton Scattering (ICS) experiment, which will investigate nonlinear properties of scattering utilizing a terawatt CO2laser system with various polarizations, is ongoing at the UCLA Neptune Laboratory. When the normalized amplitude of the incident lasers vector potential a0is larger than unity the scattering occurs in the nonlinear region; therefore, higher harmonics are also produced. ICS can be used, e.g., for a polarized positron source by striking a thin target (such as tungsten) with the polarized X-rays. As such, it is critical to demonstrate the production of polarized scattered photons and to investigate the ICS process as it enters the nonlinear regime. We present the description of the experimental set up and equipment utilized, including diagnostics for electron and photon beam detection. We present the current status of the experiment.

Beam shaping and compression scheme for the UCLA neptune laboratory

We have recently added a dispersionless translating section to the UCLA Neptune linear accelerator beamline. This new section of beamline will serve as a venue for beam shaping and compression experiments using the l4MeV electron beam produced by the UCLA Neptune PWT linac and newly installed photoinjector. An examination of the first and second-order optics indicates that when certain nonlinear effects are minimized through the use of sextupole magnets, the longitudinal dispersion is dominated by a negative RS6 which, for an appropriately chirped initial beam, can be used to create a ramped beam of a few picosecond duration that would be ideal for driving large amplitude wake fields in a plasma and producing high transformer ratios. The beamline is now in operation. Preliminary data indicate that the beamline optics are well-predicted by simulation and that sextupoles can be used successfully to eliminate nonlinear horizontal dispersion. Future experiments are planned for measuring beam compression (using CTR autocorrelation) and doing longitudinal phase space tomography (using a transverse deflecting cavity).

Longitudinal Beam Shaping and Compression Scheme for the UCLA Neptune Laboratory

We are developing a new beamline which will serve as a venue for future beam‐plasma interaction experiments using the 14MeV electron beam produced by the UCLA Neptune 1.625‐cell photoinjector and PWT linac. An examination of the first and second‐order optics indicates that when certain nonlinear effects are minimized through the use of sextupole magnets, the longitudinal dispersion is dominated by a negative R56. Simulations using the matrix transport code ELEGANT indicate that for an appropriately chirped initial beam, this beamline can be used to create a ramped picosecond to sub‐picosecond beam that is ideal for driving large amplitude wake fields in a plasma and producing high transformer ratios.

Velocity Bunching Experiment at the Neptune Laboratory

In this paper we describe the ballistic bunching compression experiment at the Neptune photoinjector at UCLA. We have compressed the beam by chirping the beam energy spectrum in a short S‐band high gradient standing wave RF cavity and then letting the electrons undergo velocity compression in the subsequent rectilinear drift. Using a standard Martin Puplett interferometer for coherent transition radiation measurement, we have observed bunch length as short as 0.4 ps with compression ratio in excess of 10 for an electron beam of 7 MeV and charge up to 0.3 nC. We also measured slice transverse emittance via quad scan technique. The observed emittance growth agrees with the predictions and the simulations. Extension of this scheme to a future advanced accelerator injector system where solenoidal magnetic field can compensate the emittance growth is studied.

Beam dynamics and wakefield simulations of the double grating accelerating structure

Laser-driven acceleration in dielectric structures can provide gradients on the order of GeV/m. The small transverse dimension and tiny feature sizes introduce challenges in design, fabrication, and simulation studies of these structures. In this paper we present the results of beam dynamic simulation and short range longitudinal wakefield simulation of the double grating structure. We show the linear trend of acceleration in a dielectric accelerator design and calculate the maximum achievable gradient equal to 0.47E0 where E0 is maximum electric field of the laser excitation. On the other hand, using wakefield simulations, we show that the loss factor of the structure with 400nm gap size will be 0.12GV/m for a 10fC, 100as electron bunch which is an order of magnitude less than expected gradient near damage threshold of the device.