R. Yoder

Experiments on laser driven beatwave acceleration in a ponderomotively formed plasma channel

A 10 ps long beam of 12 MeV electrons is externally injected into a ∼3-cm long plasma beatwave excited in a laser ionized hydrogen gas. The electrons have been accelerated to 50 MeV with a gradient of ∼1.3 GeV/m. It is shown that when the effective plasma wave amplitude-length product is limited by ionization-induced defocusing (IID), acceleration of electrons is significantly enhanced by using a laser pulse with a duration longer than the time required for ions to move across the laser spot size. Both experiments and two-dimensional simulations reveal that, in this case, self-guiding of the laser pulse in a ponderomotively formed plasma channel occurs. This compensates for IID and drives the beatwave over the longer length compared to when such a channel is not present.

Velocity bunching experiment at the Neptune laboratory

In this paper we describe the rectilinear compression experiment at the Neptune photoinjector at UCLA. The electron bunches have been shortened to sub-ps pulse length 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 following rectilinear drift. Using a standard Martin Puplett interferometer to characterize coherent transition radiation from the beam, we measured bunch lengths as short as 0.4 ps with compression ratio in excess of 10 for an electron beam of 7 MeV energy and charge up to 300 pC.

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).

Energy Loss of a High Charge Bunched Electron Beam in Plasma: Nonlinear Plasma Response and Linear Scaling

There has been much experimental and theoretical interest in blowout regime of plasma wakefield acceleration (PWFA), which features ultra‐high accelerating fields, linear transverse focusing forces, and nonlinear plasma motion. Using an exact analysis, we examine here a fundamental limit of nonlinear PWFA excitation, by an infinitesimally short, relativistic electron beam. The beam energy loss in this case is shown to be linear in charge even for nonlinear plasma response, where a normalized, unitless charge exceeds unity, and relativistic plasma effects become important or dominant. The physical bases for this persistence of linear response are pointed out. As a byproduct of our analysis, we re‐examine the issue of field divergence as the point‐charge limit is approached, suggesting an important modification of commonly held views of evading unphysical energy loss. Deviations from linear behavior are investigated using simulations with finite length beams. The peak accelerating field in the plasma wave e...

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.

Very High Energy Gain at the Neptune Inverse Free Electron Laser Experiment

We report the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in an undulator strongly tapered in period and field amplitude. The IFEL driver is a CO2 10.6 μm laser with power larger than 400 GW. The Rayleigh range of the laser, ∼ 1.8 cm, is much shorter than the undulator length so that the interaction is diffraction dominated. A few per cent of the injected particles are trapped in a stable accelerating bucket. Electrons with energies up to 35 MeV are measured by a magnetic spectrometer. Three‐dimensional simulations, in good agreement with the measured electron energy spectrum, indicate that most of the acceleration occurs in the first 25 cm of the undulator, corresponding to an energy gradient larger than 70 MeV/m. The measured energy spectrum also indicates that higher harmonic Inverse Free Electron Laser interaction takes place in the second section of the undulator.

Status of the inverse free electron laser experiment at the Neptune Laboratory

We report on the status of the Inverse Free Electron Laser accelerator experiment under construction at the UCLA Neptune Laboratory. This experiment will use a 400 GW CO/sub 2/ laser to accelerate through a tapered undulator an electron beam from 14.5 MeV up to 55 MeV. The scheme proposed is the diffraction dominated IFEL interaction where the Rayleigh range of the laser beam is 3.5 cm, much shorter than the interaction length (the undulator length is 50 cm). The undulator is strongly tapered in both field and period. The present status of the experiment is reported.

High Energy Gain IFEL at UCLA Neptune Laboratory

We report on the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected in a 50 cm long undulator strongly tapered both in period and field amplitude. A CO210.6 µ m laser with power > 400 GW is used as the IFEL driver. The Rayleigh range of the laser (∼ 1.8 cm) is shorter than the undulator length so that the interaction is diffraction dominated. Few per cent of the injected particles are trapped in stable accelerating buckets and electrons with energies up to 35 MeV are detected on the magnetic spectrometer. Three dimensional simulations are in good agreement with the electron energy spectrums observed in the experiment and indicate that substantial energy exchange between laser and electron beam only occurs in the first 25-30 cm of the undulator. An energy gradient of > 70 MeV is inferred. In the second section of the undulator higher harmonic IFEL interaction is observed.

The UCLA/SLAC Ultra-High Gradient Cerenkov Wakefield Accelerator Experiment

An experiment is planned to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range. This new UCLA/SLAC/USC collaboration will take advantage of the unique SLAC FFTB electron beam and its demonstrated ultra-short pulse lengths and high currents (e.g., σz= 20 μm at Q = 3 nC). The electron beam will be focused down and sent through varying lengths of fused silica capillary tubing with two different sizes: ID = 200 μm / OD = 325 μm and ID = 100 μm / OD = 325 μm. The pulse length of the electron beam will be varied in order to alter the accelerating gradient and probe the breakdown threshold of the dielectric structures. In addition to breakdown studies, we plan to collect and measure coherent Cerenkov radiation emitted from the capillary tube to gain information about the strength of the accelerating fields.

Acceleration of injected electrons in a laser beatwave experiment

A 10-ps beam of 12 MeV electrons was loaded in a 1-cm long plasma beat wave accelerator driven by a TW CO/sub 2/ laser pulse. CO/sub 2/ laser pulses and electron bunches were deterministically synchronized with an uncertainty of 20 ps. At the resonant electron plasma density of /spl sim/10/sup 16/ cm/sup -3/ the electrons have been accelerated to 22 MeV with a gradient of /spl sim/ 1 GeV/m.