We present recent experiments on the characterization of Betatron radiation in the blowout regime of laser-wakefield acceleration. We observed Betatron x-rays up to 80 keV, and the characterization of the angular dependence of the x-ray spectrum suggests anisotropic electron trajectories in the plasma. The characterization of the source opens up new possibilities for application experiments.
Betatron x-rays with multi-keV photon energies have been observed from a GeV-class laser-plasma accelerator. The experiment was performed using the 200 TW Callisto laser system at LLNL to produce and simultaneously observe GeV-class electron beams and keV Betatron x-rays. The laser was focused with two different optics (f/8 and f/20), and into various gas cells with sizes ranging from 3 to 10 mm, and containing mixed gases (He, N, CO2, Ar, Ne) to accelerate large amounts of charge in the ionization induced trapping regime. KeV betatron x-rays were observed for various concentrations of gases. Electron spectra were measured on large image plates with the two-screen method after being deflected by a large 0.42 Tesla magnet spectrometer. Betatron oscillations observed on the electron spectra can be benchmarked against a simple analytical model (Runge-Kutta algorithm solving the equation of motion of an electron in the wakefield), in order to retrieve the electron injection conditions into the wake.
Recently, strong effort has been done in exploring shock acceleration for the generation of highly energetic ion beams, with applications e.g. for medical purposes. The heating of a near-critical density plasma target with a laser, increases the electron temperature and excites ion acoustic waves, which can lead to electrostatic shock formation due to non-linear wave breaking. The higher inertia background ions are reflected and accelerated at the shock potential, showing a quasi-monoenergetic profile. For the first time, its feasibility has been demonstrated experimentally, gaining 20 MeV protons with a very narrow energy spread1 and a predicted scaling up to 200 MeV for lasers with a0 = 10.2 In the quest for high proton energies, optimal conditions for shock formation have to be found. We developed a relativistic model that connects the initial parameters with the steady state shock Mach number, which is based on the Sagdeev approach,3, 4 showing an increase of the ion energy for high upstream electron temperatures and low downstream to upstream density ratios5 and high temperature ratios, which has been confirmed by particle-in-cell simulations. In the context of producing a quasi-monoenergetic beam profile, we studied the enhancement of the Weibel instability in an electrostatic shock setup. Governing parameter regimes for the transition to an electromagnetic shock, which is associated with a broadening of the ion spectrum, were determined analytically and confirmed with simulations.
When a highly relativistic electron is injected off-axis into an ion channel, the restoring force of the radial field of the
ions will cause the electron to accelerate towards the axis, overshoot, and begin to undergo oscillations about the ioncolumn
axis at a characteristic frequency; the betatron frequency. This so-called betatron motion will cause the electron
to radiate hard x-rays in the forward direction. In two recent experiments at the Stanford Linear Accelerator Center
(SLAC), betatron x-rays in the 1-20kV range and in the 1-50MV range were produced with an electron beam with an
energy of 28.5 GeV for ion densities of about 1 x 1014 cm-3 and 1 x 1017cm-3, respectively. To make such an x-ray source
more compact, the 3km long SLAC linac would be replaced by a source of electrons from a Laser Wakefield accelerator
(LWFA). To increase the efficiency of converting laser into photons at high photon energies, we propose adding a
second stage where the LWFA electrons radiate via a second ion channel, independent of the accelerating process. This
two stage concept allows one to control the critical frequency of the emitted radiation as well as the efficiency of the
Monte Carlo simulation experiments have shown that very high energy electrons (VHEE), 150-250 MeV, have potential
advantages in prostate cancer treatment over currently available electrons, photon and proton beam treatment. Small
diameter VHEE beamlets can be scanned, thereby producing a finer resolution intensity modulated treatment than
photon beams. VHEE beams may be delivered with greater precision and accelerators may be constructed at
significantly lower cost than proton beams. A VHEE accelerator may be optimally designed using laser-plasma
technology. If the accelerator is constructed to additionally produce low energy photon beams along with VHEE, real
time imaging, bioprobing, and dose enhancement may be performed simultaneously. This paper describes a Monte Carlo
experiment, using the parameters of the electron beam from the UCLA laser-plasma wakefield accelerator, whereby dose
distributions on a human prostate are generated. The resulting dose distributions of the very high energy electrons are
shown to be comparable to photon beam dose distributions. This simple experiment illustrates that the nature of the dose
distribution of electrons is comparable to that of photons. However, the main advantage of electrons over photons and
protons lies in the delivery and manipulation of electrons, rather than the nature of the dose distribution. This paper
describes the radiation dose delivery of electrons employing technologies currently in exploration and evaluates potential
benefits as compared with currently available photon and protons beams in the treatment of prostate and other cancers,
commonly treated with radiation.
A two-frequency CO2 laser beam was used to beat-excite a large amplitude electron plasma wave in a resonant density plasma. The accelerating fields of the relativistic plasma wave were probed with collinear injected 2.1 MeV electrons from an electron linac. Some electrons gained at least 7 MeV in traversing the approximately 1 cm length of the beat wave accelerator, with the measurement limited by the 9.1 MeV high energy cut-off of the detection system. The corresponding average acceleration gradient is > 0.7 GeV/m and the average wave amplitude n1/n0 is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15 - 30% may have been achieved.
A compact 20 MeV linac with an RF laser-driven electron gun will drive a high-gain (10 cm gain length), 10.6 micrometers wavelength FEL amplifier, operating in the SASE mode. FEL physics in the high-gain regime will be studied, including start-up from noise, optical guiding, sidebands, saturation, and superradiance, with emphasis on the effects important for future short wavelength operation of FEL's. The hybrid undulator, designed and built at the Kurchatov Institute of Atomic Energy in the U.S.S.R., has forty periods, each 1.5 cm long. The magnetic material is a hybrid combination of SmCo5 blocks and Nd-Fe-B blocks, with vanadium-permendur yokes. The gap distance between pole-tips is fixed at 5 mm. On axis the peak value of the completed undulator's magnetic field was measured to be 7.3 kGauss (+/- 0.25%). Measurements during the conditioning phase of the RF gun for the electron beam's peak dark-current show 6 mA without the longitudinal magnetic focusing field in the gun and 34 mA with the focusing field active. The peak current from photoemission is calculated to be 200 A.
A novel, compact S-band LINAC has been designed and is currently under construction at UCLA. It is expected to deliver high brightness, 200 A, 20 MeV electron pulses, less than 4 ps in duration from a device that is about 1 meter long. It comprises; (1) a laser photocathode driven gun that produces 4.5 MeV electron bunches from a 1 1/2 cell cavity operating in the (pi) -mode and (2) an accelerating structure known as a plane wave transformer (PWT) designed by Swenson. The design considerations of the machine and initial operating experience of the gun are discussed. The linac will be used for free electron laser, advanced accelerator research and beam-plasma experiments.
1407_51Saturnus is an infrared FEL operating in the 10 micrometers wavelength region, driven by a compact 20 MeV linac with a photoinjector, under construction at UCLA. The 1.5 cm period, 0.6 (Tau) peak field undulator is being built at the Kurchatov IAE. The FEL is designed to operate primarily in the self-amplified spontaneous emission mode. This paper covers the start-up from noise, optical guiding, saturation, sidebands and superradiance, with emphasis on the effects important for future short wavelength operation of FELs. The photoinjector follows closely the Brookhaven design. Electrons are injected into an accelerating section based on the plane-wave transformer design developed by Swenson at SAIC. Simulation of the linac and FEL show a gain length of 10 cm, and a saturation power of 50 MW.
The feasibility of using relativistically moving plasma waves as short wavelength undulators for possible free-electron laser (FEL) and Compton scattering applications is considered. Focus is placed on the spontaneous emission emitted by a single electron bunch as it traverses a plasma wave wiggler. The basic characteristics of the radiation from such a wiggler are discussed, and attention is given to the wave-particle interaction physics. The electron trajectories in the plasma wave wiggler are simulated using a three-dimensional model which includes emittance effects, and the resulting radiation pattern produced by the electrons is calculated. The scattered-electron energies and spatial distributions are analyzed.