MPI, Muenchen, Germany
SLAC, Menlo Park, California, USA
USC, Los Angeles, California, USA
UCLA, Los Angeles, California, USA
Instituto Superior Tecnico, Lisbon, Portugal
MPI-P, München, Germany
*J. Vieira et al., Phys. Plasmas 19, 063105 (2012).
LLNL, Livermore, California, USA
UCLA, Los Angeles, California, USA
We demonstrate that it is possible to reconstruct in three dimensions the electron trajectories inside the channel of the laser-wakefield accelerator from the angular dependence of the Betatron x-ray spectrum by using an image plate-based spectrometer with differential filtering. The experiments were performed at LLNL using the 200 TW Callisto laser system. Experimental results are benchmarked against a code that solves the equation of motion of electrons oscillating in the plasma wake and by calculating the corresponding x-ray radiation spectrum and profile. This combined single-shot, simultaneous spectral and spatial x-ray analysis allows for a 3D reconstruction of electron trajectories in the plasma with micrometer resolution.
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
Instituto Superior Tecnico, Lisbon, Portugal
Tsinghua University, Beijing, People's Republic of China
In plasma based accelerators (LWFA and PWFA), the methods of injecting high qual- ity electron bunches into the accelerating wakefield is of utmost importance for various applications. Ionization injection has received much recent attention in experiments, in theory, and in simulation. Here we use 3D OSIRIS simulations to investigate generating high quality electron beams generated through ionization injection. This includes the study of two-stage ionization injected LWFA in the self-guided regime. The first, i.e., injection, stage is a mixture of 99.5% He and 0.5% N gasses, while the second, i.,e., acceleration stage is entirely composed of He. Laser intensities from 100TW to 1 PW will be modeled. In the 500TW case, energies greater than 3 GeV with 5% energy spread were observed.
UCLA, Los Angeles, California, USA
CO2 laser-plasma interaction provides a unique parameter space for particle acceleration in a gas jet plasma taking place at a critical plasma density ncr~ 1019 cm-3 and even at higher densities (3-10 ncr). Here we report the latest results of our study of electron acceleration in a wide range of plasma densities 1-10 ncr using a multi-TW CO2 laser system at the UCLA Neptune Laboratory. To gain insight into plasma density profile evolution during ~ 100 ps long CO2 laser-plasma interaction, we used laser interferometry with two 1 ps, 532 nm probe pulses separated by 5-100 ps. Electron beams recorded in our experiment had a divergence smaller than 15mrad and good shot-to-shot reproducibility. The knowledge of spatial distribution of accelerated electron beam transported through the overdense gas plasma is critically important for minimizing laser beam filamentation and for understanding influence of other laser-plasma instabilities. This should allow for optimization of CO2 laser driven shock wave acceleration of low-divergence monoenergetic ion beams*. The results on correlation between electron and ion acceleration in a hydrogen plasma will also be discussed.
* Haberberger, et al. 2012 Colissionless shocks in a laser produced plasma generate monoenergetic high energy proton beams. Nat. Phys. 8, 95–99
UCLA, Los Angeles, California, USA
CO2 laser-driven ion acceleration has been used to generate monoenergetic, MeV ion beams from a gas jet plasma*. Studies have been limited to single shot experiments even though optimization of the laser-plasma interaction may only be possible at a high repetition rate. Use of a gas jet is advantageous because it offers debris free interactions at a density around the critical plasma density for 10 μm pulses(1019 cm-3) and can be retuned for different ion species. At the UCLA Neptune Laboratory we have upgraded our laser system to run at a high repetition rate. We have demonstrated the amplification of 20 GW, 3 ps, laser pulses at 1 Hz. Final amplification is achieved in a 1atm CO2 laser where the bandwidth for ps pulse amplification is provided via field broadening from the laser pulse itself. However, peak powers on the order of 0.2 TW are required for producing 3 MeV proton beams. An increase in our peak power by a factor of 10 is possible through nonlinear chirping and broadening of these pulses in a gas filled hollow waveguide followed by pulse compression. Here we present our strategy for obtaining 300 mJ, 300 fs CO2 laser pulses for driving a 1-10 Hz laboratory ion source.
*D.J. Haberberger, et. al., Nature Phys. 8, 95-99 (2012).
UCLA, Los Angeles, California, USA
We present a new curve-fitting method based on asymptotically fitting source size contributions to the measured electron spectrum that is capable of determining the maximum energy of a dispersed electron beam from a laser wakefield accelerator regardless of its transverse size. This method is applied to experimental spectra obtained in the characterization of a new injector stage to show that Direct Laser Acceleration may be an additional acceleration mechanism in laser wakefield accelerators where the laser pulse is long enough to overlap the trapped electrons.
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.