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).
Duke ECE, Durham, North Carolina, USA
UCLA, Los Angeles, California, USA
We study the long-term evolution of plasma wakefields over multiple plasma-electron periods and few plasma-ion periods, much less than a recombination time. The evolution and relaxation of such a wakefield-perturbed plasma over these timescales has important implications on the upper limits of repetition-rates in plasma colliders. Intense fields in relativistic lasers (or intense beams) create highly non-linear space-charge wakefields by transferring energy to the plasma electrons. Synchronized charged-particle beams may be accelerated with acceleration/focusing gradients of tens of GeV/m. However, wakefields leave behind a plasma, not in equilibrium, with a relaxation time of multiple plasma-electron periods. Ion motion, over ion timescales, caused by energy transfer from the driven plasma-electrons to the plasma-ions can create interesting plasma states. Eventually, during this long-term evolution the dynamics of plasma de-coheres, thermalizing into random motion (2nd law of thermodynamics), dissipating energy away from the wakefields. Wakefield-drivers interacting with such a relativistically hot-plasma lead to plasma wakefields that differ from the wakefields in a cold-plasma.
* J. Marques et al., Phys. Rev. Lett. 76 (1996) 10.1103/PhysRevLett.76.3566
** L. Gorbunov et al., Phys. Plasma 10 (2003) 10.1063/1.1559011** A. Maksimchuk et al., Phys. Plasma 15 (2008) 10.1063/1.2856373
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
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.
UCLA, Los Angeles, California, USA
Instituto Superior Tecnico, Lisbon, Portugal
TUB, Beijing, People's Republic of China
Simulation of laser wakefield accelerator (LWFA) in the Lorentz boosted frame, in which the laser and plasma spatial scales are comparable, can lead to computational time speed-ups to several orders of magnitude. In these simulation the relativistic drifting plasma inevitably induces a high frequency numerical instability. To reduce this numerical instability, we developed an EM-PIC code, UPIC-EMMA, based on the components of UCLA PIC framework (UPIC) which uses a spectral solver to advance the electromagnetic field in the Fourier space. With a low pass or "ring" filter implemented in the spectral solver, the numerical instability can be eliminated. In this paper we describe the new code, UPIC-EMMA, and present results from the code of LWFA simulation in the Lorentz boosted frame. These include the modeling cases where there are no self-trapped electrons, and modeling the self-trapped regime. Detailed comparison among Lorentz boosted frame results and lab frame results obtained from OSIRIS are given. We have used UPIC-EMMA to study LWFA in the self-guided regime to 100 GeV and good agreement was found with analytical scaling.
USC, Los Angeles, California, USA
UCLA, Los Angeles, California, USA
MPI, Muenchen, Germany
We demonstrate experimentally for the first time the self-modulation seeding of a relativistic electron bunch in a plasma. The long (~3.2 ps) bunch available at BNL ATF drives wakefields with periods one to one sixth of the bunch length in plasmas of a 1015 to 1016 cm-3 density range, which is observed as a periodic modulation of the bunch correlated energy spectrum after the 2 cm long plasma. OSIRIS* simulations show that electron bunches with a square temporal current profile seed effectively the wakefield and the development of the transverse modulation instability. Although the self-modulation instability (SMI) does not grow significantly over the 2cm-long plasma with a 50 pC bunch, simulations show that the SMI of the 1 nC bunch grows significantly and reaches saturation over the 2 cm propagation distance. We further examine the possibility of measuring the energy spectrum experimentally to confirm the development of SMI. Initial results show that due to dephasing between the bunch particles and the wakefields, the actual energy gain/loss by drive bunch particles is much lower than expected.
＊R. A. Fonseca et al., Lect. Notes Comp. Sci. vol. 2331/2002, (Springer Berlin/Heidelberg, (2002).
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.