This Letter uncovers five distinct charge transport modes and their transitions in dual-energy electron beam diodes. We via first-principle particle-in-cell (PIC) simulations establish that the specific mode (e.g., space charge oscillations) and the current transport characteristics are essentially governed by the interplay between the electron beam energy and injected current density. A generalized analysis is conducted for n-component electron beams, and a theoretical piecewise function is for the transmitted current density proposed, which agrees well with the PIC results under designed conditions. The discovery provides a mechanistic picture of multiple electron beam transport in diodes, paving the way for novel designs of high-performance modern vacuum electronic devices.
A multi-shot target assembly and automatic alignment procedure for laser-plasma proton acceleration at high-repetition-rate are introduced. The assembly is based on a multi-target rotating wheel capable of hosting $>$5000 targets, mounted on a three-dimensional motorised stage to allow rapid replenishment and alignment of the target material between laser irradiations. The automatic alignment procedure consists of a detailed mapping of the impact positions at the target surface prior to the irradiation that ensures stable operation of the target, which alongside the purpose-built design of the target wheel, enable the operation at rates up to 10 Hz. Stable and continuous laser-driven proton acceleration is demonstrated, with observed cut-off energy stability about 15%.
We report the observation of longitudinal filamentation of an electron-positron pair plasma in a beam-driven QED cascade. The filaments are created in the "pair-reflection" regime, where the generated pairs are partially stopped and reflected in the strong laser field. The density filaments form near the center of the laser pulse and have diameters similar to the laser wavelength. They develop and saturate within a few laser cycles and do not induce sizable magnetostatic fields. We rule out the onset of two-stream instability or Weibel instability and attribute the origin of pair filamentation to laser ponderomotive forces. The small plasma filaments induce strong scattering of laser energy to large angles, serving as a signature of collective QED plasma dynamics.
In the quasi-gasdynamic high-current ion source described in this work, the plasma is sustained by high-power millimeter-wave radiation under the electron cyclotron resonance (ECR) condition. In such facilities, it is possible to achieve high volumetric energy input of up to $250$ $W/cm^3$ and obtain pure proton beams with a minimum amount of impurities and molecular ions. Experiments conducted on the GISMO facility demonstrated the possibility of a proton beam formation with a current of $50$ mA and an extremely high ($99.9$\%) content of atomic ions.
A beam of ultrarelativistic charged particles in a plasma can reach equilibrium with its own radial wakefield and then propagate with little change in shape. If some co-moving perturbation appears ahead of the beam, it may or may not destroy the beam with its wakefield, depending on the phase and amplitude of the wakefield. We numerically study which perturbations can destroy a single short bunch or a train of many short bunches at the parameters of interest for plasma wakefield acceleration in an axysimmetric configuration, and how fast. We find that there are particularly dangerous wakefield phases in which the beam can be destroyed by perturbations of very low amplitude. We also find that perturbations with an amplitude larger than the wakefield of a single bunch in the train are always destructive.
The emission of multi-MeV ($γ$-ray) photons from the interaction of a high-powered laser pulse with a dense plasma target is studied using particle-in-cell simulations. A new set of diagnostic techniques is presented and applied to analyze the intense field and ultra-relativistic electrons. Such methods elucidate the dominant processes responsible for efficient laser-to-$γ$ energy conversion, which include nonlinear Compton scattering and magneto-bremsstrahlung radiation, and provide a clear picture of the interaction on a microscopic level. We identify regions in the plasma target of high photon energy-density and obtain an energy conversion efficiency as high as 30%. The essential characteristics of the interaction are validated with full-3D simulations.
We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated \SI{1}{ms} apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of \SI{0.4}{kHz} for a period of 6.5 hours without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.