Dr Linus Feder

Abstract:

We demonstrate resonant excitation of a plasma wave by a train of short laser pulses guided in a preformed plasma channel, for parameters relevant to a plasma-modulated plasma accelerator (P-MoPA). We show experimentally that a train of N≈10 short pulses, of total energy ∼1J, can be guided through 110mm long plasma channels with on-axis densities in the range 1017-1018cm-3. The spectrum of the transmitted train is found to be strongly red shifted when the plasma period is tuned to the intratrain pulse spacing. Numerical simulations are found to be in excellent agreement with the measurements and indicate that the resonantly excited plasma waves have an amplitude in the range 3-10GVm-1, corresponding to an accelerator stage energy gain of order 1GeV.

Abstract:

We demonstrate that an ultrashort high intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates; the front of the pulse generates a waveguide that confines the rest of the pulse. A wide range of suitable initial index structures and gas densities will support this “self-waveguiding” process; the necessary feature is that the gas density on axis is a minimum. Here, we demonstrate self-waveguiding of pulses of at least 1.5 × 1017 W/cm2 (normalized vector potential a0 ∼ 0.3) over 10 cm, or ∼100 Rayleigh ranges, limited only by our laser energy and length of our gas jet. We predict and observe characteristic oscillations corresponding to mode-beating during self-waveguiding. The self-waveguiding pulse leaves in its wake a fully ionized low-density plasma waveguide which can guide another pulse injected immediately following; we demonstrate optical guiding of such a follow-on probe pulse. The method is well suited to laser wakefield acceleration and other applications requiring a long laser-matter interaction length.

Abstract:

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable laser-driven electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J 0 beam, generates the core of the waveguide, while the delayed second pulse, here a J 8 or J 16 beam, generates the waveguide cladding, enabling wide control of the guide’s density, depth, and mode confinement. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on-axis plasma densities as low as N e 0 ∼ 5 × 10 16     cm − 3 .

Abstract:

We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (𝜆=3.9 μm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to ∼12 MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (𝜆=800 nm) pulses.

Abstract:

We demonstrate laser-plasma acceleration of high charge electron beams to the ∼10  MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ∼0.5  nC is measured for energies >1  MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wave breaking, which can radiate more than ∼3% of the laser energy in a ∼1  fs bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.