Ginevra Cochran

Temporal resolution criterion for correctly simulating relativistic electron motion in a high-intensity laser field

Particle-in-cell codes are now standard tools for studying ultra-intense laser-plasma interactions. Motivated by direct laser acceleration of electrons in sub-critical plasmas, we examine temporal resolution requirements that must be satisfied to accurately calculate electron dynamics in strong laser fields. Using the motion of a single electron in a perfect plane electromagnetic wave as a test problem, we show surprising deterioration of the numerical accuracy with increasing wave amplitude a0 for a given time-step. We go on to show analytically that the time-step must be significantly less than λ/ca0 to achieve good accuracy. We thus propose adaptive electron sub-cycling as an efficient remedy.

Moderate repetition rate ultra-intense laser targets and optics using variable thickness liquid crystal films

Liquid crystal films are variable thickness, planar targets for ultra-intense laser matter experiments such as ion acceleration. Their target qualities also make them ideal for high-power laser optics such as plasma mirrors and waveplates. By controlling parameters of film formation, thickness can be varied on-demand from 10 nm to above 50 μm, enabling real-time optimization of laser interactions. Presented here are results using a device that draws films from a bulk liquid crystal source volume with any thickness in the aforementioned range. Films form within 2 μm of the same location each time, well within the Rayleigh range of even tight F/# systems, thus removing the necessity for realignment between shots. The repetition rate of the device exceeds 0.1 Hz for sub-100 nm films, facilitating higher repetition rate operation of modern laser facilities.

On-shot characterization of single plasma mirror temporal contrast improvement

We report on the setup and commissioning of a compact recollimating single plasma mirror for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without plasma mirror is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion (SRSI-ETE) at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the plasma mirror trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of ~ 1

Liquid Crystal Targets and Plasma Mirrors For Laser Based Ion Acceleration

\mu

Criterion for correctly simulating relativistic electron motion in a high-intensity laser field

m down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.

Laser-driven ion acceleration via target normal sheath acceleration in the relativistic transparency regime

Practical application of laser based ion acceleration will require advances across a wide range of technologies extending from the laser system itself to the delivery of the ion beam. We have recently shown that the liquid crystal 8CB provides an effective and relatively inexpensive new approach to target and plasma mirror fabrication and insertion for ion acceleration. 8CB is primarily hydrogen and carbon and forms in layers approximately 3 nm thick in its smectic phase. Taking advantage of these properties, we have developed a device we call the Linear Slide Target Inserter (LSTI) that can form films in situ from under 10 nm in thickness to over 50 μm. We describe this new technology and its operation as a target inserter and as a high-power plasma mirror. For proton acceleration, the LSTI readily achieves energies of 25 MeV using pulses of only a few joules by tuning the target thickness for the specific laser pulse characteristics and pre-pulse contrast. For plasma mirrors, we have demonstrated a weak field reflectivity below 0.2% and a high field reflectivity above 75%, yielding a potential pulse contrast improvement over two orders of magnitude. The LSTI can form films at a rate of several per minute for the thinnest films and we have developed a prototype based on a rotary geometry that has demonstrated a film formation rate up to 3 Hz for ultrathin films (approximatly 10 nm). We also propose the use of high repetition rate liquid crystal based plasma mirrors for debris mitigation. Taken together, these ideas and results suggest that liquid crystal technology could play a key role in the development of robust, high repetition rate, laser based ion sources.

Ion acceleration via TNSA near and beyond the relativistic transparency limit

We formulate a time-step criterion for particle-in-cell simulations of electron acceleration in an under-dense plasma by a high intensity laser. The numerical accuracy deteriorates with the increase of laser intensity due to errors in dephasing near stopping points along the electron trajectory. Adaptive electron sub-cycling is shown to be an efficient remedy at high laser intensities.