Stabilized Radiation Pressure Acceleration and Neutron Generation in Ultrathin Deuterated Foils.

Premature relativistic transparency of ultrathin, laser-irradiated targets is recognized as an obstacle to achieving a stable radiation pressure acceleration in the "light sail" (LS) mode. Experimental data, corroborated by 2D PIC simulations, show that a few-nm thick overcoat surface layer of high Z material significantly improves ion bunching at high energies during the acceleration. This is diagnosed by simultaneous ion and neutron spectroscopy following irradiation of deuterated plastic targets. In particular, copious and directional neutron production (significantly larger than for other in-target schemes) arises, under optimal parameters, as a signature of plasma layer integrity during the acceleration.

Absolute calibration of Fujifilm BAS-TR image plate response to laser driven protons up to 40 MeV.

Image plates (IPs) are a popular detector in the field of laser driven ion acceleration, owing to their high dynamic range and reusability. An absolute calibration of these detectors to laser-driven protons in the routinely produced tens of MeV energy range is, therefore, essential. In this paper, the response of Fujifilm BAS-TR IPs to 1-40 MeV protons is calibrated by employing the detectors in high resolution Thomson parabola spectrometers in conjunction with a CR-39 nuclear track detector to determine absolute proton numbers. While CR-39 was placed in front of the image plate for lower energy protons, it was placed behind the image plate for energies above 10 MeV using suitable metal filters sandwiched between the image plate and CR-39 to select specific energies. The measured response agrees well with previously reported calibrations as well as standard models of IP response, providing, for the first time, an absolute calibration over a large range of proton energies of relevance to current experiments.

Characterizing the modulation transfer function for X-ray radiography in high energy density experiments.

The Modulation Transfer Function (MTF) is an established means for characterizing imaging performance of X-ray radiography systems. We report on experiments using high energy, laser-driven X-ray radiography systems that assess performance using MTF values measured with the knife-edge projection method. The broadband, hard X-ray systems under study use line-projection imaging produced by narrowing the laser-generated X-ray source with a slit. We find that good contrast resolution can be achieved (the MTF = 0.5 at 75 µm wavelength) and that this performance is reproduced on different laser facilities. We also find that the MTF is sensitive both to the thickness of the line-projection slit and to the backing material thickness under the knife-edge. Both these sensitivities are due to a common mechanism, namely induced changes in the spectrally-averaged spatial widths of the X-ray source. The same line-projection system is also used on experimental campaigns measuring Rayleigh-Taylor instability growth by dynamically imaging sinusoidal, high Z micro-targets with wavelengths of 100 µm or less. By applying the measured MTF values to correct the ripple target contrast measurements, we can predict ripple growth to approximately 10% accuracy.

Calibration of time of flight detectors using laser-driven neutron source.

Calibration of three scintillators (EJ232Q, BC422Q, and EJ410) in a time-of-flight arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub-MeV to 20 MeV. A typical set of data obtained simultaneously by the detectors is shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil.

Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers.

A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C(6+), O(8+), etc.). Commonly used Fuji Image plates were used as detectors in the spectrometer, whose absolute response to deuterium ions over a wide range of energies was calibrated by using slotted CR-39 nuclear track detectors. A typical deuterium ion spectrum diagnosed in a recent experimental campaign is presented, which was produced from a thin deuterated plastic foil target irradiated by a high power laser.

Effects of front-surface target structures on properties of relativistic laser-plasma electrons.

We report the results of a study of the role of prescribed geometrical structures on the front of a target in determining the energy and spatial distribution of relativistic laser-plasma electrons. Our three-dimensional particle-in-cell simulation studies apply to short-pulse, high-intensity laser pulses, and indicate that a judicious choice of target front-surface geometry provides the realistic possibility of greatly enhancing the yield of high-energy electrons while simultaneously confining the emission to narrow (<5°) angular cones.