T. E. Cowan

Laser light and hot electron micro focusing using a conical target

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.

Laser accelerated ions and electron transport in ultra-intense laser matter interaction

Since their discovery, laser accelerated ion beams have been the subject of great interest. The ion beam peak power and beam emittance is unmatched by any conventionally accelerated ion beam. Due to the unique quality, a wealth of applications has been proposed, and the first experiments confirmed their prospects. Laser ion acceleration is strongly linked to the generation and transport of hot electrons by the interaction of ultra-intense laser light with matter. Comparing ion acceleration experiments at laser systems with different beam parameters and using targets of varying thickness, material and temperature, some insight on the underlying physics can be obtained. The paper will present experimental results obtained at different laser systems, first beam quality measurement on laser accelerated heavy ions, and ion beam source size measurements at different laser parameters. Using structured targets, we compare information obtained from micro patterned ion beams about the accelerating electron sheath, and the influence of magnetic fields on the electron transport inside conducting targets.

Increased efficiency of short-pulse laser-generated proton beams from novel flat-top cone targets

Ion-driven fast ignition (IFI) may have significant advantages over electron-driven FI due to the potentially large reduction in the amount of energy required for the ignition beam and the laser driver. Recent experiments at the Los Alamos National Laboratory’s Trident facility employing novel Au flat-top cone targets have produced a fourfold increase in laser-energy to ion-energy efficiency, a 13-fold increase in the number of ions above 10MeV, and a few times increase in the maximum ion energy compared to Au flat-foil targets. Compared to recently published scaling laws, these gains are even greater. If the efficiency scales with intensity in accordance to flat-foil scaling, then, with little modification, these targets can be used to generate the pulse of ions needed to ignite thermonuclear fusion in the fast ignitor scheme. A proton energy of at least 30MeV was measured from the flat-top cone targets, and particle-in-cell (PIC) simulations show that the maximum cutoff energy may be as high as 40–45MeV...

Study of saturation of CR39 nuclear track detectors at high ion fluence and of associated artifact patterns

The occurrence of saturation in CR39 solid state nuclear track detectors has been systematically studied as a function of the incident ion (alpha particles and laser-accelerated protons) fluence and the etching time. When overexposed (i.e., for fluences above approximately 10(8) particles/cm(2)) and/or overetched, the CR39 detectors enter a saturated regime where direct track counting is not possible anymore. In this regime, optical measurements of saturated CR39 detectors become unreliable as well, since the optical response of the saturated detectors with respect to the ion fluence is highly nonlinear. This nonlinear optical response is likely due to scattering from the surface of irregular clumping patterns which have a diameter approximately 20 microm, i.e., ten times larger than the diameter of individual tracks. These patterns, which aggregate many individual tracks, are observed to develop in highly saturated regimes. For fluences typical of high energy short pulse laser experiments, saturation occurs, inducing the appearance of artifact ringlike structures. By careful microscopic analysis, these artifact ring patterns can be distinguished from the genuine rings occurring below saturation and characteristic of low energy laser accelerated proton beams.

Comparative spectra and efficiencies of ions laser-accelerated forward from the front and rear surfaces of thin solid foils

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850fs), and for high laser intensities [(1−6)×1019W∕cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018Wcm−2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100fs) or longer (1–10ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous ...

Proton probing measurement of electric and magnetic fields generated by ns and ps laser-matter interactions

The use of laser-accelerated protons as a particle probe for the detection of electric fields in plasmas has led in recent years to a wealth of novel information regarding the ultrafast plasma dynamics following high intensity laser-matter interactions. The high spatial quality and short duration of these beams have been essential to this purpose. We will discuss some of the most recent results obtained with this diagnostic at the Rutherford Appleton Laboratory (UK) and at LULI - Ecole Polytechnique (France), also applied to conditions of interest to conventional Inertial Confinement Fusion. In particular, the technique has been used to measure electric fields responsible for proton acceleration from solid targets irradiated with ps pulses, magnetic fields formed by ns pulse irradiation of solid targets, and electric fields associated with the ponderomotive channelling of ps laser pulses in under-dense plasmas.

Isochoric heating in heterogeneous solid targets with ultrashort laser pulses

We study ultrafast heating of thin plastic foils by intense laser irradiation theoretically using collisional two-dimensional particle-in-cell simulations. We find that the laser-generated hot electrons are confined laterally by self-generated resistive magnetic fields, heating the laser focal area beyond keV electron temperatures isochorically in a few picoseconds. Using this confinement one can excite shock waves that compress the plasma beyond solid density and achieve keV thermal plasmas before the plasma disassembles. Such shocks can be launched at material interfaces inside the target where jumps in the average ionization state and thus electron density lead to gigabar pressure. They propagate stably over picoseconds accompanied by multi-megagauss magnetic fields, and thus have a potential for various applications in high energy density physics.

Ti Kα radiography of Cu-doped plastic microshell implosions via spherically bent crystal imaging

We show that short pulse laser generated Ti Kα radiation can be used effectively as a backlighter for radiographic imaging. This method of x-ray radiography features high temporal and spatial resolution, high signal to noise ratio, and monochromatic imaging. We present here the Ti Kα backlit images of six-beam driven spherical implosions of thin-walled 500-μm Cu-doped deuterated plastic (CD) shells and of similar implosions with an included hollow gold cone. These radiographic results were used to define conditions for the diagnosis of fast ignition relevant electron transport within imploded Cu-doped coned CD shells.

Investigation of Magnetic Fields in 1-MA Wire Arrays and

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of Al low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of Al wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma

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The acceleration of ions by ultra-intense lasers has attracted great attention due to the unique properties and the unmatched intensities of the ion beams. In the early days the prospects for applications were already studied, and first experiments have identified some of the areas where laser accelerated ions can contribute to the ongoing inertial confinement fusion (ICF) research. In addition to the idea of laser driven proton fast ignition (PFI) and its use as a novel diagnostic tool for radiography the strong dependence on the electron transport in the target was found to be helpful in investigating the energy transport by electrons in fast ignitor scenarios. More recently an additional idea has been presented to use laser accelerated ion beams as the next generation ion sources, and taking advantage of the luminosity of the beams, to develop a test bed for heavy ion beam driven inertial confinement fusion physics. We review our recent experiments and simulations relevant to ICF research presenting a possible scenario for PFI as well as the prospects for next generation ion sources.