S.J. Rose

Experimental studies of the advanced fast ignitor scheme

Guided compression offers an attractive route to explore some of the physics issues of hot electron heating and transport in the fast ignition route to inertial confinement fusion, whilst avoiding the difficulties associated with establishing the stability of the channel formation pulse. X-ray images are presented that show that the guided foil remains hydrodynamically stable during the acceleration phase, which is confirmed by two-dimensional simulations. An integrated conical compression/fast electron heating experiment is presented that confirms that this approach deserves detailed study.

XUV OPACITY MEASUREMENTS AND COMPARISON WITH MODELS

The opacities of aluminium, iron and holmium were measured spectrally resolved in the energy range of 70 to 280 eV. For this purpose the iodine laser ASTERIX IV at the MPQ (200J0.4 ns at 440 nm) was focused into a spherical gold Hohlraum with a dia of 3 mm. The generated radiation with a temperature of 60 eV heated thin tampered absorber foils. The transmission was measured spectrally and temporally resolved by means of a backlighter source with a delay of 0.8 ns to the main beam. At this delay time the density in the sample material is about 0.01 g/cm3 and the temperature is about 20 eV. We have compared the experimental data with the opacity codes IMP, JIMENA, OPAL, SAPHIR and STA.

Collisionless shock and supernova remnant simulations on VULCAN

The VULCAN [C. N. Danson et al., Opt. Commun. 103, 392 (1993)] laser at the UK Central Laser Facility is being used for laboratory-based simulations of collisionless shocks. By ensuring that key dimensionless parameters in the experiments have values similar to those of supernova remnants (SNRs), the hydrodynamics and magnetic field of the experiment are scaled to those of a SNR. This makes it possible to investigate experimentally the physics of collisionless magnetized shocks in such objects. The experiments are providing data against which to test current theory. Collisionless shock formation and the interaction of two counterpropagating colliding plasmas permeated by a strong magnetic field are discussed.

The Oxford electron‐beam ion trap: A device for spectroscopy of highly charged ions

An electron‐beam ion trap (EBIT) has just been completed in the Clarendon Laboratory, Oxford. The design is similar to the devices installed at the Lawrence Livermore National Laboratory. It is intended that the Oxford EBIT will be used for x‐ray and UV spectroscopy of hydrogenic and helium‐like ions, laser resonance spectroscopy of hydrogenic ions and measurements of dielectronic recombination cross sections, in order to test current understanding of simple highly charged ions.

The non-LTE excitation/ionization code GALAXY

In this paper we describe the computer code GALAXY which calculates the excitation and ionization of a plasma, including the effect of the radiation field, whether incident on the plasma or generated internally through the optical depth of the lines. In the absence of atomic data provided by the user, GALAXY calculates energy levels and the electron collisional and radiative rates which link them, using simple scaled-hydrogenic expressions. For relatively highly ionized systems this approach is found to be sufficiently accurate to be useful in studies of high-power laser-produced plasmas.

X‐ray spectroscopic studies of hot, dense iron plasma formed by subpicosecond high intensity KrF laser irradiation

The time‐integrated x‐ray emission from a hot, dense iron plasma has been recorded. The iron plasma was created when a target with a 1000‐A‐thick iron layer buried beneath 1000 A of plastic was irradiated by a 300 fs pulse of 249 nm laser light at an intensity of approximately 1017 W cm−2. Two models have been used to construct a synthetic x‐ray spectrum. The first employs detailed, spectroscopically accurate atomic data and the second uses a local thermodynamic equilibrium opacity model. The detailed model shows fairly good agreement with experiment whereas the opacity model only shows agreement in the gross features.

MEASUREMENT AND DETAILED ANALYSIS OF SINGLE PASS GAIN AT 81-A IN A RECOMBINING LASER-PRODUCED FLUORINE PLASMA

Abstract XUV amplication at 81 A was observed in a freely expanding adiabatically cooled plasma produced by laser irradiation of LiF coated carbon fibre targets. Measurements included time resolved on- and off-axis spectroscopy, target length variation and the variation of the absorbed laser energy. Detailed analysis of the experimental data has been compared with numerical simulations.

Gain scaling relationships for Ne-like Ge slab targets

Abstract The gain coefficient of the strongest 3p→3s, J = 2→1 lasing transition at 23.6 nm in the Ne-like Ge collisional excitation scheme has been measured, using the fundamental wavelength from a Nd: glass laser (1.06 μm), for a range of incident intensities on massive stripe targets up to 2.2 cm in length. From a threshold incident laser intensity of ∼ 6 x 10 12 W/cm 2 , the gain coefficient rises to ∼ 4.5 cm -1 for an irradiation intensity of ∼ 2.5 x 10 13 W/cm 2 , tending towards still higher gain coefficients at higher incident intensities. For targets of maximum length, a gain-length product gL ≈ 10 was reached with a resultant output power at 23.6 nm estimated to be at the ∼ kW level. The beam divergence decreased with length to a minimum of ∼ 7 mrad but no significant trend in beam pointing with plasma length was observed. From the trend in the gain coefficient, it appears that for a fixed energy laser irradiating a ∼ 100 μm wide slab targets, an incident incident of I i ∼ 1.2 x 10 13 W/cm 2 represents an optimum working level, assuming that plasma length is not limited by refractive effects. In addition to the usual valence electron excited 3p→3s transitions, the gain coefficient for the core excited 1s 2 2s2p 6 3d→1s 2 2s2p 6 3p transition at 19.9 nm has been measured to be ∼ 1.5 cm -1 for an incident irradiance of ∼2.5 x 10 13 W/cm 2 .

Simulation of X-ray line transfer in a cylindrically expanding plasma

Abstract A method for calculating the X-ray resonance line emission from a cylindrically expanding laser-produced plasma is presented. A multi-frequency line transfer algorithm, incorporated as a post-processor to a time-dependent 1-D hydrodynamics and non-LTE atomic physics calculation, is used to compute spatially-resolved spectral profiles of the line emission. Coupling of photon re-absorption to ionic state populations is achieved by use of the escape probability method. We use the code to investigate the effects of high expansion velocities, and the influence of neighbouring lines, on the opacity and detailed shape of optically thick lines. As an example we consider the profile of the Lyman-α (1s-2p) resonance line of hydrogenic aluminium.

A review of opacity workshops

Abstract Two Workshops, at which detailed comparisons were made of the radiative opacity calculated by different groups were held at the Rutherford Appleton Laboratory, U.K. in 1989 and subsequently at CECAM (Centre European de Calcul Atomique et Moleculaire), France in 1991. Each meeting was organized by Dr Frank Serduke of Lawrence Livermore National Laboratory. The material temperature and density of a number of test cases (both single elements and mixtures) were specified prior to the meetings and results from different groups were plotted together for comparison. At the meetings, the differences and similarities were examined in detail. This paper reports, from the viewpoint of its participants, on what was learned from this detailed comparison.