Single-module analysis and experimental scale model testing indicate that a new design of the pulse-forming and transformer sections of the Particle Beam Fusion Accelerator II (PBFA II) will improve the energy transport efficiency of this machine by greater than 50%. However, these results have not considered multimodule, cross-coupling effects. This paper presents an approximate analysis of these cross-coupling effects, predicts problems that will occur, and indicates how to overcome them. Although this analysis is directed specifically at PBFA II, the techniques presented apply to the design of pulsed power machines with open-transmission lines whenever abrupt junctions occur.

Inertial confinement fusion (ICF) has made great progress. In fact several significant scientific firsts have been achieved in the last year. These developments have presented the ICF community with an opportunity to embark on a new phase in ICF research. The key issues of laser fusion are to attain a high absorption of laser light in a plasma, to prevent preheating of fuel during the compression, and to achieve highly efficient implosion by uniform compression of fuel due to the homogeneous deposition of laser energy on the pellet surface. Direct drive and indirect drive have been investigated. The progress in both schemes is remarkable. The neutron yield by the stagnation free compression of the LHART target has attained 1013 which corresponds to a pellet gain of 1/500. The plastic shell target has reached a fuel density as large as 600 times the liquid density which is measured by the Si activation method as well as the D knockon method. A cryogenic foam target is now under investigation.

Photoionization of single-charged heavy ions has been proposed recently by Rubbia (1989) as a non-Liouvillean injection scheme from the linac into the storage rings of a driver accelerator for inertial confinement fusion. The main idea of this scheme is the accumulation of high currents of heavy ions without the usually inevitable increase of phase space. Here we suggest the use of the photoionization idea in an alternative scheme: if it is applied at the final stage of pulse compression (replacing the conventional bunch compression by an rf voltage, which always increases the momentum spread), there is a significant advantage in the performance of the accelerator. We show, in particular, that this new compression scheme can potentially relax the tough stability limitations, which were identified in the heavy-ion fusion reactor study HIBALL (Badger et al. 1984). Moreover, it is promising for achieving higher beam power, which is suitable for indirectly driven fusion targets (1016 W/g, in contrast with 1014 W/g for the directly driven targets in HIBALL).

The idea of non-Liouvillean bunch compression is to stack a large number of bunches (typically 50–100) in the same phase-space volume during a change of charge state of the ion. A particular feature of this scheme with regard to beam dynamics is its transient nature, since the time required is one revolution per bunch. After the stacking the intense bunch is ejected and directly guided to the target. The present study is a first step in exploring the possibly limiting effect of space charge under the parameter conditions of a full-size driver accelerator. Preliminary results indicate that there is a limit to the effective stacking number (non-Liouvillean “compression factor”), which is, however, not prohibitive. Requirements on the power of the photon beam from a free-electron laser are also discussed. It is seen that resonant cross sections of the order of 10−15 cm2 lead to photon beam powers of a few megawatts.

Coherent X rays can be produced by Compton scattering of laser light on a relativistic electron beam, whose current is periodically modulated by the same laser radiation in a suitable miniundulator. If electron and laser photon energy are properly matched, the modulated electron beam acts as a moving diffraction grating and the X rays show a high degree of coherence. A practical implementation of this scheme is shown.

The development of hydrodynamic instability in laser targets is studied by means of the 2D numerical code “ATLANT.” At the linear stage, perturbations grow as At the nonlinear stage, the growth rate of Rayleigh-Taylor instability is reduced and new harmonics are generated. The effect of the nonuniformity of laser irradiation has been investigated for long- and shortwave perturbations. The growth rate of short-wave perturbations may be effectively decreased by means of symmetrical prepulses.

Arbitrarily heavy cluster ion beams (CIB) are examined for the purpose of driving directly or indirectly an ICF compression. This paper provides an overview of cluster production, and of their interaction with cold and hot matter. Fragmentation is also considered. Concepts and ideas of interest for further investigations are identified and delineated. A connection is made with a CIB interaction experiment planned at Orsay. It is conjectured that CIB should experience enhanced stopping when compared to their atomic counterparts under similar conditions.

University research in the UK with high power lasers is carried out at the SERC's Central Laser Facility with a multi-terawatt neodymium glass laser, VULCAN, and a developmental KrF laser, SPRITE. These systems are briefly described together with the design of a new KrF laser to supersede VULCAN. The new laser design, SUPERSPRITE, is based on optical and Raman multiplexing which is being developed with the present SPRITE system. The specification of SUPERSPRITE is for 3.5 kJ in 1 ns and a peak power of 300 TW in short pulses. The new technology is seen as highly cost effective in relation to neodymium glass lasers. A resume of the development of XUV lasers in the UK in collaboration with laboratories overseas is given. The work is based on laser action through recombination in highly ionized ions and recent progress includes collaborative experiments on the GEKKO XII facility in Japan which have demonstrated laser action at the shortest wavelength to date at 45 A in Mg XII. The physics of energy transport in short pulses is fundamental to the extrapolation of recombination lasers to shorter wavelengths and is being studied from a more basic standpoint using both the VULCAN and SPRITE facilities. Some details of this work are given.

The hydrodynamic efficiency of conversion of the energy of fast charged particles into the kinetic energy of the bulk motion of plane-parallel shells is investigated in the framework of two simple models–one based on the stepwise density profile and the other employing a self-similar solution. The analytical estimates obtained are substantiated with ID hydrodynamic calculations. In case of spherical shells, the three key dimensionless parameters determining the values of the hydrodynamic efficiency are pointed out; the dependence of the hydrodynamic efficiency on these parameters has been explored numerically. The effects of a nonuniform energy deposition (increasing by the end of the fast particle ranges) and of a nonuniform absorber composition are also discussed.

A critical test of direct-drive laser-fusion has been conducted with the demonstration of DT compression to densities in the range of 100–200 times liquid density in experiments on the University of Rochester's OMEGA laser facility. The high-density cyrogenic experiments used 351-nm laser pulses with energies of 1500–1800 J and pulse widths in the range of 600–700 ps.

We have made the first observation of Kα X-ray satellites from a target heated by an intense ion beam. The satellites are produced when thermal ionization due to beam heating is accompanied by inner-shell ionization from beam ion impact. The Particle Beam Fusion Accelerator II was used to irradiate a conical aluminum target with a proton beam. The nominal beam parameters were 50–75 kJ in a 1-cm spot, 15–20-ns pulse length, and 4–5-MeV protons at peak power. An elliptical crystal X-ray spectrograph inside a 1000-kg tungsten shield was used to record the spectra. The peak ion stage reached by the aluminum target was +8. Collisional radiative calculations were performed, which indicate a peak electron temperature of 20–60 eV.

The main goals of the German program are the study of key issues of inertial fusion with intense beams of heavy ions. The completion of the new heavy-ion synchrotron and storage ring facility SIS/ESR at GSI opens new directions for experimental investigations on beam dynamics at high intensity and on beam/target interaction. In addition, new accelerator scenarios will be investigated based on non-Liouvillean beam-handling techniques.

The uniform side-on emission of second harmonic radiation has recently been measured from the corona of a laser irradiated plasma with large electron density gradients. A theoretical model has been developed to study the internal electric fields and double layers in such strongly inhomogeneous plasmas. The analytical model shows immediately a density-independent second harmonic term which reproduces very closely the measured intensity of the second harmonic. A detailed numerical evaluation of the term, for a plasma with a linear density profile (permitting Airy solutions), results in a net intensity with small spatial variation but also with both long and short wavelength periodicities exactly as measured. The observed vanishing of the periodicity, when broad band laser sources are used, can then immediately be explained from integration of the standing-wave like oscillations.

Coupling of the core with the surrounding corona of hot electrons produced around the plasma critical surface in a spherically symmetric laser-imploded Z-layered plasma target has been analyzed. Considering that the energy equipartition exists between the cold electrons of the core and the hot coronal electrons in the core–corona overlapping region, the analytic expression for core–corona coupling has been derived. The efficiency of heat transfer from the hot corona to the cold core depends on the laser wavelength, mean electron temperature in the ablation region, and the width of the Z-layer in the plasma pellet. Numerical results indicate that short wavelength lasers are favorable for efficient heating of the core by the surrounding hot corona. The core-corona coupling increases primarily with the mean electron temperature up to a certain extent and beyond that further laser flux transfer to the hot corona results in decoupling of the core from the corona. The presence of Z-layer is likely to reduce the electron mean free path in the ablation region and affects the laser wavelength scaling of the core-corona coupling. It is also found to have positive influence on the maximum coupling efficiency of the core with the hot corona.

The experimental setup developed at Orsay for studying ion beam-plasma interactions of relevance to inertial confinement fusion has been significantly improved. In this connection, the fast valves displayed on both sides of the plasma discharge play an essential role. The amount of residual gas is then reduced by a factor of 3. New and accurate results for the stopping and the effective charges of S7+ and Br6+ in the MeV/u energy range are presented.

The interaction of externally injected charged particles (electrons) with plasma waves moving with a phase velocity that is very close to the speed of light is examined. Such plasma waves form the basis of at least three collective accelerator schemes: the plasma beat wave accelerator (PBWA), the plasma wake-field accelerator (PWFA), and the laser wake-field accelerator (LWFA). First, the electron trapping threshold, energy gain and acceleration length are examined using a 1-D model. This model elucidates how the final energies of the injected test electrons depend upon their injection and extraction phases and phase slippage. Phase energy diagrams are shown to be extremely useful in visualizing wave-particle interactions in 1-D. Second, we examine, using a two-dimensional model, the effects of radial electric fields on focusing or defocusing the injected particles depending upon their radial positions and phases in the relativistically moving potential well. Finally, we extend the model to 3-D so that the effect of injected particles' emittance on the acceleration process may be determined. This simple 3-D model will be extremely useful in predicting the electron energy spectra of several current experiments designed to demonstrate ultrahigh gradient acceleration of externally injected test particles by relativistic plasma waves.

This paper reviews recent developments and achievements in the program of planar foil instability experiments being performed at the AWE HELEN laser. Point projection Xray backlighting, with spectroscopy, is used to measure hydrodynamic mix in radiatively accelerated ablator/foil packages; the mix is identified in the experimental radiograph from the overlap of distinguishable spectral absorption features associated with each of the constituent materials.

The first part of the paper describes the backlighting technique, and briefly summarizes progress made in the past two years, leading to the first results being obtained on a “high mix” Parylene-C ablator/molybdenum payload package. The second part considers the full analysis of one such ‘high mix’ shot (Shot 7772), describing how the spatial distribution of mix has been quantified and considering the various sources of error. Comparisons are made with both one-dimensional and two-dimensional hydrocode simulations. Finally, various improvements and extensions to the experiment and codes are indicated.

Kinematical relations of nonlinear interaction of laser beams with free charged particles are analyzed. General expressions are found for the number of scattered photons as a function of the number of simultaneously absorbed and/or emitted photons and the kinetic parameters of the charged particle-photon system. The results are applied to the processes of particle acceleration by lasers, frequency conversion, solitonic propagation, and others.

The progress of the experimental research program at GSI for studying beam-plasma interaction phenomena is reported. Heavy-ion beams from the new accelerator facility SIS/ESR at GSI-Darmstadt are now available for experiments, and will soon deliver ≥ 109 particles per pulse in 100 ns. Focused on a small sample of matter, the beams will be able to produce a high-density plasma and to permit investigation of interaction processes of heavy ions with hot ionized matter.

For the intense beam from the new heavy-ion synchrotron (SIS), a fine-focus system has been designed to produce a high specific deposition power beam for target experiments with a beam-spot radius of 100 μm. We further discuss improvements of this lens system by nonconventional focusing devices such as plasma lenses.

Intense-beam experiments at the RFQ Maxilac accelerator at GSI have already produced the first heavy-ion-induced plasma with a temperature of 0.75 eV. New diagnostic techniques for investigating ion-beam-induced plasmas are presented. The low-intensity beam from the GSI UNILAC has been used to measure energy deposition profiles of heavy ions in hot ionized matter. In this experiment an enhancement of the stopping power for heavy ions was observed. The current experimental research program tests basic plasma theory and addresses key issues of inertial confinement fusion driven by intense heavy-ion beams.