Compressed ion beams are being studied as a driver for inertial confinement fusion energy and for the creation of matter in the high-energy-density regime. In order to facilitate compression of a positive ion charge bunch longitudinally and transversely beyond the limit determined by the space-charge field of the bunch, a source of charge-neutralizing electrons must be provided. Plasma sources have been developed for the NDCX-I and NDCX-II experimental facilities, both for the 2-m-long, field-free drift regions, and for the small-diameter interior of the multi-Tesla final focus solenoid. Barium titanate based cylinders with a high dielectric coefficient are used to line the wall of the 2-m-long drift region and by applying a 9 kV pulse between the inner and outer surfaces of the cylinders, plasma with a density in the 1010 cm−3 range is formed. Results are presented from experiments using this plasma source on NDCX-I. A compact plasma source 5.1 cm long and 3.8 cm in diameter, also made using the barium titanate based material, has been developed for use in the bore of the final focus solenoid. Plasma generated near the wall of the plasma source will follow the fringing magnetic field lines of the solenoid and help to fill the bore of the magnet with plasma. Improved designs for the barium titanate plasma sources are being considered that use different inner-surface electrode materials and structures, and also use a modified electrical driver employing a spark gap crowbar switch. In addition, plasma source designs using so-called flashboard technology have been developed. In the flashboard plasma source, high density plasma is formed when the applied high voltage pulse causes a series of breakdowns between isolated copper patches aligned in rows along the surface of the 0.2 mm thick flashboard.

Recent heavy ion fusion target studies show that it is possible to achieve ignition with direct drive and energy gain larger than 100 at 1 MJ. To realize these advanced, high-gain schemes based on direct drive, it is necessary to develop a reliable beam smoothing technique to mitigate instabilities and facilitate uniform deposition on the target. The dynamics of the beam centroid can be explored as a possible beam smoothing technique to achieve a uniform illumination over a suitably chosen region of the target. The basic idea of this technique is to induce an oscillatory motion of the centroid for each transverse slice of the beam in such a way that the centroids of different slices strike different locations on the target. The centroid dynamics is controlled by a set of biased electrical plates called “wobblers.” Using a model based on moments of the Vlasov-Maxwell equations, we show that the wobbler deflection force acts only on the centroid motion, and that the envelope dynamics are independent of the wobbler fields. If the conducting wall is far away from the beam, then the envelope dynamics and centroid dynamics are completely decoupled. This is a preferred situation for the beam wobbling technique, because the wobbler system can be designed to generate the desired centroid motion on the target without considering its effects on the envelope and emittance. A conceptual design of the wobbler system for a heavy ion fusion driver is briefly summarized.

We have identified a general final compression section for HIF drivers, the section between accelerator and the target. The beams are given a head to tail velocity tilt at the beginning of the section for longitudinal compression, while going through bends that direct it to the target at specific angle. The aim is to get the beams compressed while maintaining a small centroid off-set after the bends. We used a specific example, 1 MJ driver with 500 MeV Rubidium + 1 ion beams. We studied the effect of minimizing dispersion using different bend strategies, and came up with a beamline point design with adiabatic bends. We also identified some factors that lead to emittance growth as well as the minimum pulse length and spot size on the target.

Kα radiation from low charge chlorine is examined for cold dense plasma diagnostics. A relativistic atomic structure calculation shows that the transition energies of Kα lines are slightly shifted to a higher-energy side according to the degree of M-shell ionization. Total spectral shift from Cl+ Kα lines up to those with a fully stripped M-shell is about 10 eV. With an assumption that collisional-radiative-equilibrium is valid, spectral calculations were carried out for a C2H3Cl-plasma heated by an ion beam, and clear deformation among spectral line-shapes is found in the range of the electron temperatures of ≤~ 30 eV. Contribution to the composite spectra of Kα lines with an excited electron in the outer-shell is also briefly discussed. Cl-Kα lines with M-shell electrons can be expected to give us distinct understandings for energy deposition by an ion beam in cold dense plasma.

We have focused our attention on the stopping mechanisms involved in the recently proposed ion beam-target US program. This mechanism emphasizes out production of warm dense matter through pulsed ion beams, linearly accelerated, and interacting with thin foils in Bragg peak conditions. We reviewed the relevant energy loss mechanisms involved at moderate and low velocity ion projectile. Small velocities close to zero are given some attention.

The interaction process between fast heavy ions and dense plasma was experimentally investigated. We injected 4.3-MeV/u or 6.0-MeV/u iron ions into a z-pinch-discharge helium plasma and measured the energy loss of the ions by the time of flight method. The energy loss of 4.3-MeV/u ions fairly agreed with theoretical prediction when the electron density of the target was on the order of 1018 cm−3. With increasing electron density beyond 1019 cm−3, the difference between the experiment and the theory became remarkable; the experimental energy loss was 15% larger than the theoretical value at the peak density. For 6.0-MeV/u ions, the deviation from the theory appeared even at densities below 1019 cm−3. These discrepancies indicated that density effects such as ladderlike ionization caused the enhancement of the projectile mean charge in the target.

Ion–electron two-stream instabilities in high intensity heavy ion fusion beams, described self-consistently by the nonlinear Vlasov–Maxwell equations, are studied using a three-dimensional multispecies perturbative particle simulation method. Large-scale parallel particle simulations are carried out using the recently developed Beam Equilibrium, Stability, and Transport (BEST) code. For a parameter regime characteristic of heavy ion fusion drivers, simulation results show that the most unstable mode of the ion–electron two-stream instability has a dipole-mode structure, and the linear growth rate decreases with increasing axial momentum spread of the beam particles due to Landau damping by the axial momentum spread of the beam ions in the longitudinal direction.

We consider beams that are described by a four-dimensional (4D) transverse distribution f (x, y, x′, y′), where x′ ≡ px /pz and z is the axial coordinate. A two-slit scanner is commonly employed to measure, over a sequence of shots, a two-dimensional (2D) projection of such a beam's phase space, for example, f (x, x′). Another scanner might yield f (y, y′) or, using crossed slits, f (x, y). A small set of such 2D scans does not uniquely specify f (x, y, x′, y′). We have developed “tomographic” techniques to synthesize a “reasonable” set of particles in a 4D phase space having 2D densities consistent with the experimental data. We briefly summarize one method and describe progress in validating it, using simulations of the High Current Experiment at Lawrence Berkeley National Laboratory.

The final beam transport in the reactor chamber for heavy ion fusion in preformed plasma channels offers many attractive advantages compared to other transport modes. In the past few years, experiments at the Gesellschaft für Schwerionenforschung (GSI) accelerator facility have addressed the creation and investigation of discharge plasmas, designed for the transport of intense ion beams. Stable, self-standing channels of 50 cm length with currents up to 55 kA were initiated in low-pressure ammonia gas by a CO2-laser pulse along the channel axis before the discharge is triggered. The channels were characterized by several plasma diagnostics including interferometry and spectroscopy. We also present first experiments on laser-guided intersecting discharges.

The Heavy Ion Fusion Virtual National Laboratory High Current Experiment (HCX) is exploring transport issues such as dynamic aperture, effects of quadrupole rotation, and the effects on the beam of nonideal distribution function, mismatch, and electrons, using one driver-scale 0.2 μC/m, 2–10 μs coasting K+ beam. Two- and three-dimensional simulations are being done, using the particle-in-cell code WARP to study these phenomena. We present results which predict that the dynamic aperture in the electrostatic focusing transport section will be set by particle loss.

The numerical simulation of the driving beams in a heavy ion fusion power plant is a challenging task, and simulation of the power plant as a whole, or even of the driver, is not yet possible. Despite the rapid progress in computer power, past and anticipated, one must consider the use of the most advanced numerical techniques, if we are to reach our goal expeditiously. One of the difficulties of these simulations resides in the disparity of scales, in time and in space, which must be resolved. When these disparities are in distinctive zones of the simulation region, a method which has proven to be effective in other areas (e.g., fluid dynamics simulations) is the mesh refinement technique. We discuss the challenges posed by the implementation of this technique into plasma simulations (due to the presence of particles and electromagnetic waves). We present the prospects for and projected benefits of its application to heavy ion fusion, in particular to the simulation of the ion source and the final beam propagation in the chamber. A collaboration project is under way at Lawrence Berkeley National Laboratory between the Applied Numerical Algorithms Group (ANAG) and the Heavy Ion Fusion group to couple the adaptive mesh refinement library CHOMBO developed by the ANAG group to the particle-in-cell accelerator code WARP developed by the Heavy Ion Fusion–Virtual National Laboratory. We describe our progress and present our initial findings.

The longitudinal dynamics of drift compression and pulse shaping for a space-charge-dominated heavy ion fusion beam is studied. A nonperiodic quadrupole lattice is designed for a beam undergoing drift compression, and an adiabatically matched solution is found for the transverse envelope equations in the nonperiodic lattice.

The U.S. Heavy Ion Fusion Virtual National Laboratory is continuing research into ion sources and injectors that simultaneously provide high current (0.5–1.0 A) and high brightness (normalized emittance better than 1.0 π-mm-mr). The central issue of focus is whether to continue pursuing the traditional approach of large surface ionization sources or to adopt a multiaperture approach that transports many smaller “beamlets” separately at low energies before allowing them to merge. For the large surface source concept, the recent commissioning of the 2-MeV injector for the High Current eXperiment has increased our understanding of the beam quality limitations for these sources. We have also improved our techniques for fabricating large diameter aluminosilicate sources to improve lifetime and emission uniformity. For the multiaperture approach, we are continuing to study the feasibility of small surface sources and a RF-induced plasma source in preparation for beamlet merging experiments, while continuing to run computer simulations for better understanding of this alternate concept. Experiments into both architectures will be performed on a newly commissioned ion source test stand at Lawrence Livermore National Laboratory called STS-500. This stand test provides a platform for testing a variety of ion sources and accelerating structures with 500-kV, 17-μs pulses. Recent progress in these areas is discussed as well as plans for future experiments.

A longitudinal compression is indispensable in a final stage of the heavy ion driver system using a particle accelerator. For the beam compression, an induction buncher applies the bunching voltage so as to make a considerable velocity tilt between the head and the tail of the beam bunch. However, the velocity tilt induces mismatch oscillation of the beam envelope due to the nonconformity between the beam particle velocity and the confinement force of the lens system. At the first step of design works for the beam transport line, the initial phase advance is decided to avoid resonance lines on the tune diagram. However, as the phase advance changes in the buncher by the velocity tilt, the beam bunch reenters in the resonance condition under certain circumstances. The transport region for avoiding the resonance condition is discussed on a phase diagram, as a function of the phase advance and the velocity tilt.

In heavy ion fusion, the compression of the DT pellet requires high intensity beams of ions in the gigaelectron volt energy range. Charge-changing collisions due to intrabeam scattering can have a high impact on the design of adequate accelerator and storage rings. Not only do intensity losses have to be taken into account, but also the deposition of energy on the beam lines after bending magnets, for example, may be nonnegligible. The center-of-mass energy for these intrabeam collisions is typically in the kiloelectron volt range for beam energies in the order of several gigaelectron volts. In this article, we present experimental cross sections for charge transfer and ionization in homonuclear collisions of Ar4+, Kr4+, and Xe4+, and for charge transfer only in homonuclear collisions of Pb4+ and Bi4+. Using a hypothetical 100-Tm synchrotron as an example, expected particle losses are calculated based on the experimental data. The results are compared with expectations for singly charged Bi+ ions, which are usually considered for heavy ion fusion.

A new efficient injection method from a laser ion source to a Radio Frequency Quadropole (RFQ) was proposed and is being tested in RIKEN, Japan. A laser plasma is induced just before the entrance of the RFQ and is injected directly into the RFQ channel. Using an existing RFQ, first verification tests have been completed successfully. Finally, the preliminary specifications for the first RFQ dedicated to the new injection scheme are presented.

This article presents analytical and simulation studies of intense heavy ion beam propagation, including the injection, acceleration, transport and compression phases, and beam transport and focusing in background plasma in the target chamber. Analytical theory and simulations that support the High Current Experiment (HCX), the Neutralized Transport Experiment (NTX), and the advanced injector development program, are being used to provide a basic understanding of the nonlinear beam dynamics and collective processes, and to develop design concepts for the next-step Integrated Beam Experiment (IBX), an Integrated Research Experiment (IRE), and a heavy ion fusion driver. Three-dimensional nonlinear perturbative simulations have been applied to collective instabilities driven by beam temperature anisotropy, and to two-stream interactions between the beam ions and any unwanted background electrons; three-dimensional particle-in-cell simulations of the 2-MV electrostatic quadrupole (ESQ) injector have clarified the influence of pulse rise time; analytical studies and simulations of the drift compression process have been carried out; syntheses of a four-dimensional particle distribution function from phase-space projections have been developed; and studies of the generation and trapping of stray electrons in the beam self-fields have been performed. Particle-in-cell simulations, involving preformed plasma, are being used to study the influence of charge and current neutralization on the focusing of the ion beam in NTX and in a fusion chamber.

A model for the stopping power for strong beam–plasma coupling is presented. The model calculates the stopping power through a T-matrix approach which correctly describes close binary collisions. A velocity-dependent screening length is used in the beam particle–plasma electron interaction to include dynamic screening effects. The model has the correct limiting behavior for low and high beam particle velocities and agrees well with simulation data up to very high coupling strengths. Compared to other theories, which include strong correlations and/or dynamic screening only approximately, a smaller energy loss has been found; this leads to a considerably longer stopping range.

The High Current Experiment (HCX) is being assembled at Lawrence Berkeley National Laboratory as part of the U.S. program to explore heavy ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge dominated heavy ion beams at high space-charge intensity (line-charge density ∼ 0.2 μC/m) over long pulse durations (>4 μs). This machine will test transport issues at a driver-relevant scale resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, lost-particle induced electron effects, and longitudinal bunch control. We present the first experimental results carried out with the coasting K+ ion beam transported through the first 10 electrostatic transport quadrupoles and associated diagnostics. Later phases of the experiment will include more electrostatic lattice periods to allow more sensitive tests of emittance growth, and also magnetic quadrupoles to explore similar issues in magnetic channels with a full driver scale beam.

For the intense beams in heavy ion fusion accelerators, details of the beam distribution as it emerges from the source region can determine the beam behavior well downstream. This occurs because collective space-charge modes excited as the beam is born remain undamped for many focusing periods. Traditional studies of the source region in particle beam systems have emphasized the behavior of averaged beam characteristics, such as total current, rms beam size, or emittance, rather than the details of the full beam distribution function that are necessary to predict the excitation of the collective modes. Simulations of the beam in the source region and comparisons to experimental measurements at Lawrence Berkeley National Laboratory and the University of Maryland are presented to illustrate some of the complexity in beam characteristics that has been uncovered as increased attention has been devoted to developing a detailed understanding of the source region. Also discussed are methods of using the simulations to infer characteristics of the beam distribution that can be difficult to measure directly.