A.W. Molvik

Large acceptance angle retarding‐potential analyzers

Electrostatic retarding‐potential gridded analyzers have been used to measure the current and the axial energy distributions of ions escaping along magnetic field lines in the 2XIIB magnetic fusion experiment at Lawrence Livermore National Laboratory (LLNL). Two analyzers are discussed: a large scanning analyzer with a movable entrance aperture that can measure ion losses from a different segment of the plasma diameter on each shot, and a smaller analyzer that mounts in 5‐cm‐diam. ports. Careful electromagnetic shielding and grounding were necessary in order to reduce spurious signals. Accurate measurements of ion current require grids that attenuate the plasma flow and isolate it from the analyzer grid potentials, several techniques to suppress secondary and primary electrons, and consideration of the space‐charge limits as well as techniques to increase these limits. Accurate measurements of ion energy require, in addition, confining large‐angle ion orbits within the analyzer by an axial magnetic field,...

Simulating Electron Clouds in Heavy-Ion Accelerators

Contaminating clouds of electrons are a concern for most accelerators of positively charged particles, but there are some unique aspects of heavy-ion accelerators for fusion and high-energy density physics which make modeling such clouds especially challenging. In particular, self-consistent electron and ion simulation is required, including a particle advance scheme which can follow electrons in regions where electrons are strongly magnetized, weakly magnetized, and unmagnetized. The approach to such self-consistency is described, and in particular a scheme for interpolating between full-orbit (Boris) and drift-kinetic particle pushes that enables electron time steps long compared to the typical gyroperiod in the magnets. Tests and applications are presented: simulation of electron clouds produced by three different kinds of sources indicates the sensitivity of the cloud shape to the nature of the source; first-of-a-kind self-consistent simulation of electron-cloud experiments on the high-current experim...

Experimental demonstration of compact torus compression and acceleration

Tests of compact torus (CT) compression on the RACE device [Phys. Rev. Lett. 61, 2843 (1988)] have successfully demonstrated stable compression by a factor of 2 in radius, field amplification by factors of 2–3 to 20 kG, and compressed densities exceeding 1016 cm−3. The results are in good agreement with two‐dimensional magnetohydrodynamic simulations of the CT dynamics. The CT is formed between a pair of coaxial conical conductors that serve as both a flux conserver for stable, symmetric formation and as electrodes for the compression and acceleration phases. The CT is compressed by J×B forces (poloidal current, toroidal field) when a 120 kV, 260 kJ capacitor bank is discharged across the electrodes. The CT reaches two‐fold compression to a radius of 8 cm and a length of 20–30 cm near the time of peak current, 10 μsec (many Alfven times) after the accelerator fire time, and is subsequently accelerated in a 150 cm straight coaxial section to velocities in the range 1.5–6.5×107 cm/sec. A new set of accelera...

Magneto-hydrodynamically stable axisymmetric mirrorsa)

Making axisymmetric mirrors magnetohydrodynamically (MHD) stable opens up exciting opportunities for using mirror devices as neutron sources, fusion-fission hybrids, and pure-fusion reactors. This is also of interest from a general physics standpoint (as it seemingly contradicts well-established criteria of curvature-driven instabilities). The axial symmetry allows for much simpler and more reliable designs of mirror-based fusion facilities than the well-known quadrupole mirror configurations. In this tutorial, after a summary of classical results, several techniques for achieving MHD stabilization of the axisymmetric mirrors are considered, in particular: (1) employing the favorable field-line curvature in the end tanks; (2) using the line-tying effect; (3) controlling the radial potential distribution; (4) imposing a divertor configuration on the solenoidal magnetic field; and (5) affecting the plasma dynamics by the ponderomotive force. Some illuminative theoretical approaches for understanding axisymm...

Overview of US heavy ion fusion research

Significant experimental and theoretical progress has been made in the U.S. heavy ion fusion program on high-current sources, injectors, transport, final focusing, chambers and targets for high energy density physics (HEDP) and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of present research is the beam physics associated with quadrupole focusing of intense, space-charge dominated heavy-ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in present research is the introduction of background plasma to neutralize the space charge of intense heavy ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high energy density conditions as well as for inertial fusion energy.

Progress in heavy ion fusion research

The U.S. Heavy Ion Fusion program has recently commissioned several new experiments. In the High Current Experiment [P. A. Seidl et al., Laser Part. Beams 20, 435 (2003)], a single low-energy beam with driver-scale charge-per-unit-length and space-charge potential is being used to study the limits to transportable current posed by nonlinear fields and secondary atoms, ions, and electrons. The Neutralized Transport Experiment similarly employs a low-energy beam with driver-scale perveance to study final focus of high perveance beams and neutralization for transport in the target chamber. Other scaled experiments—the University of Maryland Electron Ring [P. G. O’Shea et al., accepted for publication in Laser Part. Beams] and the Paul Trap Simulator Experiment [R. C. Davidson, H. Qin, and G. Shvets, Phys. Plasmas 7, 1020 (2000)]—will provide fundamental physics results on processes with longer scale lengths. An experiment to test a new injector concept is also in the design stage. This paper will describe th...

High-Performance Beam-Plasma Neutron Sources for Fusion Materials Development

The design and performance of a relatively low-cost, plasma-based, 14-MeV deuterium-tritium neutron source for accelerated end-of-life testing of fusion reactor materials are described. An intense flux (up to 5 [times] 10[sup 18] n/m[sup 2][center dot]s) of 14-MeV neutrons is produced in a fully ionized high-density tritium target (n[sub e] [approx] 3 [times] 10[sup 21] m[sup [minus]3]) by injecting a current of 150-keV deuterium atoms. The tritium plasma target and the energetic D[sup +] density produced by D[sup 0] injection are confined in a [<=] 0.16-m-diam column by a linear magnet set, which provides magnetic fields up to 12 T. Energy deposited by transverse injection of neutral beams at the midpoint of the column is transported along the plasma column to the end regions. Three variations of the neutron source design are discussed, differing in the method of control of the energy transport. Emphasis is on the design in which the target plasma density is maintained in a region where electron thermal conduction along the column is the controlling energy-loss process.

The high current experiment: First results

The High Current Experiment (HCX) is being assembled at Lawrence Berkeley National Laboratory as part of the US 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 spacecharge intensity (line-charge density {approx} 0.2 {micro}C/m) over long pulse durations (>4 {micro}s). 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{sup +} 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.

Planning for an integrated research experiment

We describe the goals and research program leading to the Heavy Ion Integrated Research Experiment (IRE). We review the basic constraints which lead to a design and give examples of parameters and capabilities of an IRE. We also show design tradeoffs generated by the systems code IBEAM.

A 14 MeV fusion neutron source for material and blanket development and fission fuel production

Fusion development will require materials capable of withstanding extensive harsh bombardment by energetic neutrons and plasma. The plasma-based gas dynamic trap neutron source concept is capable of testing and qualifying materials and fusion blanket sub-modules for eventual deployment in fusion energy systems. In this paper we describe the suitability of this source to assess thermal fatigue in fusion blanket components caused by the small normal variability of neutron flux inherent in fusion energy concepts. A second part of the paper considers the requirements for a fusion–fission hybrid suitable for producing fissile fuel. Both solid and molten salt fuel from blanket designs are described which emphasize non-proliferation and passive safety.