Inertial Fusion Driver Research Articles

Transport of electron beams and stability of optical windows in high-power e-beam-pumped krypton fluoride lasers

Two of the key issues of a krypton fluoride (KrF) laser driver for inertial fusion energy are the development of long life, high transparency pressure foils (to isolate vacuum in the electron beam diode from a working gas in the laser chamber), and the development of durable, stable, optical windows. Both of these problems have been studied on the single-pulse e-beam-pumped KrF laser installation GARPUN. We have measured the transport of electron beams (300 keV, 50 kA, 100 ns, 10 × 100 cm) through aluminum-beryllium and titanium foils and compared them with Monte Carlo numerical calculations. It was shown that 50-μm thickness Al-Be and 20-μm Ti foils had equal transmittance. However, in contrast to Ti foil, whose surface was strongly etched by fluorine, no surface modification nor fatal damages were observed for Al-Be foils after ∼1000 laser shots and protracted fluorine exposure. We also measured the 8% reduction in the transmission of CaF2 windows under irradiation by scattered electrons when they were set at 8.5 cm apart from the e-beam-pumped region. However an applied magnetic field of ∼0.1 T significantly reduced electron scattering both across and along the laser cell at typical pumping conditions with 1.5 atm pressure working gas. Thus the e-beam-induced absorption of laser radiation in optical windows might be fully eliminated in an e-beam-pumping scheme with magnetic field guiding.

The University Maryland Electron Ring (UMER)

A detailed understanding of the physics of space-charge dominated beams is vital in the design of heavy ion inertial fusion (HIF) drivers. In that regard, low-energy, high-intensity electron beams provide an excellent model system. The University of Maryland Electron Ring (UMER), currently under construction, has been designed to study the physics of space-charge dominated beams with extreme intensity in a strong focusing lattice with dispersion. At 10 keV, 100 mA, the UMER beam has a generalized perveance in the range of 0.0015, corresponding to that of proposed HIF drivers. Though compact (11 m in circumference), UMER is a very complex device. In this paper, the unique design features of this research facility, the beam physics to be investigated, and recent experimental results in the prototype injector as well as simulation studies will be reviewed.

A generalized scaling law for the ignition energy of inertial confinement fusion capsules

The minimum energy needed to ignite an inertial confinement fusion capsule is of considerable interest in the optimization of an inertial fusion driver. Recent computational work investigating this minimum energy has found that it depends on the capsule implosion history, in particular, on the capsule drive pressure. This dependence is examined using a series of LASNEX simulations to find ignited capsules which have different values of the implosion velocity, fuel adiabat and drive pressure. It is found that the main effect of varying the drive pressure is to alter the stagnation of the capsule, changing its stagnation adiabat, which, in turn, affects the energy required for ignition. To account for this effect a generalized scaling law has been devised for the ignition energy, Eign ∝ αif1.88±0.05v-5.89±0.12P-0.77±0.03.This generalized scaling law agrees with the results of previous workin the appropriate limits.

High current injectors for heavy ion driven inertial fusion

Conceptual heavy ion driven inertial fusion (HIF) drivers typically have an array of up to 100 parallel beams each supplying a beam current of ≈0.25 A. According to space-charge limitations in beam extraction and in the low energy beam transport section, there are two options in building injectors for HIF drivers. The traditional way is to use low current density, large aperture, contact ionization sources. The major disadvantage of this approach is the very large size of the injector and matching section. The other option is to use high current density, multiple beamlet ion sources. From various scaling rules, it is found that the multiple beamlet approach is the more attractive one because it can be smaller, and more efficient, although the requirements on the ion source are more demanding.

Alternative approaches: concept improvements in magnetic fusion research

While the conventional tokamak, as embodied in JET, is the front–runner in the magnetic–confinement approach to fusion, other concepts are being developed that might, in the long term, prove more attractive for power generation (‘concept improvement’ research). Also, in the shorter term, these concepts might offer a more–rapid and cheaper way of studying ignited or near–ignited plasmas. Their advantages, and disadvantages, are described. The stellarator offers steady–state operation at the cost of coil complexity, new advanced stellarators with overall size comparable to JET are either under construction or being commissioned in Japan and Germany. ‘Spherical–torus’ configurations, of which the most promising is currently the spherical tokamak, offer high–pressure containment in a compact device, but the high power density may mean power–plant technology will be more challenging (e.g. suitable materials). New spherical tokamaks will soon come into operation in the UK, USA and Russia and will test properties in larger higher–current longer–pulse plasmas. Meanwhile, other concepts such as the ‘reversed–field pinch’, magnetic–mirror systems and the dense Z –pinch, have their own advantages, though they are less well developed. The status of the various concepts are summarized as are their potential fusion applications that include electricity generation, acting as a fusion neutron source, and providing a driver for inertial fusion.

Driver Technology for Inertial Fusion Research Introduction. 2. Inertial Fusion Driver development and Future Prospects in the United States.

A brief review is given of the requirements of inertial fusion energy drivers and their status and prospects in the U.S. Drivers based on lasers (diode-pumped solid-state and KrF) and heavy ions are discussed.

Studies of the physics of space-charge-dominated beams for heavy ion inertial fusion

At the University of Maryland our research on beam physics focuses on space-charge effects and collective phenomena in high-intensity, high-brightness beams for advanced accelerator applications such as drivers for heavy ion inertial fusion. In this paper, we present results of recent beam physics experiments including study of the resistive-wall instability and beam transport in a FODO lattice. We also report on the status of an electron ring project to study the recirculation of beam currents well beyond the normal tune-shift limit.

Progress of the two-beam funneling experiment

The injector for a heavy ion inertial fusion (HIIF) driver has to provide a high current and small emittance ion beam which cannot be provided by one single ion source. The required beam will be reached by several funneling stages, where two identically bunched ion beams are combined into a single beam with twice the frequency, current and brightness. For the most critical first funneling stage a two-beam radio-frequency quadrupole (RFQ), where two beams are bunched and accelerated in a single RF cavity, and a novel scheme for an RF funneling deflector, operating at low voltages, has been developed. With the use of convergent incoming beams, the funneling structure will be placed around the beam crossing position. The progress of the experiment, consisting of a combination of a two-beam RFQ with an RF deflector for funneling of two He +-beams at low energies will be presented.

The status of the HIDIF study

The HIDIF Collaboration, set up in 1994 by several European laboratories, has reviewed the possibilities of designing a heavy-ion driver for inertial fusion and explored in particular the limits of phase-space density that can be achieved with a single-ionized heavy-ion beam from a “linac plus storage ring” configuration. This overall report gives the aims and achievements of the collaboration at the time of the 1997 HIIF Symposium, where in several papers on specific aspects of the work were presented by members of the collaboration.

Beam charge and current neutralization of high-charge-state heavy ions

High-charge state heavy ions may reduce the accelerator voltage and cost of heavy-ion inertial fusion drivers, if ways can be found to neutralize the space charge of the highly charged beam ions as they are focused to a target in a fusion chamber. Using 2-D particle-in-cell simulations, we have evaluated the effectiveness of two different methods of beam neutralization: (1) by redistribution of beam charge in a larger diameter, pre-formed plasma in the chamber, and (2), by introducing a cold-electron-emitting source within the beam channel at the beam entrance into the chamber. We find the latter method to be much more effective for high-charge-state ions.

A two-beam radio-frequency quadrupole (RFQ) for funnelling of ion beams at low energies

In a heavy ion inertial fusion (HIIF) driver the strongest current limitations are space charge forces in the low energy part of the linac. For this reason the required high current and small emittance ion beam will be reached by several funnelling stages where two identically bunched ion beams are combined into a single beam with twice the frequency, current and brightness. For the first funnelling stage a new two-beam radio-frequency quadrupole (RFQ) could be used where two beams are bunched and accelerated in a single r.f. cavity. The r.f. structure development and the experimental set-up for beam funnelling with He + will be presented.

Physics design and scaling of Elise

Elise is an electrostatically focused heavy-ion accelerator being designed and constructed at Lawrence Berkeley Laboratory. The machine is intended to be the first half of the four-beam Induction Linear Accelerator (LINAC) Systems Experiment (ILSE), which will ultimately test the principal beam dynamics issues and manipulations of induction heavy-ion drivers for inertial fusion. Elise will use an existing 2 MeV injector and will accelerate space-charge-dominated pulses to greater than 5 MeV. The design objective of Elise is to maximize the output beam ‘rules of thumb’ used for choosing the beam and lattice parameters for heavy-ion induction accelerators, and we discuss incorporating these relations in a spreadsheet program that generates internally consistent lattice layouts and acceleration schedules. These designs have been tested using a one-dimensional particle simulation code (SLIDE), a three-dimensional fluid/envelope code (CIRCE) and a three-dimensional particle-in-cell code (WARP3d). Sample results from these calculations are presented. Results from these dynamics codes are also shown, illustrating sensitivities to beam and lattice errors, and testing various strategies for longitudinal confinement of the beam ends.

Investigation of space-charge-compensated ion beams with a time-resolving ion energy spectrometer

High perveance ion beams are required for heavy-ion inertial fusion drivers. The current limit of a low energy beam transport line is enhanced by space-charge compensation. It rise time is several hundreds of microseconds under the supposition that electrons are produced by residual gas ionization only. Therefore the rise time depends on the residual gas pressure and ionization cross-section. This is of special interest for pulsed ion beams, with a pulse length close to or below that range. To study this compensation process, we developed a time-resolving residual gas ion energy spectrometer with single-particle detection; the available time resolution is about 2 μs. Experimental results are presented and compared with a theoretical prediction.

Conceptual design of compact heavy-ion inertial fusion driver with an r.f. LINAC with high acceleration rate

The interdigital-H-type (IH) linear accelerator (LINAC) is well known for its high shunt impedance at low and medium particle velocities. Therefore, it can be used to operate efficiently with a high acceleration gradient. The IH LINAC cavity is able to generate 10 MV m −1 (average acceleration gradient) with focusing of the particles by a superconducting solenoid and quadrupole. The LINAC can accelerate particles with a charge to mass ratio ( q/ A) greater than 1/250 from 0.3 MeV a.m.u. −. In a compact heavy-ion inertial fusion driver design, the total effective length of the IH LINAC cavities is about 1250 m.

Engineering development for a small-scale recirculator experiment

Lawrence Livermore National Laboratory (LLNL) is evaluating the physics and technology of recirculating induction accelerators for heavy ion inertial fusion drivers. As part of this evaluation we are building a small-scale recirculator to demonstrate the concept and to use as a test bed for the development of recirculator technologies. System designs have been completed and components are presently being designed and developed for the small-scale recirculator. The hardware being developed includes both mechanical and electrical components of the beam line. Our present development efforts are focused on two areas: (1) the design of the modular beam line component called a “half-lattice module” which must satisfy challenging space and vacuum requirements and (2) the development of an advanced solid state modulator which will generate precisely tailored electrical pulses at repetition rates exceeding 100 kHz for acceleration. This paper will discuss results of the design and development activities that are presently being conducted to implement the small-scale recirculator experiments. An overview of the system design will be presented along with a discussion of the implications of this design on the mechanical and electrical hardware. The paper will focus primarily on discussions of the development and design of the half-lattice period hardware and the advanced solid state modulator.

ILSE Heavy Ion Accelerator

Lawrence Berkeley Laboratory (LBL) and Lawrence Livermore National Laboratory (LLNL) propose to build at LBL the Induction Linac Systems Experiments (ILSE), the next logical step towards the eventual goal of a heavy-ion induction accelerator powerful enough to implode or {open_quotes}drive{close_quotes} inertial-confinement fusion targets. ILSE is a 10 MeV heavy ion accelerator system that will establish the beam dynamics understanding required for a heavy ion IFE driver. Space charge dominated beams with driver scale dimensions will be employed. ILSE, although much smaller than a driver, will be the first experiment at full driver scale in several important parameters. Most notable among these are line charge density and beam cross section. Nearly all accelerator components and beam manipulations needed for an inertial fusion energy (IFE) driver will be tested. The ILSE accelerator and research program will permit experimental study of those beam manipulations required of an induction linac inertial fusion driver that have not been tested sufficiently in previous experiments. ILSE is also an important step in driver technology development for heavy ion drivers.

Plans for driver and target studies for heavy-ion inertial fusion by intense heavy-ion linac and cooler synchrotron

Basic experiments for heavy-ion inertial fusion by using an intense heavy-ion linac and a heavy-ion cooler synchrotron are proposed. The planned accelerator complex consists of nine heavy-ion linacs, a heavy-ion cooler synchrotron, two storage rings, 24 beam buncher rings and a chamber for target experiments. In a first phase an old accelerator system will be improved to make it able to accelerate intense beams of heavy ions, such as Xe, with charge to mass ratio ofq/A=1/6, up to 2.4 MeV/a.m.u.

Modulators for heavy-ion recirculating induction linacs

We have begun a two-year study on the problem of designing high-power pulse modulators for the induction cavities of a recirculating heavy-ion linac driver for inertial fusion. Recent studies have shown the potential of a recirculator for major cost reduction. However, important issues exist including beam stability and control, and severe tolerances. We are addressing the need for efficient, cost-effective modulators. Our example conceptual design uses a 3 ms, linearly rising dipole field and constant energy/turn for 96 turns. The modulator pulses must vary in duration and spacing by a factor of 4. For each 264 kV module we propose to use 50–100 Metglas cores, each driven by a number recently developed, solid-state, on-off switches. Several circuit methods are being studied, including delayed triggering of groups of switches and selective charging of pulse networks. Here we report on initial circuit studies and on preliminary experiments to characterize promising switches and related components. Plans are underway to assemble a scaled accelerating module and simulate an ion beam with a separte high-voltage modulator.

Low-energy beam transport of intense and partially space-charge-neutralized ion beams

High-perveance ion beams are required for heavy-ion inertial-fusion drivers and cannot be transported without a reduction of space charge forces by space charge neutralization. The rearrangement of beam ions due to the self-field of partly neutralized beams is one source of r.m.s. emittance growth in low-energy sections. Calculations including the influence of compensating electrons have been carried out. A low-energy beam line is being set up to investigate the influence of partial space charge neutralization on the transverse beam emittance of high-perveance beams (typically 9keV, 3.5 mA He+). Theoretical assumptions and results of numerical simulations are presented and compared with first experimental results.

Overview of nuclear-pumped lasers

The first demonstration of a nuclear-pumped laser (NPL) dates back to the mid-1970s while theoretical studies started earlier. This history will be briefly reviewed, followed by a discussion of the present status of various NPL concepts. Both physics and engineering issues are noted. Most attention has been given to gaseous media due to problems associated with radiation damage in liquid- and solid-state media. Various pumping methods, ranging from direct pumping of the laser medium to the use of nuclear-pumped flashlamps, will be discussed. Recent experimental results on pumping of O2(1Δ), the first demonstration of a nuclear-pumped flashlamp-driven laser XeBr*-I, and the new 3He-Ne-H2 NPL will be presented. Potential applications and corresponding NPL requirements, e.g., the long delay time requirements for an inertial fusion driver based upon NPLs, will be reviewed.