Heavy Ion Fusion Accelerator Research Articles

Improving the neutralization of a pulsed ion beam by electron cooling and accumulation: A kinetic study

Direct electron injection, such as through electron-emitting filaments, is usually difficult to neutralize ion beams to a very high degree. In this paper, the possibility of a pulsed ion beam achieving very high neutralization through the continuous accumulation of cold electrons is investigated using a two-dimensional particle-in-cell code. Three schemes of electron injection, namely, single-point injection, periodic point-source injection, and periodic line-source injection, are numerically studied and compared. The simulations show that even if an excess of electrons are injected, the single-point electron source is difficult to neutralize the ion beam pulse to exceed 90%, consistent with existing experiments. It is found that the spontaneous cooling mechanism of neutralizing electrons is able to improve the neutralization of the ion beam to a certain extent, but it requires a lot of time. By using a smaller injection current, the latter two injection schemes not only effectively suppress solitary waves, but more importantly, they continuously provide cold electrons that can accumulate inside the ion beam, thereby significantly improving the neutralization of the ion beam in a short period of time. The results show that periodic line-emission sources can neutralize the ion beam to over 99%, but periodic point-emission sources exhibit relatively poor neutralization performance due to their higher virtual-cathode potential. The research results can provide a reference for the design of neutralizing sources in applications that pursue very high neutralization of ion beam pulses, such as heavy ion fusion accelerators.

Multiple beam induction accelerators for heavy ion fusion

Induction accelerators are appealing for heavy-ion driven inertial fusion energy (HIF) because of their high efficiency and their demonstrated capability to accelerate high beam current (≥10kA in some applications). For the HIF application, accomplishments and challenges are summarized. HIF research and development has demonstrated the production of single ion beams with the required emittance, current, and energy suitable for injection into an induction linear accelerator. Driver scale beams have been transported in quadrupole channels of the order of 10% of the number of quadrupoles of a driver. We review the design and operation of induction accelerators and the relevant aspects of their use as drivers for HIF. We describe intermediate research steps that would provide the basis for a heavy-ion research facility capable of heating matter to fusion relevant temperatures and densities, and also to test and demonstrate an accelerator architecture that scales well to a fusion power plant.

Research and development toward heavy ion driven inertial fusion energy

We describe near-term heavy ion fusion (HIF) research objectives associated with developing an inertial fusion energy demonstration power plant. The goal of this near-term research is to lay the essential groundwork for an intermediate research experiment (IRE), designed to demonstrate all the key driver beam manipulations at a meaningful scale, and to enable HIF relevant target physics experiments. This is a very large step in size and complexity compared to HIF experiments to date, and if successful, it would justify proceeding to a demonstration fusion power plant. With an emphasis on accelerator research, this paper is focused on the most important near-term research objectives to justify and to reduce the risks associated with the IRE. The chosen time scale for this research is 5--10 years, to answer key questions associated with the HIF accelerator drivers, in turn enabling a key decision on whether to pursue a much more ambitious and focused inertial fusion energy research and development program. This is consistent with the National Academies of Sciences Review of Inertial Fusion Energy Systems Interim Report, which concludes that ``it would be premature at the present time to choose a particular driver approach\dots{}'' and encouraged the continued development of community consensus on critical issues, and to develop ``options for a community-based roadmap for the development of inertial fusion as a practical energy source.''

Halogens for negative ion beams and ion–ion plasmas

Negative ions have attractive features as drivers for inertial confinement fusion, because they will avoid electron cloud effects, and could be efficiently photodetached to neutrals after the final focus, which could also be beneficial in heating warm dense matter targets. The halogens have large electron affinities, and thus should be able to produce high current densities of relatively robust negative ions. Recent experiments comparing chlorine beams to argon beams using the same source, extraction optics, and diagnostics have demonstrated that Cl − beams can be produced with similar emittance to Ar + beams, and with about 3 4 the current density from the same configuration. The observed effective beam temperature of about 1 3 eV , and the similarity of current densities show that negative halogen beams can meet the current density and emittance requirements of heavy ion fusion. The near equivalence of the Cl − and Cl ++Cl 2 + current densities reaching the Faraday cup after passage through a substantial line density of effluent gas demonstrates that beam losses in the higher vacuum of a heavy ion fusion accelerator should be acceptable. A number of lines of evidence show that negative ion–positive ion plasmas (hereafter ion–ion plasmas), composed primarily of negative and positive ions with a small population of electrons, were produced in the sources near the extractor plane. Since Cl, F, I, and Br should all show similar chemistry, any of these halogens should be suitable as fusion driver beams, and heating thin iodine or bromine foils may produce ion–ion plasmas in the warm dense matter regime.

Scientific issues in future induction linac accelerators for heavy-ion fusion

The worldwide heavy-ion fusion effort has made a large amount of progress over almost three decades of research on the intense beam physics and concepts needed for an inertial fusion driver. In the 1970s, ‘80s, and ‘90s in the US, most experiments used driver parameters scaled to low energy and low line charge density in order to study, most often with a single beam, the physics of high-perveance beams. More recently, the US program has introduced higher line charge density experiments, where the beam space charge potential is significant. This allows investigation of the production, and effect on the beam, of stray electrons. Increasingly, the possibilities for neutralizing plasmas are also being investigated, both just upstream and in the target chamber. This talk will outline the scientific issues that remain to be examined. These include issues of longitudinal compression, dynamic aperture, electron/ion instabilities, effect of electrons and gas on the beam, longitudinal emittance evolution, halo production, inductive effects at high energy, multiple beams, innovative final focus approaches, and driver issues for solenoidal transport. An outline of future experiments to address these issues will be presented and discussed.

Simulating electron cloud effects in heavy-ion beams

Stray electrons can be introduced in heavy-ion fusion accelerators as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize here results from several studies of electron-cloud accumulation and effects: (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss and the importance of ion scattering is shown. (2) We simulate the effect of specified electron cloud distributions on ion beam dynamics. We find electron cloud variations that are resonant with the breathing mode of the beam have the biggest impact on the beam (larger than other resonant and random variations), and that the ion beam is surprisingly robust, with an electron density several percent of the beam density required to produce significant beam degradation in a 200-quadrupole system. We identify a possible instability associated with desorption and resonance with the breathing mode. (3) We carry out preliminary investigations of a long-timestep algorithm for electron dynamics in arbitrary magnetic fields.

Electron-cloud simulation and theory for high-current heavy-ion beams

Stray electrons can arise in positive-ion accelerators for heavy-ion fusion or other applications as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize the distinguishing features of electron-cloud issues in heavy-ion-fusion accelerators and a plan for developing a self-consistent simulation capability for heavy-ion beams and electron clouds (also applicable to other accelerators). We also present results from several ingredients in this capability. (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss, which illustrates the importance of retaining ion reflection at the walls. (2) We simulate the effect of specified electron-cloud distributions on ion beam dynamics. We consider here electron distributions with axially varying density, centroid location, or radial shape, and examine both random and sinusoidally varying perturbations. We find that amplitude variations are most effective in spoiling ion beam quality, though for sinusoidal variations which match the natural ion beam centroid oscillation or breathing-mode frequencies, the centroid and shape perturbations can also have significant impact. We identify an instability associated with a resonance between the beam-envelope ``breathing'' mode and the electron perturbation. We estimate its growth rate, which is moderate (compared to the reciprocal of a typical pulse duration). One conclusion from this study is that heavy-ion beams are surprisingly robust to electron clouds, compared to a priori expectations. (3) We report first results from a long-time-step algorithm for electron dynamics, which holds promise for efficient simultaneous solution of electron and ion dynamics.

A node-centered local refinement algorithm for Poisson's equation in complex geometries

This paper presents a method for solving Poisson's equation with Dirichlet boundary conditions on an irregular bounded three-dimensional region. The method uses a nodal-point discretization and adaptive mesh refinement (AMR) on Cartesian grids, and the AMR multigrid solver of Almgren. The discrete Laplacian operator at internal boundaries comes from either linear or quadratic (Shortley–Weller) extrapolation, and the two methods are compared. It is shown that either way, solution error is second order in the mesh spacing. Error in the gradient of the solution is first order with linear extrapolation, but second order with Shortley–Weller. Examples are given with comparison with the exact solution. The method is also applied to a heavy-ion fusion accelerator problem, showing the advantage of adaptivity.

Development and Testing of the Improved Focusing Quadrupole for Heavy Ion Fusion Accelerators

An improved version of the focusing magnet for a heavy ion fusion (HIF) accelerator was designed, built and tested in 2002-2003. This quadrupole has higher focusing power and lower error field than the previous version of the focusing quadrupoles successfully built and tested in 2001. We discuss the features of the new design, selected fabrication issues and test results.

Induction Accelerator Technology Choices for the Integrated Beam Experiment (IBX)

Over the next three years the research program of the Heavy Ion Fusion Virtual National Laboratory (HIF-VNL), a collaboration among LBNL, LLNL, and PPPL, is focused on separate scientific experiments in the injection, transport and focusing of intense heavy ion beams at currents from 100 mA to 1 A. As a next major step in the HIF-VNL program, we aim for a complete “source-to-target” experiment, the Integrated Beam Experiment (IBX). By combining the experience gained in the current separate beam experiments IBX would allow the integrated scientific study of the evolution of a single heavy ion beam at high current (~1 A) through all sections of a possible heavy ion fusion accelerator: the injection, acceleration, compression, and beam focusing.This paper describes the main parameters and technology choices of the planned IBX experiment. IBX will accelerate singly charged potassium or argon ion beams up to 10 MeV final energy and a longitudinal beam compression ratio of 10, resulting in a beam current at target of more than 10 Amperes. Different accelerator cell design options are described in detail: Induction cores incorporating either room temperature pulsed focusing-magnets or superconducting magnets.

Thick-Liquid Blanket Configuration and Response for the HIF Point Design

This paper describes the thick-liquid blanket system of the Robust Point Design (RPD-2002). RPD-2002 is the first self-consistent description of a heavy-ion fusion accelerator, final focus, target, magnet shielding, and thick-liquid blanket design. The 120 beams are delivered to the target from two sides, in 9x9 arrays, with 5.4° between rows giving a maximum beam angle from the target axis of 24°. The chamber employs thick-liquid protection, using liquid jets that have been demonstrated to have the required geometric precision in scaled water experiments. Other aspects of the chamber design, not directly related to the beam-line shielding, have been kept the same as the HYLIFE-II design.

Beam Dynamics Simulation in Final Beam Bunching of Heavy Ion Inertial Fusion.

Bunching beam dynamics and emittance growth in a final buncher of heavy ion fusion accelerator system are studied using a particle-in-cell simulation. The results indicate that while the beam current is increased by longitudinal compression, the beam particles dilute in phase space depending on the bunching history.

End-to-end simulation: The front end

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.

Final focus shielding designs for modern heavy-ion fusion power plant designs

Recent work in heavy-ion fusion accelerators and final focusing systems shows a trend towards less current per beam, and thus, a greater number of beams. Final focusing magnets are susceptible to nuclear heating, radiation damage, and neutron activation. The trend towards more beams, however, means that there can be less shielding for each magnet. Excessive levels of nuclear heating may lead to magnet quench or to an intolerable recirculating power for magnet cooling. High levels of radiation damage may result in short magnet lifetimes and low reliability. Finally, neutron activation of the magnet components may lead to difficulties in maintenance, recycling, and waste disposal. The present work expands upon previous, three-dimensional magnet shielding calculations for a modified version of the HYLIFE-II IFE power plant design. We present key magnet results as a function of the number of beams.

Development of quadrupole arrays for heavy-ion fusion

Extensive studies have been performed to optimize the design of superconducting quadrupole arrays for beam transport in future heavy-ion fusion accelerators. In these arrays, 20 or more quadrupole coils are densely packed with their axes aligned in parallel. Field strengths between 3 and 5 tesla at the inner coil diameters have been investigated. The aperture of the individual quadrupoles has been varied between 60 mm and 240 mm. The coils have a typical length of 1 m, and special effort has been made to minimize the length of the interconnect region between the array and adjacent accelerator components. The relative orientation of quadrupoles in the array is chosen to optimize field sharing between neighboring cells. In the presented design, field uniformity in the boundary cells is maintained by placing concentric correction coils onto these cells. A novel approach for the mechanical stabilization and alignment of the individual quads in the array has been developed. Since Inertial Fusion Energy has to compete with other long-term energy sources, cost and reliability are major issues of the magnet array. The presented design uses a round NbTi mini-cable, which allows for a cost-effective, highly automated manufacturing of the required superconducting coils. A flexible, 37-strand mini-cable has been developed and prototype manufacturing has started.

Heavy Ion Fusion Accelerator Research

This paper describes an economical program for heavy ion fusion accelerator research. The heavy ion accelerator research program is part of a larger national research program to develop inertial fusion for commercial power production.

Transient beam dynamics in the Lawrence Berkeley Laboratory 2 MV injector

A driver-scale injector for the heavy ion fusion accelerator project has been built at LBL. This machine has exceeded the design goals of high voltage (above 2 MV), high current (more than 0.8 A of K +) and low normalized emittance (less than 1 π mm mrad). The injector consists of a 750 keV gun pre-injector followed by an electrostatic quadrupole accelerator which provides strong (alternating gradient) focusing for the space-charge-dominated beam, and simultaneously accelerates the ions to 2 MeV. A matching section is being built to match the beam to the electrostatic accelerator ELISE. The gun pre-injector, designed to hold up to 1 MV with minimal breakdown risks, consists of a hot alumino-silicate source with a large curved emitting surface surrounded by a thick “extraction electrode”. During beam turn-on the voltage at the source is biased from a negative potential, enough to reverse the electric field on the emitting surface and to avoid emission, to a positive potential to start extracting the beam; it stays constant for about 1 μs, and is reversed to turn off the emission. Since the Marx voltage applied on the accelerating quadrupoles and the main pre-injector gap is a long, constant pulse (several microseconds), the transient behavior is dominated by the extraction pulser voltage time profile. The transient longitudinal dynamics of the beam in the injector was simulated by running the particle-in-cell codes gymnos and warp3 d in a time-dependent mode. The generalization and its implementation is WARP3D of a method proposed by Lampel and Tiefenback to eliminate transient oscillations in a one-dimensional planar diode will be presented.

Space-charge-neutralized ion induction linacs with cusp insulation

The space charge of ions limits the current that can be accelerated in a conventional ion linear accelerator (linac). Furthermore, the accelerating electric field must be kept low enough to avoid the generation and acceleration of counterstreaming electrons. These limitations have resulted in heavy ion fusion accelerator designs that employ long beam lines and would be expensive to build. Space charge neutralization and magnetic insulation of the acceleration gaps could substantially reduce these two limitations, but at the expense of increasing the complexity of the beam physics. We consider it necessary to keep all structures out of the beam path to maintain beam symmetry and to avoid beam-induced outgassing. Using only field coils outside of the beam results in a cusp field to insulate the accelerating gaps and a solenoidal field between acceleration gaps, which provides a containment force. Electrons can enter the beam region along the magnetic field lines and provide fairly good space charge neutralization. We have performed three-dimensional particle-in-cell simulations, which indicate that electrons remain tied to magnetic field lines, despite the development of electromagnetic fluctuations. Thus, the center of the beam is not neutralized, resulting in a non-linear net focusing force. We have developed a simple model to determine the beam emittance induced by the non-linearity of the focussing force. We discuss the constraints this model puts on the choice of ion and the number of stages required for the accelerator.

Experimental studies of space-charge waves and resistive-wall instability in space-charge-dominated electron beams

We present results of experiments on space-charge waves and longitudinal instabilities in space-charge-dominated beams, which are important to the design and development of induction accelerators as heavy-ion fusion drivers. By employing localized space-charge waves, the geometry factor-an important beam parameter for heavy-ion fusion accelerator design-has been measured and its correct formula has been verified. Space-charge waves have been found to reflect back on the shoulder of an eroded rectangular beam bunch. This phenomenon has significant implications for the understanding and analysis of longitudinal instabilities. A resistive-wall instability experiment has clearly shown the growth of slow waves and the decay of fast waves. For localized perturbations, the amplitude change that results from the instability is smaller than expected.

Beam loss in heavy-ion fusion accelerators due to charge exchange scattering

Beam loss due to charge exchange scattering leads to a fundamental limit for the acceptable storage time. Recently measured cross-sections for Bi1+ and Au1− are evaluated in a computer program, which takes into account the variation of beam temperature and density in the storage ring due to strong focusing. In particular we have found that in case of Bi1+ the loss is increasing with emittance, whereas for Au1− the behaviour is the opposite due to a different dependence of the ion cross-sections on collision energy. Examples are presented for driver storage rings. Several examples of synchrotrons for an experimental facility leading beyond the threshold of radiation conversion physics have been studied.