VNL Contributions to Related Research Activities
The HIF-VNL intense beam science program contributes to and benefits from related research programs worldwide in accelerator science for high-energy physics, defense, biology, medicine, and materials science. The wide disparity of time and length scales in accelerators, as well as the complexity of the physics, has also necessitated advances in computer science, many of which are finding use outside of HIFS. The program complements foreign accelerator-research programs for high-energy-density physics (HEDP) and heavy-ion fusion (HIF) at GSI in Germany, RIKEN in Japan, and ITEP in Russia. We have both sent researchers to and hosted them from GSI and RIKEN for collaborations on specific physics areas of common interest, such as bunch-compression physics, code development, and induction-modulator engineering. Since 1997, we have participated in US/Japan HIF workshops, as well as in exchanges involving short working visits to the other country. We also maintain close contact with researchers at GSI, and in particular a GSI physicist is a standing member of the HIF-VNL Program Advisory Committee, and the VNL director is a standing member of the GSI Plasma physics/HEDP advisory committee. The biannual International HIF Symposium (for which the US sponsors about half the meetings) helps maintain vigorous contact between the HIF centers in Germany, Japan, Russia, and other international and US HIF researchers.
The European and Japanese HEDP/HIF accelerator facilities (GSI and RIKEN) represent complementary approaches in both technology choices and physics regime to the US approach. GSI and RIKEN accelerate ions to high energy but modest current. Thus, they concentrate on ions with large range, and hence relatively large (~cm scale in length) heated experimental volumes, for HEDP research at energies well beyond the Bragg peak. The VNL approach to HEDP is to pursue the use of ion beams of lower kinetic energy but higher current, with smaller ranges, operating near the Bragg peak, so that heating of matter can be more uniform (d2E/dx2 → 0 at the Bragg peak), and accomplished in smaller volumes (~100 micron scale in length), with more efficient use of beam power, and expected higher temperatures at a given pulse energy.
The VNL's approach to HEDP also has the potential to complement the work of other HEDP facilities using long-pulse lasers (eg Omega at Rochester's and NIF at LLNL), short-pulse lasers (Vulcan at RAL, CUOS at the Univerisity of Michigan, and the L'OASIS facilities at LBNL), as well as the Z-pinch (Sandia), to name a few of the current facilities engaged in HEDP research. The heavy-ion facility would also offer beam availability dedicated to science users rather than to other programs, and at a site more accessible to visiting scientists and students than many of the laser facilities. The VNL is studying the relative advantages that induction accelerators at moderate ion energies can provide for HEDP studies, including uniform ion energy deposition in solid matter near the Bragg peak, a high repetition rate, and flexible current waveforms. For example, heating that is uniform within a few percent would help reduce the error bars in comparing equation-of-state data to various competing ab initio plasma theories.
Connection with NNSA-Funded Work
The research in the HIF-VNL is closely coordinated with the DOE's National Nuclear Security Administration (NNSA) Defense Program's target science (ICF program), with OFES-sponsored HIF target design, and with enabling target and chamber technology research for both energy and defense applications. The HIF target work has progressed well, and is feeding back to the larger target physics program with such innovations as "cocktails" of hohlraum wall material to improve the target energetics, and shims on the capsule to improve the implosion symmetry. This year the first tests of the shim concept were made on the Z-machine at Sandia, with encouraging results. In addition, advanced hohlraum designs are being considered for the NIF; these use certain design features pioneered in HIF distributed radiator target designs: low-density low-Z foams to hold the hohlraum walls back, and hohlraum walls with low-density inner layers.
The induction accelerator research carried out within the HIF-VNL has enjoyed considerable cross-fertilization with the induction accelerator research carried out in support of Defense Program's x-ray radiographic diagnostics mission, which includes the Dual Axis Radiographic Hydrodynamic Test (DARHT) accelerator (an electron induction linac) and research toward even more advanced systems. For example, pulse modulator development for the radiography program has benefited significantly from HIF work.
A beneficial connection to the US magnetic-fusion program includes significant overlap on simulation methods and code-development tools, materials, magnets, and basic plasma physics studies, eg non-neutral plasma physics.
Connection with the Particle-Accelerator Community
In addition, we interact strongly with other colleagues in the particle accelerator community. In addition to our LLNL and LBNL radiography colleagues, we are in frequent contact with our LBNL Center for Beam Physics colleagues, who undertake research on a variety of accelerator systems including those on the Spallation Neutron Source (SNS), the Advanced Light Source (ALS), and the Large Hadron Collider (LHC), and possible future accelerators such as the Next Linear Collider (NLC) and the muon collider. We are strong participants in the biannual Particle Accelerator Conferences and International Linac Conference, and the United States Particle Accelerator School. We have particularly strong ties to the specialized communities of electron-cloud studies and pressure-rise studies, as detailed below.
Understanding the effects of electrons on positively charged ion beams is a topic of high importance in the accelerator research community, since these effects limit luminosity in high-energy accelerators and those for nuclear physics, and electrons can have large effects in any accelerator with intense beams. The VNL has a parallel interest in understanding electron effects in intense heavy-ion beams. Examples of these studies can be found in the work of HIF-VNL researchers on the ion-electron two-stream modes in proton storage ring/synchrotron facilities like SNS at ORNL and PSR at LANL. The VNL work includes both experimental measurements on multiple facilities using specially designed detectors, and the development and application of computer models. Another example is the modeling of electron effects in the LHC at CERN, being carried out by HEP-funded LBNL researchers, with whom collaborations are ongoing through a funded LLNL/LBNL internal research task. We have participated significantly in international workshops on electron-cloud effects and related topics, such as pressure rise in rings, and have presented (and are about to present) a number of invited talks in those venues, as well as at the international Particle Accelerator Conference. Collaborations on the modeling have also been established with researchers at Tech-X Corporation, who are funded by SBIR grants to carry out related research, and with researchers on the UC Berkeley campus with extensive experience in modeling discharges for plasma processing.
This work began as part of our electron-cloud work, because we expected ionization of gas to be the dominant source of electrons in long-pulse HIF beams. In discussing the unexpectedly large desorption coefficients with atomic scientists, we learned of "electronic sputtering," which is capable of producing large desorption yields from ions impinging on frozen gases. For ions in the MeV range, electron excitation and ionization are the dominant contribution to the stopping power dE/dx, rather than the elastic or "nuclear" collision processes that dominate at lower energies. With this clue, we used STS-500 and HCX to study desorption scaling for K+ ions from 50 keV to 1 MeV inpinging on stainless steel. We found that desorption scaled as (dE/dx)1.5, similar to the scaling observed for beam desorption of frozen gases. Identifying the mechanism for desorption is important for the accelerator community, where the luminosities of several major accelerators, such as RHIC, GSI-Upgrade, and LHC, are limited or threatened by pressure rise.
This issue is also important in astrophysics. Complex carbon molecules are observed in the Orion nebula that may be the building blocks for life. However, it is not clear how they are formed. One possible pathway is Fischer-Tropsch catalysis, the iron/nickel catalyzed conversion of CO and H2 to hydrocarbons, which modeling has shown to function well in the pressure and temperature profiles of solar nebula until the accumulation of carbon (in graphitic form) on the metal surface poisons the reaction. Adding a desorption process, like we describe here, to the model could scrub carbon or other deposits from circumstellar metallic iron dust to maintain its catalytic properties. The efficient desorption of gas from metals by electronic sputtering thus becomes a crucial step in the chemistry of forming the building-blocks for life in the universe.
Connection with GSI Upgrade
GSI has proposed a new facility upgrade, which would greatly increase the achievable ion intensity, and this potentially would make future collaboration with GSI on electron effects in intense ion beams mutually beneficial. These explorations have resulted in a new annex to the ongoing collaboration agreement in the field of dense plasma physics between DOE and the German Federal Ministry of Education and Research (with the HIF-VNL and GSI the participants in the agreement). We have found that research in electron effects in intense ion beam accelerators, because it is expected to become more important to both the HIF and high-energy and high-power accelerator communities in the future, presents a good opportunity for cooperation and cross-fertilization of new ideas, diagnostics, theory and simulation.
Source and Injector Experiments
Sources and injectors for other applications are benefiting from HIF research. The injector for the second axis of DARHT was developed using the HIF 2-MV injector as its main knowledge base. The injector for the Relativistic Klystron Two-Beam Accelerator proof-of-principle experiment (RTA) at LBNL also drew heavily on HIF-derived knowledge. A DC high-current low-energy ESQ injector has been proposed for an industrial application (doping), as well as for Boron Neutron Capture Therapy (BNCT). No such DC machine has yet been built, but there are no known fundamental flaws to the concept. Our simulation tools are used in support of the VENUS injector project aimed at the proposed RIA, as described below. The merging-beamlet concept, in conjunction with electrostatic-quadrupole post-acceleration, may have application to high-energy (MV-level) neutral-beams for MFE current drive and heating for ITER and other experiments.
Experiments with Non-Neutral Plasmas
The HIF-VNL is involved directly in research employing particle "traps," specifically the Paul Trap Simulator at PPPL. In addition, the LLNL/LBNL group is working with faculty and students at UCB, to apply the HIF-VNL's discrete-particle driver simulation code WARP to pure-electron plasmas in axisymmetric traps, and to studies of quadrupole traps for the confinement of antimatter.
Superconducting Magnet Development
Because of economics, the quadrupole array focusing unit for HIF must be compact. In the radial direction, compact designs that minimize the structure between adjacent beams (coils, supports) are required to minimize the focusing array size and in turn the amount of induction core material needed for economical acceleration. In the longitudinal direction, the transition from the cold mass to the acceleration gap should be minimized to allow the most room for acceleration structures. (This is most important at low kinetic energy, where the lattice period is short.) Our prototype focusing doublet is compatible with high intensity transport at injection energy, and with transitions from the ends of the cold mass to the outer surface of the cryostat of about 5 cm.
There are two aspects of our magnet development program that overlap with other fields:
We are developing new or improved diagnostics, building on existing accelerator diagnostic capabilities. Our unique experimental regime of high intensity, high space charge, and high mass beams drives new developments that have applicability to a broad range of particle beams in accelerators. The following list gives some of our developments:
In addition, we have submitted an invention disclosure to the LBNL patent department on a new high-voltage measurement technique using the voltage-current relationship of a heavy ion beam diode. This technique may find commercial application in the high-voltage measurement industry.