NDCX-II, A New Induction Linear Accelerator for Warm Dense Matter Research

NDCX-II, A NEW INDUCTION LINEAR ACCELERATOR FOR WARM DENSE MATTER RESEARCH* M. Leitner # , F. Bieniosek, J. Kwan, G. Logan, W. Waldron, LBNL, Berkeley, CA 94720, U.S.A. J.J. Barnard, A. Friedman, B. Sharp, LLNL, Livermore, CA 94550, U.S.A. E. Gilson, R. Davidson, PPPL, Princeton, NJ 08543, U.S.A. Abstract The Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL), a collaboration between Lawrence Berkeley National Laboratory (LBNL), Lawrence Livermore National Laboratory (LLNL), and Princeton Plasma Physics Laboratory (PPPL), is currently constructing a new induction linear accelerator, called Neutralized Drift Compression eXperiment NDCX-II. The accelerator design makes effective use of existing components from LLNL’s decommissioned Advanced Test Accelerator (ATA), especially induction cells and Blumlein voltage sources that have been transferred to LBNL. We have developed an aggressive acceleration “schedule” that compresses the emitted ion pulse from 500 ns to 1 ns in just 15 meters. In the nominal design concept, 30 nC of Li + are accelerated to 3.5 MeV and allowed to drift-compress to a peak current of about 30 A. That beam will be utilized for warm dense matter experiments investigating the interaction of ion beams with matter at high temperature and pressure. Construction of the accelerator will be complete within a period of approximately two and a half years and will provide a worldwide unique opportunity for ion-driven warm dense matter experiments as well as research related to novel beam manipulations for heavy ion fusion drivers. accelerator based driver system, including fast diagnostics, to experimentally probe WDM equations of state. In simplest terms, an equation of state attempts to describe the relationship between temperature, pressure, density, and internal energy for a given substance or mixture of substances. The HIFS-VNL plans experiments with targets in the density range between 10 21 to 10 23 ions/cm 3 (solid aluminum density: 6·10 22 atoms/cm 3 ) around a temperatures of 1 eV (11,000 K). Heavy Ion-Driven WDM Research Heavy ion beams have a number of advantages as drivers for warm dense matter experiments. First, heavy ions have a range exceeding the mean free path of thermal x-rays, so that they can penetrate and deposit most of their energy deep inside the targets. Second, the range of heavy ion beams in dense plasma targets is determined primarily by Coulomb collisions with the target electrons. The rate of energy loss in the target, dE/dx, is dependent on the energy of the incoming projectile and displays a pronounced peak, which occurs at higher energies for higher (atomic number) Z projectiles. These properties make heavy ions an excellent candidate for warm dense matter physics studies, where thin (!m) target plasmas could be uniformly heated by locating the energy deposition peak (“Bragg peak”) near the target center (see reference [2] for a more detailed description). To achieve the most uniform target heating volume (in contrast to non-uniform heating with laser or X-ray heating) the main strategy is to pick a target thickness and beam energy such that the ion beam enters the target slightly above (~1.5 E peak ) the energy of maximum dE/dx, deposits most of its energy inside the thin target foil, and exits the target slightly below the dE/dx peak at ~0.5 E peak . From an accelerator standpoint, the most cost-effective way of heating targets at the Bragg peak is to use lighter ion projectiles (e.g. Li + ) on low- mass target foils (e.g. aluminum) where the maximum dE/dx occurs at rather low energies. For NDCX-II, a combination of a Li + ion beam and an Aluminum target foil, the Bragg peak is located at 1.8 MeV, and the ion range is ~ 5 !m. In summary, the advantages of such a low-range ion heating approach (“Bragg heating”) are: The target is heated isochorically (uniformly). By placing the center of the target foil at the Bragg peak the heating uniformity is maximized and the accelerator beam energy is used most efficiently. Bragg heating requires low energy (~MeV) and thus much smaller accelerators. INTRODUCTION Warm Dense Matter (WDM) Research A US National Task Force [1] has identified exploration of fundamental properties of “warm dense matter” (WDM) as a major, future research area. Warm dense matter consists of extreme states of matter that are neither in a “cold, condensed-matter” state, nor in a “hot, plasma” state, but rather somewhere intermediate. Warm dense matter is typically a strongly-coupled, many-body charged particle system with energy density exceeding 10 10 J/m 3 , conditions that are extremely difficult to study analytically and by numerical simulation. However, many astrophysical systems as well as common laboratory experimental conditions, where plasma is created quickly from a solid, fall into this regime. Because of the short timescales involved, attempts to isolate warm dense matter for study have proven to be a major challenge. The U.S. Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL) is currently developing an ion- * This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. MLeitner@lbl.gov