E. Boggasch

Energy loss of fast heavy ions in plasmas

Abstract Interaction processes of heavy ions and ionized matter have been investigated experimentally. A Monte Carlo code to simulate charge exchange and energy loss processes in a plasma environment has been developed to plan the experiments, to analyze the results, and to investigate the influence of magnetic fields on the particle trajectories inside the plasma target. The first results of the beam plasma interaction experiments are: — The stopping power of ionized matter is increased compared to that of cold, non-ionized matter. This effect is especially pronounced at low ion energies (E 1MeV/u) this effect is less dramatic, but is still on the order of 2–3, depending on the ion species. — The charge state of ions traversing a fully ionized plasma is increased due to reduced capture cross sections of free plasma electrons. — It is possible to focus intense heavy ion beams of high magnetic rigidity using a plasma lens. This ideal focusing device achieves a high focusing power for ion beams almost comparable to optical systems for intense beams from high power lasers. — Energetic ion beams are very well suited to heat cylindrical target volumes and to create a dense plasma. The characteristic inner-shell target and projectile X-rays provide a tool to analyze the hydrodynamic behavior of beam heated targets with space and time resolution.

Beam–plasma interaction experiments with heavy-ion beams

The progress of the experimental research program at GSI for studying beam-plasma interaction phenomena is reported. Heavy-ion beams from the new accelerator facility SIS/ESR at GSI-Darmstadt are now available for experiments, and will soon deliver ≥ 10 9 particles per pulse in 100 ns. Focused on a small sample of matter, the beams will be able to produce a high-density plasma and to permit investigation of interaction processes of heavy ions with hot ionized matter. For the intense beam from the new heavy-ion synchrotron (SIS), a fine-focus system has been designed to produce a high specific deposition power beam for target experiments with a beam-spot radius of 100 μm. We further discuss improvements of this lens system by nonconventional focusing devices such as plasma lenses. Intense-beam experiments at the RFQ Maxilac accelerator at GSI have already produced the first heavy-ion-induced plasma with a temperature of 0.75 eV. New diagnostic techniques for investigating ion-beam-induced plasmas are presented. The low-intensity beam from the GSI UNILAC has been used to measure energy deposition profiles of heavy ions in hot ionized matter. In this experiment an enhancement of the stopping power for heavy ions was observed. The current experimental research program tests basic plasma theory and addresses key issues of inertial confinement fusion driven by intense heavy-ion beams.

Heavy-ion beam focusing with a wall-stabilized plasma lens

Focusing of heavy-ion beams is an important issue for ion beam-driven inertial confinement fusion. For the experimental program to investigate matter at high energy densities at GSI, the application of a plasma lens has attractive features compared to standard quadrupole lenses. A plasma lens using a wall-stabilized discharge has been systematically investigated and optimized for this purpose. Different lenses were tested in several runs at the GSI linear accelerator UNILAC and at the SIS-synchrotron. A remarkably high accuracy and reproducibility of the focusing were found. The focal spot size was mainly limited by the beam emittance. A summary of experimental results and important limitations of the focal spot size is given.

Improvement of the active cylindrical plasma lens concept by a tapered discharge geometry

Recently an active current carrying plasma lens was introduced using a wall-stabilized discharge mode with remarkable focusing properties for high energy heavy ion beams. In this paper, a further improvement of focusing performance of a wall-stabilized plasma lens will be reported using a tapered geometry. The gradient of the focusing field is increased in z direction and the focusing power is increased compared to a cylindrical lens with the same length and discharge current. Aberration effects of this tapered lens were minimized by optimizing both shaping and discharge conditions. With the tested lens an argon ion beam of 15.4 MeV/amu was focused from an initial diameter of 10 mm down to a spot size of 160 /spl mu/m. The results of the focusing experiments are compared with optical plasma diagnostics data allowing for an interpretation of the observed focusing behavior. >

Interaction of heavy ions with plasma

Abstract The accelerator facilities at GSI, consisting of a high current radio frequency quadrupole (RFQ) accelerator, a rf-linac (UNILAC), a heavy ion synchrotron (SIS), and an experimental storage ring (ESR) provide the unique opportunity to study beam-target interaction phenomena over a wide range of beam energy. This range extends from 45 keV/u up to, and above 1 GeV/u. Thus rf accelerator and beam-target interaction physics issues for heavy-ion-beam driven inertial confinement fusion can be studied. Recent results of the experimental program such as the enhanced energy loss and the increased effective charge state of heavy ions in ionized matter, the generation of dense plasma by intense heavy ion beams, and new advanced focussing methods for intense beams are discussed.

Focusing behaviour of plasma lenses compared to conventional quadrupole systems

Abstract Intense heavy ion beams focused to small spots are powerful tools to deposit a high specific deposition power in a target. A hot dense target plasma is formed which is available for further experimental investigation and to study beam-target interaction phenomena with relevance for the physics of inertial confinement (ICF) fusion. For a given set of beam parameters the minimum focused spot size is mainly limited by the focusing strength of the final lens system. Besides systems consisting of conventional quadrupoles also cylindrically symmetric, “active” plasma lenses have been built and tested at GSI attempting to overcome the limitations of quadrupole focusing. Superiour fine-focusing properties of plasma lenses have been demonstrated and a remarkable precision has been found. In this paper a quantitative comparison of the focusing strength of plasma lenses with conventional magnetic quadrupoles is presented.

The plasma lens solution for heavy-ion beam focusing

SummaryA new type of plasma lens using a «wall-stabilized» discharge has been investigated concerning its potential to fine-focus heavy-ion beams. The lens was tested in several runs at the GSI linear accelerator UNILAC with a beam energy of 2.2 GeV and a magnetic rigidity of 1.6 Tm. A remarkably high accuracy and reproducibility of the focusing was found. With a peak current of 22 kA a beam of 1 cm initial diameter was focused down to a spot diameter of below 300 μm at a distance of 90 mm behind the lens, in agreement with analytical calculations and Monte Carlo simulations of the focusing process. The focal spot size was mainly limited by the beam emittance and no lens-related aberrations could be detected. Plasma lenses are under consideration for use at the SIS/ESR facility of GSI to produce small beam spots on targets in order to achieve high energy density in matter. A design for a plasma lens focusing the beam from the SIS-synchrotron with a typical rigidity of 6 T m is proposed.

Development of a plasma lens as a fine focusing lens for heavy-ion beams

SummaryWithin the framework of the «High Energy Density in Matter» program, an experiment has been designed to focus a heavy-ion beam onto a very small spot, using a plasma lens. A spot diameter of 200 μm (FWHM) was desired to get the necessary energy density on the target to create a plasma. A flexible system has been designed in order to explore the various discharge, mechanisms. Design considerations and a beam test with the first stage of the system are discussed.

Diagnostics methods for beam-plasma experiments

SummaryWe report the first optical diagnostics of the dense plasma of a Z-pinch helium plasma which is used as a plasma target for investigation of beam-plasma interaction. Information about dynamics of the plasma and the relevant plasma parameters was obtained by spatially, temporally and spectrally resolved measurements of its visible light emission. The Stark broadening of the HeIIPα andPβ line yielded the number density of free electrons, temperature was deduced from the line-to-continuum intensity ratio of this lines. Maximal densities up to 1.14·1019 cm−3 at a temperature around 23 eV have been determined.

Energy loss and charge state measurements of heavy ions passing a hydrogen plasma

The accelerator facilities at GSI, consisting of a high current radio frequency quadrupole accelerator (MAXILAC), a rf‐linac (UNILAC), a heavy ion synchrotron (SIS) and an experimental storage ring (ESR) provide unique opportunities to study beam target interaction phenomena over a wide range of beam energy. This range extends from 45 keV/u up to 2 GeV/u. Thus rf‐accelerator and beam‐target interaction physics issues for heavy‐ion‐beam driven inertial confinement fusion can be studied. Recent results of stopping power experiments at UNILAC verified an enhanced energy loss of heavy ions in a plasma of a factor three at beam energies of 1–6 MeV/u. Even bigger differences in stopping power and charge state between ionized and cold matter are expected for beam velocities of the same order of magnitude as the thermal electron velocities in a plasma. A new energy loss experiment in this parameter range has been started at MAXILAC. First results indicate a tremendous enhancement of energy loss in a plasma at bea...