A. Tauschwitz

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.

Density Diagnostics of an Argon Plasma by Heavy Ion Beams and Spectroscopy

Experiments have been performed to investigate charge state and energy loss of 5.9-MeV/u uranium ions in a partially ionized dense argon plasma with 10 17 ≤ n e 10 19 cm -3 and 1 ≤ T e ≤ 10 eV. The charge state of the ions was found to be equal to the equilibrium charge state in cold gas. The energy loss offered a useful tool for plasma density diagnostics. Spectroscopic measurements using the electron collision broadening of ArII- and ArIII-lines in the visible and UV spectral region confirm this plasma density data.

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.

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.

Pulsed, high-current and iron-free ion optical systems for beam transport in ICF driver accelerators

SummaryTo drive ICF implosion targets, pulsed ion beams will be used. Therefore the corresponding beam guiding systems do not have to provide the magnetic flux density in a d.c. mode, but only in a pulsed mode on a short time scale. The operation of steady-state magnetic fields in accelerator-based ICF driver scenarios as outlinede.g. in the HIBALL reactor study would lead to a waste of a significant fraction of the produced electrical energy. We suggest making use of pulsed, high-current and iron-free magnetic lenses for this purpose. In a number of experiments at the UNILAC accelerator at GSI-Darmstadt, pulsed quadrupole lenses have already proven their ability and reliability to focus and transport highly energetic heavy-ion beams.

Shape Optimization of a Wall-Stabilized Plasma Lens (*).

SummaryThe idea of optimizing the shape of a wire wall-stabilized plasma lens for final focusing of ion beams has been considered for the first time. A theoretical analysis of the properties of such a lens has been developed, showing that the “ideal” focusing properties of wire lenses are preserved. The focusing performance of the optimized lens has been numerically evaluated with a code especially developed for the purpose. Considerable increase of focusing efficiency is predicted, since it is theoretically possible to obtain the same focal spot size with a factor of four less current. First experimental results, confirming the improvement, are shown.

Adiabatic focusing and channel transport for heavy ion fusion

The final focus lens in an ion beam driven inertial confinement fusion reactor is important since it sets limiting requirements for the quality of the driver beam. Improvements of the focusing capabilities can facilitate the construction of the driver significantly. A focusing system that is of interest both for heavy ion and for light ion drivers is an adiabatic, current carrying plasma lens. This lens is characterized by the fact, that it can slowly (adiabatically) reduce the envelope radius of a beam over several betatron oscillations by increasing the focusing magnetic field along a tapered high current discharge. A reduction of the beam diameter by a factor of 3 to 4 seems feasible with this focusing scheme. Such a lens can be used for an ignition test facility where it can be directly coupled to the fusion target. For use in a repetitively working reactor chamber the lens has to be located outside of the reactor and the tightly focused but strongly divergent beam must be confined in a high current t...

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.