Plasma diagnostics of discharge channels for neutralized ion beam transport

• Main Author: Niemann, Christoph
• Format: Others
• Language: English; en
• Published: 2002
• Online Access: http://tuprints.ulb.tu-darmstadt.de/224/1/phd.pdf; Niemann, Christoph <http://tuprints.ulb.tu-darmstadt.de/view/person/Niemann=3AChristoph=3A=3A.html> : Plasma diagnostics of discharge channels for neutralized ion beam transport. [Online-Edition] Technische Universität, Darmstadt [Ph.D. Thesis], (2002)

Most of the future accelerators will be high intensity machines delivering mega-watt beams for applications such as spallation neutron production, muon colliders, neutrino factories, nuclear-waste transmutation or inertial confinement fusion energy (IFE). Especially in the field of heavy ion driven inertial confinement fusion, where space charge dominated multi kilo-ampere beams have to be transported over several meters through a reactor chamber to a mm-size target, some kind of beam neutralization is required. Among the possible solutions is the plasma-channel based final transport. High-current discharge channels offer unique properties for ion-beam transport. They can neutralize both current and space charge of very intense ion beams and provide a strong azimuthal magnetic field all the way along the channel. This work describes experiments performed at the Gesellschaft fuer Schwerionenforschung (GSI) in Darmstadt and the Lawrence Berkeley National Laboratory (LBNL) to create and diagnose high current discharge channels designed for the transport of intense ion beams. In both experiments, stable channels are created by a laser pulse which is fired into the chamber along the desired path of breakdown before the discharge is triggered. While in Berkeley a UV-laser creates an initial seed of electrons by two-photon ionization of a small admixture of Benzene molecules to the background gas, at GSI an IR-laser is used to heat ammonia gas by resonant absorption and to produce a gas-rarefaction channel with preferred conditions for a breakdown. An alternate technique, using an ion beam to initiate stable, free-standing channels has been tested successfully for the first time at the UNILAC- linear accelerator at GSI. All channels, created by either of these three methods show no signs of hydrodynamic instabilities in a wide parameter range. Several plasma diagnostics were developed and used to gain a deeper understanding of the dynamics and stability of the discharges. In order to measure the electron density with spatial resolution a two-color imaging interferometer was developed. Peak densities around 1017 to 1018 cm-3 were inferred, depending on the discharge conditions. The interferometer also showed the existence of a rarefaction channel and a radially expanding gas wall which is believed to enhance the stability of the discharge. This pressure wave was studied in detail by means of schlieren diagnostics. In Berkeley a Faraday polarimeter was developed to determine the current density distribution and thus the magnetic field inside the plasma, from the rotation of the plane of polarization of a CO2 laser beam passing through the plasma. The plasma-self emission was investigated by spectroscopy in the visible wavelength range. Measurements of the Stark broadening of hydrogen Balmer lines were used for an alternate electron-density measurement. To fit theoretical spectra to the measurements, a rate model for a nitrogen plasma was developed and a maximum electron temperature of 7 eV was derived. For an application of plasma channels to the final focus and transport inside an inertial-confinement fusion reactor with two-sided target illumination it is required to produce intersecting discharges to provide a focusing magnetic field for two ion beams propagating from opposite sides towards the target. Experiments at GSI demonstrated that X-shaped, T-shaped and L-shaped discharges can be produced by CO2 laser initiation. The ion optical properties of such T-discharges were investigated with a 660 MeV Ni+12 heavy-ion beam from the UNILAC-linear accelerator at GSI.