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Title: Development of a rf negative-ion source for ITER neutral beam injection

Abstract

The development at IPP of a large-area rf source for negative hydrogen ions, an official EFDA task agreement, aims to demonstrate ITER-relevant source parameters. This implies a current density of 20 mA cm{sup -2} accelerated D{sup -} at a source-filling pressure of <0.3 Pa, an electron to ion ratio of <1, and for pulse lengths of up to 1 h. The principle suitability concerning current density, pressure, and electron content has been demonstrated with the test facility Bavarian Test Machine for Negative Ions but with only small extraction area (70 cm{sup 2}) and for pulse length of <6 s. The further development concentrates now on long pulse operation at the test stand Multi-Ampere Negative Ion Test Unit (MANITU), which became operational this spring. For source size extension from 70 to 1000 cm{sup 2} MANITU and a third test facility, currently under development, called RADI will be used. This article will report on the latest results of the work in progress. A critical issue for ITER is reliable source operation at high current densities. Therefore the procedure used to obtain in a reproducible manner source operation at the ITER target values will be detailed and discussed.

Authors:
; ; ; ; ; ; ; ; ;  [1]
  1. Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Postfach 1533, D-85740 Garching (Germany)
Publication Date:
OSTI Identifier:
20779004
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 77; Journal Issue: 3; Conference: 11. international conference on ion sources, Caen (France), 12-16 Sep 2005; Other Information: DOI: 10.1063/1.2166246; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ANIONS; CURRENT DENSITY; DEUTERIUM IONS; ELECTRONS; HYDROGEN IONS; ION SOURCES; ITER TOKAMAK; OPERATION; PLASMA BEAM INJECTION; PULSES; TEST FACILITIES

Citation Formats

McNeely, P., Falter, H.-D., Fantz, U., Franzen, P., Froeschle, M., Heinemann, B., Kraus, W., Martens, Ch., Riedl, R., and Speth, E. Development of a rf negative-ion source for ITER neutral beam injection. United States: N. p., 2006. Web. doi:10.1063/1.2166246.
McNeely, P., Falter, H.-D., Fantz, U., Franzen, P., Froeschle, M., Heinemann, B., Kraus, W., Martens, Ch., Riedl, R., & Speth, E. Development of a rf negative-ion source for ITER neutral beam injection. United States. doi:10.1063/1.2166246.
McNeely, P., Falter, H.-D., Fantz, U., Franzen, P., Froeschle, M., Heinemann, B., Kraus, W., Martens, Ch., Riedl, R., and Speth, E. Wed . "Development of a rf negative-ion source for ITER neutral beam injection". United States. doi:10.1063/1.2166246.
@article{osti_20779004,
title = {Development of a rf negative-ion source for ITER neutral beam injection},
author = {McNeely, P. and Falter, H.-D. and Fantz, U. and Franzen, P. and Froeschle, M. and Heinemann, B. and Kraus, W. and Martens, Ch. and Riedl, R. and Speth, E.},
abstractNote = {The development at IPP of a large-area rf source for negative hydrogen ions, an official EFDA task agreement, aims to demonstrate ITER-relevant source parameters. This implies a current density of 20 mA cm{sup -2} accelerated D{sup -} at a source-filling pressure of <0.3 Pa, an electron to ion ratio of <1, and for pulse lengths of up to 1 h. The principle suitability concerning current density, pressure, and electron content has been demonstrated with the test facility Bavarian Test Machine for Negative Ions but with only small extraction area (70 cm{sup 2}) and for pulse length of <6 s. The further development concentrates now on long pulse operation at the test stand Multi-Ampere Negative Ion Test Unit (MANITU), which became operational this spring. For source size extension from 70 to 1000 cm{sup 2} MANITU and a third test facility, currently under development, called RADI will be used. This article will report on the latest results of the work in progress. A critical issue for ITER is reliable source operation at high current densities. Therefore the procedure used to obtain in a reproducible manner source operation at the ITER target values will be detailed and discussed.},
doi = {10.1063/1.2166246},
journal = {Review of Scientific Instruments},
number = 3,
volume = 77,
place = {United States},
year = {Wed Mar 15 00:00:00 EST 2006},
month = {Wed Mar 15 00:00:00 EST 2006}
}
  • IPP Garching has successfully developed a RF driven negative ion source for the ITER neutral beam injection system. The RF source was chosen recently as the reference source for ITER due to its in principle maintenance-free operation. Current densities of 330 A/m{sup 2} and 230 A/m{sup 2} have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio of less than 1 for a small extraction area (7.0x10{sup -3} m{sup 2}) and short pulses (<4 s). The development concentrates now on extending the pulse length and extending the size of the source onmore » two dedicated test facilities. The pulse length can be extended up to one hour at the long pulse test facility having an extraction area of 0.02 m{sup 2}. The large source test facility is equipped a large RF source with the width and half the height of the ITER beam source in order to demonstrate the homogeneity of a large RF plasma. The paper will give a short overview on the results achieved at the three test facilities of IPP; the underlying physical mechanisms are discussed with an emphasis on electron suppression.« less
  • The international fusion experiment ITER requires for the plasma heating and current drive a neutral beam injection system based on negative hydrogen ion sources at 0.3 Pa. The ion source must deliver a current of 40 A D{sup -} for up to 1 h with an accelerated current density of 200 A/m{sup 2} and a ratio of coextracted electrons to ions below 1. The extraction area is 0.2 m{sup 2} from an aperture array with an envelope of 1.5x0.6 m{sup 2}. A high power rf-driven negative ion source has been successfully developed at the Max-Planck Institute for Plasma Physics (IPP)more » at three test facilities in parallel. Current densities of 330 and 230 A/m{sup 2} have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio below 1 for a small extraction area (0.007 m{sup 2}) and short pulses (<4 s). In the long pulse experiment, equipped with an extraction area of 0.02 m{sup 2}, the pulse length has been extended to 3600 s. A large rf source, with the width and half the height of the ITER source but without extraction system, is intended to demonstrate the size scaling and plasma homogeneity of rf ion sources. The source operates routinely now. First results on plasma homogeneity obtained from optical emission spectroscopy and Langmuir probes are very promising. Based on the success of the IPP development program, the high power rf-driven negative ion source has been chosen recently for the ITER beam systems in the ITER design review process.« less
  • Cesium seeded sources for surface generated negative hydrogen ions are major components of neutral beam injection systems in future large-scale fusion experiments such as ITER. The stability and delivered current density depend highly on the work function during vacuum and plasma phases of the ion source. One of the most important quantities that affect the source performance is the work function. A modified photocurrent method was developed to measure the temporal behavior of the work function during and after cesium evaporation. The investigation of cesium exposed Mo and MoLa samples under ITER negative hydrogen ion based neutral beam injection relevantmore » surface and plasma conditions showed the influence of impurities which result in a fast degradation when the plasma exposure or the cesium flux onto the sample is stopped. A minimum work function close to that of bulk cesium was obtained under the influence of the plasma exposition, while a significantly higher work function was observed under ITER-like vacuum conditions.« less
  • The neutral beam injection (NBI) system of ITER is based on a large RF driven negative hydrogen ion source. For good beam transmission ITER requires a beam homogeneity of better than 10%. The plasma uniformity and the correlation with the beam homogeneity are being investigated at the prototype ion sources at IPP. Detailed studies are carried out at the long pulse test facility MANITU with a source of roughly 1/8 of the ITER source size. The plasma homogeneity close to plasma grid is measured by optical emission spectroscopy and by fixed Langmuir probes working in the ion saturation region. Themore » beam homogeneity is measured with a spatially resolved H{sub {alpha}} Doppler-shifted beam spectroscopy system. The plasma top-to-bottom symmetry improves with increasing RF power and increasing bias voltage which is applied to suppress the co-extracted electron current. The symmetry is better in deuterium than in hydrogen. The boundary layer near the plasma grid determines the plasma symmetry. At high ion currents with a low amount of co-extracted electrons the plasma is symmetrical and the beam homogeneity is typically 5-10%(RMS). The size scaling and the influence of the magnetic field strength of the filter field created by a plasma grid current is studied at the test facility RADI (roughly a 1/2 size ITER source) at ITER relevant RF power levels. In volume operation in deuterium (non-cesiated source), the plasma illumination of the grid is satisfying.« less
  • An injector of hydrogen atoms with an energy of 0.5–1 MeV and equivalent current of up to 1.5 A for purposes of controlled fusion research is currently under design at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. Within this project, a multiple-aperture RF surface-plasma source of negative hydrogen ions is designed. The source design and results of experiments on the generation of a negative ion beam with a current of >1 A in the long-pulse mode are presented.