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Title: A Negative Hydrogen-Ion Source for SNS Using a Helicon Plasma Generator

Abstract

The Spallation Neutron Source (SNS) at Oak Ridge National Labo ratory is a world-class facility for materials research based on neutron scattering. It consists of a negative hydrogen (H-) ion source, linear accelerator, proton accumulator ring, and liquid Hg target. A power up grade is planned for the device, which will require significant improvements in the negative ion source, including the production of H- beam currents of 70-95 mA (~2 the present SNS source value), with a pulse length of 1 ms and duty factor of ~ 7%. No H- sources currently in existence meet these combined requirements. A proof-of-principle experiment is being constructed in which the rf inductive plasma generator in the present source is replaced by a helicon plasma generator. This is expected to produce a factor of three or better in crease in the maximum source plasma density at a reduced rf power level, resulting in significantly increased negative ion current with reduced heat removal requirements.

Authors:
 [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1128948
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Conference
Resource Relation:
Conference: 17th Topical Conference on Radio Frequency Power in Plasmas, Clearwater, FL, USA, 20070507, 20070509
Country of Publication:
United States
Language:
English

Citation Formats

Goulding, Richard Howell, Welton, Robert F, Baity Jr, F Wallace, Crisp, Danny W, Fadnek, Andy, Kang, Yoon W, Murray Jr, S N, Sparks, Dennis O, and Stockli, Martin P. A Negative Hydrogen-Ion Source for SNS Using a Helicon Plasma Generator. United States: N. p., 2007. Web.
Goulding, Richard Howell, Welton, Robert F, Baity Jr, F Wallace, Crisp, Danny W, Fadnek, Andy, Kang, Yoon W, Murray Jr, S N, Sparks, Dennis O, & Stockli, Martin P. A Negative Hydrogen-Ion Source for SNS Using a Helicon Plasma Generator. United States.
Goulding, Richard Howell, Welton, Robert F, Baity Jr, F Wallace, Crisp, Danny W, Fadnek, Andy, Kang, Yoon W, Murray Jr, S N, Sparks, Dennis O, and Stockli, Martin P. Mon . "A Negative Hydrogen-Ion Source for SNS Using a Helicon Plasma Generator". United States. doi:.
@article{osti_1128948,
title = {A Negative Hydrogen-Ion Source for SNS Using a Helicon Plasma Generator},
author = {Goulding, Richard Howell and Welton, Robert F and Baity Jr, F Wallace and Crisp, Danny W and Fadnek, Andy and Kang, Yoon W and Murray Jr, S N and Sparks, Dennis O and Stockli, Martin P},
abstractNote = {The Spallation Neutron Source (SNS) at Oak Ridge National Labo ratory is a world-class facility for materials research based on neutron scattering. It consists of a negative hydrogen (H-) ion source, linear accelerator, proton accumulator ring, and liquid Hg target. A power up grade is planned for the device, which will require significant improvements in the negative ion source, including the production of H- beam currents of 70-95 mA (~2 the present SNS source value), with a pulse length of 1 ms and duty factor of ~ 7%. No H- sources currently in existence meet these combined requirements. A proof-of-principle experiment is being constructed in which the rf inductive plasma generator in the present source is replaced by a helicon plasma generator. This is expected to produce a factor of three or better in crease in the maximum source plasma density at a reduced rf power level, resulting in significantly increased negative ion current with reduced heat removal requirements.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Conference:
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  • The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is a world-class facility for materials research based on neutron scattering. It consists of a negative hydrogen (H-) ion source, linear accelerator, proton accumulator ring, and liquid Hg target. A power upgrade is planned for the device, which will require significant improvements in the negative ion source, including the production of H-beam currents of 70-95 mA ({approx}2xthe present SNS source value), with a pulse length of 1 ms and duty factor of {approx}7%. No H-sources currently in existence meet these combined requirements. A proof-of-principle experiment is being constructed in whichmore » the rf inductive plasma generator in the present source is replaced by a helicon plasma generator. This is expected to produce a factor of three or better increase in the maximum source plasma density at a reduced rf power level, resulting in significantly increased negative ion current with reduced heat removal requirements.« less
  • Helicon plasma generators are widely used for plasma processing applications due to their long life-time and capability of creating high-density plasmas efficiently. The aim of the helicon plasma generator-assisted negative ion source project at Los Alamos Neutron Science Center (LANSCE) is to use these features for producing intense beams of H- ions. Our development work builds upon pioneering experiments previously conducted at Lawrence Berkeley National Laboratory (LBNL) with a 2.45 GHz electron cyclotron resonance plasma generator. In the new approach a helicon plasma generator is used as a plasma cathode injecting electrons into a multi-cusp H- ion source. The secondarymore » source can be operated without filaments or any other consumable parts and, consequently, the life-time of the ion source can be extended significantly. The development of the ion source is aimed to meet the beam production goals of the LANSCE 800 MeV linear accelerator refurbishment project i.e. 20 mA of H- beam with normalized area emittance (95 % of the beam) less than 1.1 {pi}{center_dot}mm{center_dot}mrad and a duty factor of 12 %. The operation principle of the source, the test stand design and the status of the development work will be presented in this article.« less
  • In order to develop very high energy (>1 MeV) neutral beam injection systems for applications, such as plasma heating in fusion devices, it is necessary first to develop high throughput negative ion sources. For the ITER reference source, this will be realised using caesiated inductively coupled plasma devices, containing either hydrogen or deuterium discharges, operated with high rf input powers (up to 90 kW per driver). It has been suggested that due to their high power coupling efficiency, helicon devices may be able to reduce power requirements and potentially obviate the need for caesiation due to the high plasma densities achievable. Here,more » we present measurements of negative ion densities in a hydrogen discharge produced by a helicon device, with externally applied DC magnetic fields ranging from 0 to 8.5 mT at 5 and 10 mTorr fill pressures. These measurements were taken in the magnetised plasma interaction experiment at the Australian National University and were performed using the probe-based laser photodetachment technique, modified for the use in the afterglow of the plasma discharge. A peak in the electron density is observed at ∼3 mT and is correlated with changes in the rf power transfer efficiency. With increasing magnetic field, an increase in the negative ion fraction from 0.04 to 0.10 and negative ion densities from 8 × 10{sup 14 }m{sup −3} to 7 × 10{sup 15 }m{sup −3} is observed. It is also shown that the negative ion densities can be increased by a factor of 8 with the application of an external DC magnetic field.« less
  • The converter-type negative ion source currently employed at the Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H{sup -} ion beams in a filament-driven discharge. In this kind of an ion source the extracted H{sup -} beam current is limited by the achievable plasma density which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which deposits on the H{sup -}more » converter surface and degrades its performance. Therefore, we have started an ion source development project focused on replacing these thermionic cathodes (filaments) of the converter source by a helicon plasma generator capable of producing high-density hydrogen plasmas with low electron energy. In our studies which have so far shown that the plasma density of the surface conversion source can be increased significantly by exciting a helicon wave in the plasma, and we expect to improve the performance of the surface converter H{sup -} ion source in terms of beam brightness and time between services. The design of this new source and preliminary results are presented, along with a discussion of physical processes relevant for H{sup -} ion beam production with this novel design. Ultimately, we perceive this approach as an interim step towards our long-term goal, combining a helicon plasma generator with an SNS-type main discharge chamber, which will allow us to individually optimize the plasma properties of the plasma cathode (helicon) and H{sup -} production (main discharge) in order to further improve the brightness of extracted H{sup -} ion beams.« less
  • Existing linear plasma materials interaction (PMI) facilities all use plasma sources with internal electrodes. An rf-based helicon source is of interest because high plasma densities can be generated with no internal electrodes, allowing true steady state operation with minimal impurity generation. Work has begun at Oak Ridge National Laboratory (ORNL) to develop a large (15 cm) diameter helicon source producing hydrogen plasmas with parameters suitable for use in a linear PMI device: n(e) >= 10(19)m(-3), T(e) = 4-10 eV, particle flux Gamma(p) > 10(23) m(-3) s(-1), and magnetic field strength |B| up to I T in the source region. Themore » device, whose design is based on a previous hydrogen helicon source operated at ORNL[1], will operate at rf frequencies in the range 10 - 26 MHz, and power levels up to similar to 100 kW. Limitations in cooling will prevent operation for pulses longer than several seconds, but a major goal will be the measurement of power deposition on device structures so that a later steady state version can be designed. The device design, the diagnostics to be used, and results of rf modeling of the device will be discussed. These include calculations of plasma loading, resulting currents and voltages in antenna structures and the matching network, power deposition profiles, and the effect of high |B| operation on power absorption.« less