Radiofrequency and 2.45 GHz electron cyclotron resonance H⁻volume production ion sources

Abstract

The volume production of negative hydrogen ions ( -H ) in plasma ion sources is based on dissociative electron attachment (DEA) to rovibrationally excited hydrogen molecules (H 2 ), which is a two-step process requiring both, hot electrons for ionization, and vibrational excitation of the H 2 and cold electrons for the -H formation through DEA. Traditionally -H ion sources relying on the volume production have been tandem-type arc discharge sources equipped with biased filament cathodes sustaining the plasma by thermionic electron emission and with a magnetic filter separating the main discharge from the -H formation volume. The main motivation to develop ion sources based on radiofrequency (RF) or electron cyclotron resonance (ECR) plasma discharges is to eliminate the apparent limitation of the cathode lifetime. In this paper we summarize the principles of -H volume production dictating the ion source design and highlight the differences between the arc discharge and RF/ECR ion sources from both, physics and technology point-of-view. Furthermore, we introduce the state-of-the-art RF and ECR -H volume production ion sources and review the challenges and future prospects of these yet developing technologies. Volume production of -HModern -H plasma ion sources rely on two ion formation processes, resonant-tunneling ionization on low work function surfaces [4] and volumetric ionization occurring in the plasma discharge [5]. Only the volume production will be discussed hereafter.

Production of hydrogen negative ions in an ECR volume source: balance between vibrational excitation and ionization

“…This can be attributed to the high dissociation degree of the source (about 12% for this source compared with 3% for typical filament sources [20]). The different dissociation degree is most likely due to the different EEDFs in the two discharge types, favoring triplet state excitation and subsequent dissociation in the case of microwave heating as discussed in [55]. Mutual neutralization is another important loss process, being about three times lower than dissociative attachment.…”

An overview of negative hydrogen ion sources for accelerators

“…PKU is currently developing a microwave driven volume source with a cooled RF window. On an ion source test stand, they have recently reported over 25 mA of CW H − beam current without the addition of caesium [38]. The previously highest reported CW currents for this type of source are 5 mA at Argonne National Laboratory [39] and 3.8 mA CEA Saclay [40].…”

Focus on sources of negatively charged ions

“…The production of a negative hydrogen ion via dissociative attachment of a low energy electron to an excited hydrogen molecule is the essence of the so-called 'volume production' [15]. Tarvainen and Peng review volume sources based on radio frequency (RF) and 2.45 GHz electron cyclotron resonance plasma heating; these H − sources demonstrated maintenance-free and high reliability operation [16]. The kinetics of electrons and neutrals are key aspects of negative hydrogen volume sources-Chung presents detailed analysis and experimental investigation of a 13 MHz RF transformer-coupled plasma driven by a spiral antenna with focus on the local electron energy distribution function and transport of excited hydrogen molecules form their production (driver) to the dissociative attachment and extraction regions [17].…”