Extraction of negative hydrogen ions using a plasma electrode covered by Ta or Ti

Abstract

We compared the effect of a tantalum (Ta) fresh coating with that of a Ta thin sheet covering the plasma electrode of a negative hydrogen ion source on the ratio of extracted negative hydrogen (H) ion current to extracted electron current. Fresh Ta evaporation from a thin hot Ta filament showed about 22 % increase of extracted H ion current compared to the operation with tungsten (W) hot filament. On the other hand, a thin Ta foil covering the plasma electrode decreased the extracted electron current without any large influence on H ion current. Meanwhile, when a titanium (Ti) foil covered the plasma electrode, both the extracted H ion current and electron current decreased substantially. At a plasma electrode bias higher than the plasma potential, the ratio of H ion current to the electron current for the Ti covered plasma electrode exceeded 80%. INTRODUCTION Neutral beam injection (NBI) heating requires a negative hydrogen (H) ion beam since the neutralization efficiency at higher beam energy becomes negligible for a positive hydrogen ion beam [1]. It is common to deposit cesium (Cs) on the plasma electrode surface as a method for improving H ion production efficiency and reduce coextracted electron current [2]. However, introducing Cs to an H source can cause Cs leakage to high voltage holding system of the accelerator leading to a breakdown. Thus, the use of Cs is preferred to be avoided and materials which could be used to realize Cs-free ion source operation are currently investigated [3,4]. Source operation with tantalum (Ta) filaments as the arc discharge cathode is known to increase the production of negative hydrogen ions while reducing the co-extracted electron current [5,6]. High-temperature cathode filaments made of Ta as compared to tungsten (W), may improve the production rate of hydrogen vibrationally excited molecules (H2) and thus that of H ions in a hydrogen plasma [7]. There is another possible explanation of the observed enhanced H ion production. A plasma electrode coated with Ta may efficiently absorb hydrogen atoms which destroy the hydrogen negative ions by associative detachment [8]. Meanwhile, when a Ta foil covered the plasma electrode surface, it did not show an enhancement as large as that observed by evaporation [9]. Titanium (Ti) and Ti alloys also absorbs hydrogen atoms at elevated temperature [10]. Thus, a thin foil of Ti could be placed on the surface of the plasma electrode if it improves the ratio of H ion current to electron current. In this experiment, the negative hydrogen ion density near the plasma electrode (nH) is measured by the laser photodetachment method [11,12], and the measured density is correlated to the extracted H ion current (IH). The H ion current and the co-extracted electron current (Iext) are measured simultaneously by sweeping the bias potential applied to the plasma electrode [13], and the electron current is estimated to be equal to Iext as the Faraday cup and the extractor are biased separately. The electron density (ne), electron temperature (Te) and plasma potential (Vp) are determined from the Langmuir probe measurements. EXPERIMENTAL SETUP A single filament serves as the cathode to sustain an arc discharge in a cylindrical ion source. Figure 1 shows a schematic diagram of the ion source chamber. The chamber has a diameter of 150 mm and a height of 200 mm. Connecting a turbo molecular pomp and a rotary pump in series, ultimate vacuum pressure reaches 5.0×10 Pa in the ion source chamber. A stainless-steel plate with four pumping orifices of 12.7 mm diameter separates the main chamber and the manifold connected to the turbo molecular pump which stably keeps the H2 gas pressure of plasma production region at 0.5 Pa. Sixteen rows of samarium cobalt magnets equipped on the outside chamber wall forms a hexadecapole multi-cusp magnetic field. The chamber side wall has twelve ICF-34 flange ports aligned on three planes with four ports arranged to observe the axis of the chamber at right angles. A water cooling system protects permanent magnets from the discharge heating. The filament dimensions are 0.5 mm in diameter and 90 mm long. A single filament is set at the bottom position of the chamber where the distance from the bottom flange is 45 mm. The filament is negatively biased at 80 V against the chamber wall to drive a DC plasma at 1 A discharge current. Figure 2 shows a diagram of the details of the extraction structure. A pair of permanent magnet forms the filter magnetic field, and creates a volume of low temperature plasma in front of the extraction hole. A 145 mm diameter, 2 mm thick stainlesssteel plasma electrode (PE) is introduced into the chamber separated by 25 mm from the top flange. The PE has an extraction hole of 5 mm diameter. A floating electrode (FE), separated by 2 mm from the PE, has a center hole of 30 mm diameter and 1.5 mm thickness. The FE made of stainless-steel masks the PE surface to protect the PE surface from direct exposure to the plasma except the center 30 mm diameter region. A Ta or a Ti foil which has the area of 50×50 mm and a thickness of 0.1 mm is attached between the PE and the FE as shown in Fig. 3. Electrons and negative ions are extracted by biasing the extraction electrode at 800 V electrical potential. The H ion current is measured by the Faraday cup also biased at 800 V with respect to the ion source chamber ground. A pair of permanent magnets on the top flange provides a magnetic field in the extraction region. The extraction electrode collects only the electron current, Iext and the Faraday cup collects only the negative ion current, IHbecause of the magnetic field present in the region between the extraction hole and the entrance of the Faraday cup; this field serves as the electron suppression field. An L-shaped 0.5 mm diameter 12 mm long Langmuir probe made of W is inserted into the top part of the chamber. The distance between the PE and the Langmuir probe is 12 mm. FIGURE 2. A diagram of the extraction system. Broken lines show magnetic field lines of force. FIGURE 1. The diagram of the experimental setup. FIGURE 3. A schematic showing how to cover PE with a foil. Figure 4 represents the schematic diagram of the electrical connection of the experimental system. Devices not shown in the figure include the current monitor of the Faraday cup high voltage power supply, the current monitor of the extraction voltage, and the current monitor of the plasma electrode bias. The location of the vacuum ionization gauge is at the center plane of the ion source chamber. RESULTS Extracted H Ions and Electrons from a Hydrogen Plasma We conducted experiment by five patterns as follows; W filament with stainless steel (SUS) PE, Ta filament with SUS PE, W filament with Ta foil PE, W filament with Ti foil PE and Ta filament with Ta foil PE. Figure 5(a) shows the extracted electron current Iext , and Figure 5 (b) shows the extracted negative hydrogen ion current, IHas a function of PE bias voltage Vb. (a) (b) FIGURE 4. The schematic diagram of the experimental apparatus: (a) the overall view, (b) the details of the extraction system. (a) (b) FIGURE 5. The effect of different hot cathodes and foil materials on PE for extracted H ion and electron currents versus PE bias voltage: (a) co-extracted electron current, (b) H ion current. 3.0