Diagnostics in a single element of a matrix source of negative hydrogen ionsa) St. Lishev, D. Yordanov, and A. Shivarova Citation: Review of Scientific Instruments 85, 02B101 (2014); doi: 10.1063/1.4826338 View online: http://dx.doi.org/10.1063/1.4826338 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Basis of the discharge maintenance in a matrix source of negative hydrogen ionsa) Rev. Sci. Instrum. 85, 02B105 (2014); 10.1063/1.4826541 Wave frequency dependence of H− ion production and extraction in a transformer coupled plasma H− ion source at SNUa) Rev. Sci. Instrum. 83, 02A727 (2012); 10.1063/1.3678659 Matrix of small-radius radio-frequency discharges as a volume-production based source of negative hydrogen ionsa) Rev. Sci. Instrum. 83, 02A702 (2012); 10.1063/1.3662019 Plasma diagnostic tools for optimizing negative hydrogen ion sources Rev. Sci. Instrum. 77, 03A516 (2006); 10.1063/1.2165769 Negative hydrogen ions and evidence for a potential dip in an electron cyclotron resonance discharge for highly charged ions Rev. Sci. Instrum. 69, 1197 (1998); 10.1063/1.1148664

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02B101 (2014)

Diagnostics in a single element of a matrix source of negative hydrogen ionsa) St. Lishev,b) D. Yordanov, and A. Shivarova Faculty of Physics, Sofia University, 5 J. Bourchier Blvd., Sofia BG-1164, Bulgaria

(Presented 10 September 2013; received 2 September 2013; accepted 30 September 2013; published online 25 October 2013) A small radius discharge in hydrogen inductively driven by a planar coil is studied experimentally regarding development of a matrix source of negative hydrogen ions: a matrix of small radius discharges, each of them completed with a magnetic filter and a single aperture extraction device. Probe and laser photodetachment diagnostics are the methods employed. Results for the axial structure of the discharge show the influence of the absorbed rf power, of the magnetic filter (and its position), and of the bias applied to the first electrode of the extraction device on the spatial distribution of the plasma parameters and on the discharge modes. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826338] I. INTRODUCTION

Recent development towards non-ceasiated rf source of negative hydrogen ions (H− ) for fusion applications has provoked looking for a new design of the source which could ensure high yield of volume produced ions. Results from modeling1–3 for high H− -accumulation at the position of the maximum of the dc potential in the discharge via the flux of the ions in the dc field when the discharge radius is small (R = (2–3) cm) have led to the concept4 for the matrix source: a matrix of small radius discharges inductively driven by a planar coil. Measurements of the H− -density n– in small-scale arrangements of a two-chamber rf source have shown5, 6 higher n– in the first chamber where the rf power is applied compared to the expanding plasma region in the second chamber. The diagnostics presented here is a step in the work on the matrix source. The axial distribution of the plasma parameters is studied in a single element of the matrix. II. EXPERIMENTAL SETUP

Probe and laser photodetachment diagnostics7 have been carried out in a single discharge (Fig. 1) of the matrix: a small radius (R = 2.25 cm) discharge inductively driven (at 27 MHz) by a planar coil completed with a magnetic filter (MF, a dipole magnetic field produced by two permanent magnets (Fig. 2(a))) and a Faraday cup (FC) as a device for the H− -extraction (Fig. 1). The position z = 0 (Fig. 2(b)) of the center of the MF field is denoted further on as zMF . Aiming at control on the axial distribution of the dc potential,8 the position zMF of MF and the bias UPE applied to the first electrode (the plasma electrode (PE)) of FC have been varied: zMF = (2–12) cm and UPE = (0–60) V. The rf power Pw applied for sustaining the discharge has been also varied (Pw = (50–200) W). Axial profiles (Figs. 3–6) of the

plasma parameters (electron density ne and temperature Te , plasma potential Upl , electronegativity α = n– /ne , and negative ion density n– ) are measured by a Langmuir probe (going into the plasma through the coil) and a laser beam, both axially movable (Fig. 1). The obtained results are at gas pressure p = 6 mTorr and gas flow of 1.5 sccm. (In Figs. 3–6, z = 0 is the position of the coil (a 3.5 turn coil) and the position zPE = 12 cm of PE fixes the discharge length.) Nd:YAG laser operating at wavelength of 532 nm is used. The probe tip, inserted in the laser beam, is parallel to it. The probe and PE are biased with respect to a grounded electrode immersed into the plasma. For reducing the disturbance of the discharge by the probe, a thin ceramic tube (diameter of 1 mm) is used as a probe holder. Outside the discharge tube the probe is shielded against the influence of the rf field of the coil. The reliability of the probe configuration in Fig. 1 has been checked both with and without FC by comparison with results obtained with a probe mounted on the back flange of the source, i.e., a probe entering the discharge tube on the second-chamber side. The measurements without FC show agreement with former results.6 The results obtained with FC present and a probe passing through its aperture are in agreement with those presented in Figs. 4–6.

a) Contributed paper, published as part of the Proceedings of the 15th Interna-

tional Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:

[email protected]. 0034-6748/2014/85(2)/02B101/3/$30.00

FIG. 1. Experimental arrangements. 85, 02B101-1

© 2013 AIP Publishing LLC

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02B101-2

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Rev. Sci. Instrum. 85, 02B101 (2014)

FIG. 2. Configuration of MF (a) and axial variation of its field on the discharge axis (b); z = 0 is the position of the center of the filter.

III. RESULTS AND DISCUSSIONS

The axial profiles of n– obtained without MF show the decrease of n– along the discharge length (Fig. 3). The higher n– close to the coil at Pw = 200 W is related to the higher ne (Fig. 4(a)) there when the discharge operates in the inductive mode,9 compared to the capacitive mode at lower Pw (Pw = 100 W, Fig. 5(a)). However, in both cases n– drops almost to the same value at the position of PE (Fig. 3), and the n– profiles are only slightly influenced by the PE bias. Completing the discharge with MF causes drastic changes in the axial profiles of ne , Te , and Upl (Figs. 4 and 5). In accordance with the results from modeling,10 (i) Te drops sharply in the filter region and stays at almost the same (low) value behind it, (ii) ne , decreasing in front of the filter, reaches its minimum in the filter region followed by a maximum and further decreases behind the filter, and (iii) Upl decreases due to the reduced losses when the plasma is in an external magnetic field. This pattern of changes in the axial profiles of the plasma parameters shifts along the discharge length with shifting the position of MF from zMF = 2 cm to zMF = 12 cm. MF strongly affects also the mode of the discharge operation. The discharge sustained at Pw = 200 W is in an inductive mode, both without and with MF. However, the discharge sustained at Pw = 100 W, which without MF is in a capacitive mode, transforms into an inductive mode at zMF = (2–8) cm, i.e., when MF is close to the rf power deposition. With MF positions close to PE (zMF = (10–12) cm) the discharge is sustained in capacitive mode. Applying a bias to PE, with MF present, also causes strong changes, mainly in the axial variation of Upl (Fig. 6). With UPE applied, the total axial profiles of the plasma potential shift up. The effect of MF is to form almost a plateau in the Upl -profiles in the filter region and behind it, after the initial decrease of Upl in front of MF. Over the last centimeter before PE Upl sharply goes to the value of the potential

FIG. 3. Axial profiles of the negative ion density obtained without a magnetic filter for two values of the applied rf power and different values of the bias applied to the plasma electrode.

FIG. 4. Axial profiles of the electron density (a) and temperature (b) and of the plasma potential (c) obtained at rf power Pw = 200 W for different positions zMF of the magnetic filter without bias applied to the plasma electrode (UPE = 0 V). Results without magnetic filter are also shown.

applied to PE. The dc electric field formed in front of PE is stronger (and in a proper direction) at higher UPE (UPE = 60 V in Fig. 6). Although this means that H− -ions will reach the extraction device, due to the slight increase of Upl towards the plasma interior, only H− -ions locally produced in front of PE can do it.

FIG. 5. The same as in Fig. 4 but for applied rf power Pw = 100 W.

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02B101-3

Lishev, Yordanov, and Shivarova

Rev. Sci. Instrum. 85, 02B101 (2014)

Without MF the H− -density close to the position of the extraction is comparable and even higher than that in the inductive mode (Fig. 3). However, with MF the inductive mode is that displaying higher n– : The highest value of n– close to the position of the extraction (the zPE -position) is obtained at Pw = 200 W, UPE = 60 V, and zMF = 12 cm, i.e., when MF is positioned at PE (Figs. 7(a) and 7(b)). The results in Figs. 7(b) and 7(c) show the shift of the maximum of n– towards the position of the extraction with increasing UPE . Regardless of MF present and a potential UPE applied to PE, the high n– is still kept close to the coil (Fig. 3), at the position of the maximum of Upl , slightly influenced by MF and UPE . IV. CONCLUSION FIG. 6. Axial profiles of Upl obtained at Pw = 200 W for different zMF -positions and bias UPE = 40 V (a) and UPE = 60 V (b) applied to PE.

A strong impact of MF on the discharge structure is shown, with influence not only on the spatial distribution of the plasma parameters but also on the modes of the discharge maintenance: MF positioned close to the power deposition force a transition from capacitive to inductive mode operation. Regarding the H− -ions, an increase of the rf power in a combination with high UPE and MF close to PE increases their density in the extraction region. However, due to the dc potential decrease in the expanding plasma region of the planar-coil inductive discharges, the efficient H− yield is close to the coil. Shortening the discharge length with measurements of the extracted current densities will be the next step in the work. ACKNOWLEDGMENTS

The work has been supported by the European Atomic Energy Agency (EURATOM) and Bulgarian Science Fund through the Association EURATOM/INRNE (Task 2.1.1). Support to the experimental equipment from Bulgarian Science Fund via project D01-413 is also acknowledged. 1 Ts.

FIG. 7. Dependence of the H− -density in [m−3 ], obtained at z = 11 cm for different positions zMF of the magnetic filter, on the bias applied to the plasma electrode at Pw = 200 W (a) and on the power applied for the discharge maintenance at UPE = 60 V (b) and UPE = 0 V (c).

Paunska, A. Shivarova, and Kh. Tarnev, J. Appl. Phys. 107, 083301 (2010). 2 Ts. V. Paunska, A. P. Shivarova, and Kh. Ts. Tarnev, AIP Conf. Proc. 1390, 165 (2011). 3 Ts. V. Paunska, A. P. Shivarova, Kh. Ts. Tarnev, and D. T. Todorov, AIP Conf. Proc. 1515, 99 (2013). 4 St. Lishev, Ts. Paunska, A. Shivarova, and Kh. Tarnev, Rev. Sci. Instrum. 83, 02A702 (2012). 5 St. Lishev, A. Shivarova, and Ts. Tsankov, J. Phys.: Conf. Proc. 223, 012001 (2010). 6 St. St. Lishev, A. P. Shivarova, D. I. Iordanov, D. T. Todorov, and A. P. Demerdzhiev, AIP Conf. Proc. 1515, 270 (2013). 7 M. Bacal, Rev. Sci. Instrum. 71, 3981 (2000). 8 M. Bacal, Nucl. Fusion 46, S250 (2006). 9 St. Lishev, A. Shivarova, Kh. Tarnev, S. Iordanova, I. Koleva, Ts. Paunska, and D. I. Iordanov, J. Phys. D: Appl. Phys. 46, 165204 (2013). 10 St. Kolev, St. Lishev, A. Shivarova, Kh. Tarnev, and R. Wilhelm, Plasma Phys. Control. Fusion 49, 1349 (2007).

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Diagnostics in a single element of a matrix source of negative hydrogen ions.

A small radius discharge in hydrogen inductively driven by a planar coil is studied experimentally regarding development of a matrix source of negativ...
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