1. Optimum Plasma Grid Bias in a Negative Hydrogen Ion Source Operation with Caesium M. Bacal 1, M. Sasao 2, M. Wada 3, R. McAdams 4 1 UPMC, LPP, Ecole Polytechnique, France 2 R&D Promotion Organization, Doshisha University, Japan 3 School of Science and Engineering, Doshisha University, Japan 4 CCFE, Culham Science Center, Abingdon, Oxfordshire 0X14 3DB, UK.

2. INTRODUCTION * The plasma electrode (PE) is a major component of negative ion sources Since it contains the extraction opening. * The contemporary negative ion source is often a magnetic multipole or bucket plasma source. The figure presents schematically the filamented ion source geometry. RF or microwave discharges can replace the arc discharge for plasma generation.

3. ROLES OF THE PLASMA ELECTRODE IN PURE HYDROGEN VOLUME SOURCES

4. I.The PE Bias Controls the Curent Collected by the Plasma Electrode and thus the positive Vb depletes the Electron Density near the PE

5. Culham large filament-driven NIS for fusion

6. Plasma electrode current-voltage characteristic in the Culham ion source Holmes, Rev. Sci. Instrum. 53 (10) 1517 (1982) Note that the saturation positive ion current has been subtracted.

7. Microwave driven Camembert III ion source In a filamented version of the source, we performed the comparison of *pure hydrogen and * caesiated operations.

8. The PE Bias Controls the Curent it Collects as well as extracted electron and H- currents Svarnas et al, IEEE TPS, 35 (4) 1156 (2007 ) Plasma produced by a 2D network of ECR Dipolar Plasma Sources The maximum negative ion extraction occurs for V b slightly higher than the local plasma potential.

9. II. The Positive PE Bias Controls the Extracted Currents Leung et al in LBL found that in the presence of a magnetic filter a positive bias applied to the plasma electrode can produce a sizable increase of the extracted H- current along with a significant reduction of the co-extracted electron current. A maximum in the H - current variation with V b occurs with a strong magnetic filter. Ref: Leung et al, Rev. Sci. Instrum. 54 (1) 56 (1983)

10. Results from Leung et al, 54 (1), 56 (1983 )

11. Results from Ecole Polytechnique Bacal, Bruneteau and Devynck, Rev. Sci. Instrum., 59 (10), 2152 (1988) The 3 curves correspond to different transverse magnetic fields near the PE surface. The strongest B corresponds to the highest peak of I- and most rapid decrease of Ie.

12. III. The Positive PE Bias Controls the Negative Ion and Electron Densities in the Center of the Extraction (Target) Region of the Tandem Multipole Camembert II Results from Leung and Bacal, Rev. Sci. Instrum. 55 (3), 338 (1984) The variation of the relative negative ion density confirms the peak in the variation of the extracted current. Camembert II was equipped with a rod-type magnetic filter The relative negative ion density attains 12%

13. IV. The Positive PE Bias Controls the Negative Ion and Electron Densities near the PE, in Camembert III. Measurement effected at 8 mm from the PE, in a 50 V, 50 A, 3 mTorr discharge The relative negative ion density attains n-/n e = 12. Camembert III has a virtual magnetic filter Results from El Balghiti et al, Rev. Sci. Instrum. 67 (6) 2221 (1998)

14. V. The PE Bias Controls the Potential Distribution across the Magnetic Filter Results from Leung and Bacal, Rev. Sci. Instrum. 55 (3), 338 (1984)

15. With V b = 0 the H- ions are trapped electrostatically in the driver region, While With V b = +2.5 V the plasma potential gradient is reduced and some H- ions from the driver can traverse the filter field and reach the extraction region. Thus the PE bias controls the flow of H- ions formed in the driver region into the extraction region.

16. VI. The PE Surface Recycles Atoms to Vibrationally Excited Molecules Note that the H - ion current is 2.5 times larger when a fresh tantalum or caesium film is deposited on the PE, compared to the case when the PE is covered with tungsten.

17. This is due to the higher cross section for formation of H 2 (v’’) in high v’’ states by recombinative desorption on the tantalum covered PE, compared to the tungsten covered one. The H 2 (v’’) populations were reported by Hall et al in 1988 R.I. Hall et al, Phys. Rev. Lett. 60, 337 (1988)

18. Comparison of pure hydrogen and caesiated operation in Camembert III The data indicate a clear threshold of the surface component; i.e. a sudden decrease in the extracted H- ion current near V b =0.5 V. In caesiated operation I (H-) is enhanced at plasma potential by a factor 2.5 compared to pure hydrogen operation. From Bacal, El Balghiti-Sube, Elizarov and Tontegode, Rev. Sci. Instrum. 69 (2) 932 (1998)

19. Bacal, Elizarov and Tontegode, Rev. Sci. Instrum., 69 (2)932 (1998)From

20. Possible explanations of the H- current enhancement due to caesium seeding In the whole bias range gettering of atomic hydrogen by caesium can lead to the enhancement of H- ion density and current, due to the reduction of the recombinative detachment. * At bias voltage below plasma potential ( V b < V p ) the H- ion current enhancement is usually ascribed to the direct production of H- ions by positive ions and atoms incident onto the low work function caesiated surface. At bias voltage above plasma potential ( V b < V p ) the H- current is enhanced compared to its level in pure hydrogen, although direct production is not active since the negative ion cannot leave the surface. Two explanations for the presence of H- ion current in this range can be proposed: - gettering of atomic hydrogen by caesium - H- ion flow from the bulk plasma.

21. Effect of Bias Voltage on Negative Ion Density and Extracted D- Current In a Deuterium Discharge in BATMAN (IPP Garching) 53 kW, 0.45 Pa Discharge From Christ-Koch et al, Plasma Sources Sci. Technol. 18, 025003 (2009)

22. CHOICE OF THE OPTIMUM BIAS VOLTAGE This choice is determined by the preference of a high negative ion current, or * a low co-extracted electron current. If a high negative ion current is essential, V b should be chosen below (with Cs) or around (in pure H 2 ) the plasma potential. If a low co-extracted electron current is essential, V b should be chosen above the plasma potential, when the positively biased plasma electrode depletes the electron population in the extraction region.

23. Effect of the PE bias set voltage on bias voltage, bias current and extracted currents in BATMAN (IPP Garching) The shaded area corresponds to the Operation Point, which is chosen near the floating potential.

24. CONCLUSION 1. In small volume H- sources with a strong magnetic filter a maximum is observed in the variation of I- with V b. In large H- volume sources the maximum of I(H-) was not observed. 2. A smooth decrease of the co-extracted electron current is also observed. 3. In pure H 2 sources only the positive PE causes change in plasma parameters in the extraction region and thus improvement in extraction. 4. In Cs seeded sources H- ions are formed also by direct H- production by positive ion impact on the PE surface. Therefore negative PE also enhances the extracted negative ion current. 5. Proper PE bias realizes enhanced transport of H - ions formed by volume production in the bulk plasma toward the PE and maximizes the ratio of H - current to electron current.