CW RF cesium-free negative ion source development at SNU

Transcription

1 CW RF cesium-free negative ion source development at SNU Bong-ki Jung, Y. H. An, W. H. Cho, J. J. Dang, Y. S. Hwang Department of Nuclear Engineering Seoul National University JP-KO Workshop on Phys. and Tech. of Heating and Current Drive Jan. 29 th 2013 Dept. of Nuclear Engineering, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea

2 Outline Introduction System setup Negative hydrogen ion source at SNU Ø H- current depends on Pressure & Filter field strength Ø H- current depends on Plasma Electrode Ø H- current depends on Driving RF Frequency Summary & Conclusion Future Work 2/29

3 Introduction High Energy Neutral Beam Injector [1] [2] [3] High energy(>1mev) neutral beam injector is required for heating & current drive source of high density fusion plasma like ITER or DEMO in considering penetration length and current drive efficiency To produce high-energy(>1mev) neutral beam, development of high current negative ion source is required due to higher neutralization efficiency than positive ion source. 3/29

4 Reaction branches of negative hydrogen ion production process H- ion production process Volume production Surface production H 2 *(v ) formation H 2 *(v ) destruction H - ion creation H - ion creation Low energy electron excitation High energy electron excitation Surface assisted excitation Surface recombination excitation Excited molecules relaxation Excited moleculeground state relaxation Excited molecule-atom relaxation Ionization reaction Dissociation reaction Wall relaxation Dissociation Attachment H - ion destruction Electronic detachment Mutual neutralization Associative detachment Atomic process Ionic process 4/29

5 Advantage of Cs free negative hydrogen ion source Hydrogen ion, atom Cesium ion, atom Magnetic Filter Field Hydrogen negative ion electron Extraction electrode N S Plasma N S 5/29 Cs Layered surface Plasma electrode Production of negative ion increases with feeding Cs due to decrease of work function of surface. However, deposition of Cs on electrodes can lead to breakdown between acceleration electrodes & complex chemistry interactions between Cs and plasma or wall is still a problem. Cs less operation of RF based negative ion source with large production of negative ions and lower operating pressure(<0.3 Pa[2.3mTorr]) to reduce heat load on electrode is highly required for stable long-pulse operation of the NBI device. [2]

6 Negative hydrogen ion production Volume production Step 1: Electron impact excitation Step 2: Dissociative electron attachment e(>~20ev) + H 2 (v =0) e + H 2 (v ) e(<1.0ev) + H 2 (v ³ 5) H - + H fast electron slow electron H 2 (v=0) H 2 (v>0) H 2 (v>0) H- H [4] J.R. Hiskes, J. Appl. Phys. 51, 4592, (1980) [5] A.P. Hickman, Phys. Rev., A43, 3495, (1991) 6/29

7 Concept of RF based negative ion source Primary Ionization Region H- Formation Region Multicusp Magnet e- (cold) e- Filter Magnet Electrodes for Extraction Spiral RF Anten nna e- (hot) H 2 (v=0) H 2 (v>0) e- (hot) H 2 (v>0) H- H- ion e(>~20ev) + H 2 (v =0) e + H 2 (v ) e(<1.0ev) + H 2 (v ³ 5) H - + H 7/29

8 Experimental Setup: TCP RF Volume Negative Ion Source at SNU Acceleration Electrode Extraction Electrode Plasma Electrode Bias Electrode Plasma Quartz Window Planar Spiral Coil RF Power V / I probe (MKS Corp.) RF Matcher u Ion Source Characteristics ü Volume production H- ion source ü Longtime CW operation by RF plasma source without filament ü No contamination by external antenna ü Cs free H- ion source Discharge Chamber Size: 14 pole cusp magnets 2~3 kv Filter (Transverse) magnets Φ10cm, length 10cm (750 cm 3 ) RF frequency: 11~27.12 MHz (limited by matching capacity) 0~20 kv u Filter magnetic field ü Total: 150~210G = 60~120G (Virtual Filter) + 90G (Dipole Magnet, fixed) RF power: 500~1400W (CW) Operating Pressure: >2 mtorr RF antenna: planar spiral coil 8/29

9 H- current depends on Pressure MHz_3mTorr 13.56MHz_5mTorr 13.56MHz_7mTorr H- Current (ma) RF power (W) Stable minimum operating pressure of the TCP RF based negative ion source is 3mTorr. In contrast with previous work [2], negative ion beam current increase with lower operating pressure. 9/29

10 H- current depends on Magnetic Filter field strength H- ion Beam Current[mA] RF Power[W] Low Filtering Middle Filtering High Filtering Plasma Electrode Bias Current[A] Electron current flow to the plasma electrode Low Filtering Middle Filtering High Filtering RF Power[W] Negative ion beam current increase with lower magnetic filter field strength in low RF power regime but negative ion beam current increase with higher magnetic filter field strength in high RF power regime. Electron transport near extraction region is related to filter field strength. 10/29

11 Effect of bias electrode in negative ion source Electric schematics of bias electrode Bipolar DC Power Supply (±60V, 10A) Plasma Ceramic Electric feed-through Bias Electrode V In general ion sheath formation, negative ion is hard to transport through the extraction hole due to sheath potential. Positively biased plasma electrode can enhance negative ion current. plasma potential [V] probe position [cm] nobias 0V 5V 10V 15V 20V 25V 30V 11/29

12 Effect of bias electrode in negative ion source Plasma potential profile H - current variation Plasma potential [V] Probe position [cm] nobias 0V 5V 10V 15V 20V 25V 30V H- Current [ma] mTorr 5mTorr 7mTorr Bias Voltage [V] Plasma potential is measured for various bias voltage of plasma electrode. Negative ion beam current increase with positively biased plasma electrode but negative ion beam current decrease with too high positively biased condition due to decrease of electron density in extraction region. 12/29

13 Effect of plasma electrode materials H - current variation with different electrode materials H- Current [ma] sccm Mo 1.6sccm Mo sccm Mo 1.2sccm S/S sccm S/S 2.0sccm S/S Recombinative Desorption [4,5,6] H + H/surface H 2 (v ) H 3+ + surface H 2 (v ) + H H 2+ + surface H 2 (v ) H, H 2 +, H PE Bias Voltage [V] H 2 (v>0) Increasing of negative ion beam current is observed with different material(stain less steel Molybdenum) of plasma electrode. Different surface of material can affect to production of vibrational excited molecules which is important role of dissociative attachment(da) process. 13/29

14 Study on effects of various material plasma electrode by using additional electrode Plasma Electrode (PE) Positively biased for optimum extraction H- Secondary Electrode (SE) Negatively biased or floating for recombinative desorption + 0 H n, H e- (hot) e- H 2 (v ) H 2 (v ) H 2 (v=0) e(<1.0ev) + H 2 (v 5) H - + H e(~20ev) + H 2 e + H 2 (v ) H + n, H 0 H + H/surface H 2 (v ) H 3+ + surface H 2 (v ) + H H 2+ + surface H 2 (v ) 14/29

15 Study on effects of various material plasma electrode by using additional electrode H- Current variation by using secondary electrode H- Current [ma] 0.70 Ta Ti Mo 0.65 SS W Surface related effect H- current increase at low SE bias voltage SE Bias Voltage [V] H- current profile as a function of secondary electrode bias voltage with various SE material (3mTorr, 0V PE bias voltage) Total H- current decrease because secondary electrode may be an obstacle as an electron sink. Positive ions are converted into vibrationally excited molecules with aids of metal surfaces. H + H/surface H 2 (v ) H 3+ + surface H 2 (v ) + H H 2+ + surface H 2 (v ) Titanium, Molybdenum and Tungsten also showed enhancement of H- current compared with S/S Mo, Ti, W > Stainless Steel 15/29

16 Effect of driving rf frequency on H- production Low driving rf frequency High driving rf frequency As rf frequency decreases As rf frequency increases High energy electron Population increases Power coupling Efficiency Increases [7] Valery A. Godyak and Vladimir I. Kolobov, Phys. Rev. Lett. 81, (1998) ] [8] V. A. Godyak, R. B. Piejak, and B. M. Alexandrovich, J. Appl. Phys. 85, 703 (1999) Finding optimal driving rf frequency for TCP H- ion source Investigation of the effect of driving rf frequency on H- ion production in TCP H- ion source Plasma parameter diagnostics to find out the cause of rf frequency effect on H- production Searching for the optimal driving rf frequency of TCP H- ion source 16/29

17 Extracted negative ion beam current depends on the driving RF frequency mTorr 10mTorr H- Current (ma) RF frequency (MHz) Input RF power 500W Negative ion beam current is measured with various RF driving frequencies. Extracted negative ion beam current increase with higher RF driving frequencies at identical input power condition. 17/29

18 [ Schematic experimental setup ] TCP RF Volume Negative Ion Source with various driving RF frequencies Acceleration Electrode 14 pole cusp magnet Spiral RF Antenna RF Choke RF Matcher Extraction Electrode Plasma Electrode(PE) Quartz Window RF Amplifier Function Generator ü Operation condition : 500 rf Power ü RF Frequency of 11~27.12 MHz are used to generate plasma. ü Langmuir probe with compensation ring and rf choke circuit is used for diagnostic of plasma parameters. 18/29

19 Heating region Plasma Parameters with various driving RF frequency Extraction region Electron Temperature (ev) Electron Temperature Electron Density 2.0x x W, 10mTorr, 10 kev 8.0x RF frequency (MHz) 1.6x x x x10 11 Electron Density (#/cm 3 ) Electron Temperature (ev) Electron Temperature Electron Density 1x x x RF frequency (MHz) 500W 10mTorr 10keV 9x x x x x x10 10 Electron Density (#/cm 3 ) Electron density increase with higher driving RF frequency, whereas electron temperature decrease. Heating & extraction region has the similar characteristics of electron density and temperature. 19/29

20 Extracted negative ion beam current depends on the driving RF frequency H- Current (ma) Electron Temperature (ev) mTorr 0.05 Input RF power 500W /29 RF frequency (MHz) Electron Temperature Electron Density 1x x x x RF frequency (MHz) 500W 10mTorr 10keV 8x x x x x10 10 Electron Density (#/cm 3 ) Heating region Increased n e Increase vibrational excited molecules decreased T e decrease vibrational excited molecules Extraction region Increased n e Increase negative ion by DA process decreased T e Increase negative ion by DA process Based on the plasma parameter, negative ion current variation can be explained with various driving RF frequency.

21 Extracted negative ion beam current depends on the driving RF frequency H- Current (ma) mTorr 0.05 Input RF power 500W RF frequency (MHz) I_rf (A) I_rf RF frequency (MHz) RF antenna current is measured for various driving RF frequencies 21/29 because Inductively coupled plasma mainly depends on the RF antenna current. The result show relation between negative ion current (plasma parameters) and RF antenna current for various driving RF frequency. Higher RF driving frequency can enhance negative ion production.

22 Extracted negative ion beam current with higher driving RF frequency & input power 22/29 H- Current (ma) MHz_3mTorr 13.56MHz_5mTorr 13.56MHz_7mTorr 27.12MHz_3mTorr 27.12MHz_5mTorr 27.12MHz_7mTorr RF power (W) Negative ion beam current is measured with two different driving frequencies RF power (13.56, Mhz) as increasing input power. Higher negative ion current is obtained with the higher driving RF frequency in lower input RF power regime. However, lower negative ion current is obtained with the higher driving RF frequency in higher input RF power regime.

23 Plasma diagnostic by using bias electrode Electric circuit for measuring IV Curve by using Bias Electrode Ceramic Electric feed-through Bipolar DC Power Supply (±60V, 10A) Plasma OSC Low Pass Filter (~1.9Mhz) 23/29 10X Voltage Probe [Tektronix corp.] V Measuring Resistor(25ohm) Bias Electrode Add more bi-pass Capacitor to reduce RF perturbation

24 I/V Curve w/ plasma electrode depends on operating conditions I/V Curve for Various RF Frequency I/V Curve for Various Pressure Current(A) Mhz 13Mhz 15Mhz 17Mhz 19Mhz 21Mhz Current(A) mTorr(0.4Pa) 5mTorr(0.65Pa) 7mTorr(0.9Pa) W, 3mTorr W,11Mhz Voltage(V) Voltage(V) I/V Curve by using the bias electrode is successfully obtained for various driving frequency & Pressure condition. 24/29

25 Electron Density (#/cm 3 ) 5x x x x x10 11 Plasma Parameters with higher driving RF frequency & input power 13.56MHz_3mTorr 27.12MHz_3mTorr RF power (W) Extraction Region Electron Temperature (ev) MHz_3mTorr 27.12MHz_3mTorr RF power (W) Extraction Region Electron Temperature and Density increase as increasing input RF power. Electron Temperature at higher RF frequency is lower than that at lower RF 25/29 frequency. Electron Density at higher RF frequency is higher than that at lower RF frequency. But, above 1000W, Electron density at higher RF frequency is lower than that at lower RF frequency

26 Extracted negative ion beam current with higher driving RF frequency & input power H- Current (ma) MHz_3mTorr 27.12MHz_3mTorr RF power (W) I_rf (A) MHz 27.12MHz RF power (W) RF antenna current is measured for driving RF frequencies and input power. The result also show relation between negative ion current (plasma 26/29 parameters) and RF antenna current. In contrast to lower input power regime, unfortunately negative ion current decrease with higher driving frequency in high input power regime. Capacitive loss of ICP plasma with higher driving frequency and input power condition can cause decrease of electron density and temperature [9].

27 Summary & Conclusion To reduce heat load to acceleration electrodes, RF based negative ion source operation at low pressure(3mtorr) is achieved and Higher magnetic filter field strength shows less electron flow to plasma electrode. Effects of various plasma electrode materials are compared with negative ion current in consideration of recombinative desorption and negative ion current is enhanced with plasma electrode materials of Ti,Mo,W. Negative ion current changes with various driving RF frequencies and the results is explained by variation of plasma parameters(n e,t e ) due to effect of different power coupling with various frequencies. Higher negative ion current is obtained with the higher driving RF frequency in lower input RF power regime. However, lower negative ion current is obtained with the higher driving RF frequency in higher input RF power regime due to capacitive loss in ICP. 27/29

28 Comparison of negative ion source NNBI for ITER (RF source, Cs seeded) DESY (RF source) Total Current 40A(CW):100kW for ICP module*8 40mA(Pulse [15ms]):30kW Current density 24 ma/cm ma/cm 2 J-Parc (Filament source) 18mA(CW) 28.3 ma/cm 2 SNU 1.67 ma(cw) :1.4kW, 0.4 Pa 3.3 ma/cm 2 In comparing extraction current with other negative ion source, TCP based RF negative ion source has competitiveness in considering RF power density but still needs enhancement of total current and current density. 28/29

29 Future Work Characterization of negative ion production in various conditions more precisely by using diagnostics(measurement of negative ion density, vibrationally excited molecules) to optimize negative ion production and extraction respectively. Geometry effect of the TCP negative ion source to optimize in considering transport of plasma. Effect of more various plasma electrode materials in others approach. (LaB6, Cs-doped Metals, ) Research on the method for higher power coupling with various RF frequency to increase negative ion current efficiently. 29/29