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Effect of argon seeding on the negative ion yield of the Kamaboko III ion source
Fusion Engineering and Design 66 /68 (2003) 609 /614 www.elsevier.com/locate/fusengdes
Effect of argon seeding on the negative ion yield of the Kamaboko III ion source D. Boilson *, H.P.L. de Esch, R. Hemsworth, A. Krylov, P. Massmann, M. Rada, L. Svensson Association EURATOM-CEA, CEA/DSM/DRFC, CEA-Cadarache, 13108 St Paul-Lez-Durance, France
Abstract It has been previously reported that the addition of argon to a hydrogen plasma in an RF driven ion source can substantially increase (up to a factor 4) the extracted and accelerated negative ion (H ) current (W. Kraus et al., Development of large radio frequency negative-ion sources for nuclear fusion, Rev. Sci. Instrum. 73(2) (2002)). Realizing such an increase in the filamented arc discharge negative ion sources used for neutral beam injection systems would have significant benefits. Unfortunately the reported studies of argon addition to filamented sources have not shown a similar gain, but so far these have been carried out with arc powers and plasma densities far from those typical of the plasma in the negative ion sources used on neutral beam injectors (N. Nishiura et al., Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002); N. Curran et al. , The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen, Plasma Scources, Sci. Technol. 9 (2000)). The Kamaboko III ion source operates at the pressure and plasma density close to those anticipated in the ion source proposed for the ITER neutral beam injectors. Measurements have been made of the plasma density, electron temperature and the negative ion yield as a function of the argon seeding rate. The plasma parameters are determined with a fast spatially scanning Langmuir probe system. The effect on the H yield is determined from the effect on the current extracted and accelerated from the source. Data will be presented for source filling pressures between 0.1 and 0.5 Pa of hydrogen, additions of argon from 0 to 30%, and a discharge power of 38 kW. All these data are collected in pure volume non-caesiated discharges. Some increase in the extracted H yield is measured for small percentage additions of argon, (0 /20%), but only at the highest H2 pressure used, 0.5 Pa. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Argon seeding; Kamaboko III; MANTIS test
1. Experimental set-up
* Corresponding author. Tel.: /33-4-4225-6293; fax: /33-44225-6233. E-mail address: [email protected] (D. Boilson).
The source used at the MANTIS test stand is a small-scale version of the source designed for the ITER neutral beam injection system called the Kamaboko III ion source. It is a 30 l quasicylindrical chamber machined of oxygen free copper, which forms the anode, with 12 filaments
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00166-2
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mounted on water cooled co-axial mounts (see Fig. 1). Electron confinement is enhanced by 16 magnetic line cusps created by SmCo permanent magnets (width /height /10 /20 mm2) arranged in longitudinally machined channels on the outside of the chamber. The cooling of the source is achieved by water lines brazed into channels on the outside of the source next to the columns of magnets. The source is directly attached to, but electrically insulated from, the accelerator. Columns of large (30 /30 /20 mm3 or 50 /30 /20 mm3, length by width by height) permanent magnets mounted inside the source flange produce a horizontal magnetic field across the front of the plasma grid. This field forms the ‘standard’ magnetic filter with a strength of 1200 G cm. The plasma grid is ‘actively cooled’ by water tubes attached to the plasma facing side of the grid. It is electrically biased with respect to the source to /5 V. The electrical connections for the source and the accelerator and their power supplies are shown in Fig. 2. The maximum extraction energy is 30 keV.
The plasma parameters, electron density, electron temperature, ion density, plasma potential and the floating potential were measured with a scientific systems fast injection Langmuir probe system [2]. The stroke length of this probe is 210 mm; the probe starting position is typically 5 mm back from the inner anode surface. Results presented in this paper are all taken at a position of 95 mm inside the source. A pressure range of 0.1 /0.5 Pa was investigated. A total of 0.5 Pa was the upper limit of source operation as it became impossible to hold the arc for higher gas pressures. The flow rate of argon was adjusted accurately by use of a needle valve and the source pressure was monitored by a Baratron capacitance manometer. The negative ion current is deduced from the accelerated current.
2. Results Plasma parameters were measured with the Langmuir probe in a hydrogen volume discharge
Fig. 1. Schematic off the KAMABOKO source.
D. Boilson et al. / Fusion Engineering and Design 66 /68 (2003) 609 /614
611
Fig. 2. Electrical schematic of MANTIS.
and are shown in Fig. 3(a) and (b); the variation in the electron density (ne), temperature (Te), and floating potential (Vf) are shown for a 26 kW discharge as a function of pressure and in a 0.32 Pa filling pressure discharge as a function of the power in the discharge. The ion density (ni) is shown rather than the electron density due to saturation of the I /V characteristic before electron current saturation was reached. The probe in this case was a tungsten wire of 0.5 mm diameter and 5 mm length. The ion density is seen to increase for a source filling pressure upto 0.15 Pa, after this it remains stable at 2.5 /1012 cm 3. It can be seen that the electron temperature decreases as the electron density increases and continues to decrease to approx. 5.3 eV at 0.55 Pa.
Stable operating conditions were established before the operation of the discharge with argon. The discharge behaves differently when operating with argon, and maintaining the arc with mixed gas discharges was found to be difficult. Fig. 4 shows the effect of the addition of Ar to the negative ion density with a discharge operated at either 0.3 or 0.5 Pa source filling pressure (H2 pressure measured prior to the discharge), with an input arc power of 38 kW. An increase of 7.5% in the accelerated negative ion current can be seen for additions of Ar up to 25% when operating at a source pressure of 0.3 Pa. However with higher percentage additions the density starts to decrease. In the case of the 0.5 Pa source filling pressure (H2), the increase in the negative ion density can be
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Fig. 3. (a, b) Plasma parameters in pure hydrogen discharge.
more clearly seen, with an increase of 36% being obtained with 26% of Ar. In order to try to understand the observed increase in the accelerated H current, the effect of the argon on the plasma parameters was investigated at 0.3 and 0.5 Pa filling pressure (H2). Fig. 5(a) and (b) show the measured electron temperature, density and floating potential when the added percentage of argon increased. In these cases, the probe tip had been decreased to a diameter of 0.3 mm, and the length to 2.5 mm, therefore allowing collection of the electron saturation current, and hence an accurate determination of the electron density. It can be seen that the effect of argon addition is to increase the electron density and to decrease the electron temperature. In Fig. 5(a) it can be seen that for a 0.3 Pa H2 discharge, seeding with 26% Ar, which produced an increase of 7.5% in the H current,
causes an electron temperature decrease from 5.2 to 4.6 eV, i.e. 11%, and an electron density increase from 2.5 /1012 to 3.1 /1012 cm 3, i.e. 23%. Fig. 5(b) shows the results with a 0.5 Pa filling pressure (H2) discharge. The effect of the argon causes the electron temperature to decrease from 4.3 to 3.8 eV and increases the electron density from 2.7 /1012 to 3.1 /1012 cm 3. Fig. 5(a) and (b) show that the value of the ion density, calculated from the Langmuir probe data decreases with increasing percentages of argon. This is in contradiction to the measured increase in the electron density. The reason is that the ion mass used in the calculation of the ion density is 1.24 amu for both the hydrogen, and hydrogen / argon discharges, which is certainly incorrect as in the latter case the ionisation of some of the Ar will increase the effective ion mass. (Note that the effective mass of 1.24 amu was chosen as it
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sis of these spectra is shown in Fig. 6. In all the cases shown, 0.1, 0.3, and 0.5 Pa H2, it can be seen that with the addition of Ar, the intensity of the argon line increases, as expected, but the intensity of the Ha lines decrease.
3. Discussion/conclusion
Fig. 4. Effect of argon on negative ion current in 0.3 and 0.5 Pa discharges.
corresponds to that calculated for a ratio of 70:20:10 for H:H2:H3, which is typical of the ion ratios extracted at high current densities from positive ion source without a magnetic filter, i.e. what one might expect upstream of the magnetic filter in the Kamaboko III source.) By the use of a 0.5 m spectrometer, emission spectra of the Ar and Ha lines were collected during the aforementioned discharges. The analy-
It has been found that with a source filling pressure of 0.5 Pa (H2), the effect of argon seeding of a filamented hydrogen arc discharge with plasma parameters in the range of interest for the ITER negative ion sources is to cause an increase in the accelerated H current of 36%, which is significantly below that observed with the RF source at IPP Garching. Little increase is observed at the ITER ion source design operating pressure (0.3 Pa of D2). The maximum accelerated H current density (deduced from the drain current) was 40 A/m2. The increase in the accelerated H current cannot be simply explained by an increase in the H density in the source created by the measured increase in the electron density, as suggested by Nishiura et al., [1], as the electron density increase is well below the measured H current increase. However, adding Ar also caused Te to fall, which could result in a reduced H destruction rate,
Fig. 5. (a, b) Effect of argon on plasma parameters in 0.3 and 0.5 Pa discharges.
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Fig. 6. Relative intensities of the Ha and Ar lines for different pressures of H2.
which, combined with the density increase, might explain the observed H current increase. It is worth noting that in some aspects the measured data differ somewhat from expectations. The ionisation rate constant for Ar is substantially higher (:/4 times) than that for H2 at an electron temperature of 10 eV, and it is almost certainly higher at the temperatures in the Kamaboko source. As the source has good electron confinement, this might not lead to an increase in plasma density, but the ion species mix should change substantially. The largest effective mass deduced from the ratio of the electron and ion saturation currents is :/1.8, when the gas mixture in the source consisted of 40% Ar and 60% H2, suggesting that the Ar made up only :/20% of the ions in the plasma. (The presence of H in the plasma was not considered here, but the characteristics measured in pure H2 suggest that this has little
effect on the probe current ratios under the conditions encountered during these experiments.) Further study is required for the understanding of the effect of argon on these discharges. It is proposed to investigate the effect of argon seeding in the presence of Cs. Also the use of a quantitative measurement of the negative ion densities would be of interest, either by use of the photo detachment probe technique or by use of the cavity ringdown spectroscopy.
References [1] N. Nishiura et al. , Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002). [2] M.B. Hopkins, W.G. Graham, Langmuir probe technique for plasma parameter measurement in a medium density discharge, Rev. Sci. Instrum. 57 (1986).
Effect of argon seeding on the negative ion yield of the Kamaboko III ion source D. Boilson *, H.P.L. de Esch, R. Hemsworth, A. Krylov, P. Massmann, M. Rada, L. Svensson Association EURATOM-CEA, CEA/DSM/DRFC, CEA-Cadarache, 13108 St Paul-Lez-Durance, France
Abstract It has been previously reported that the addition of argon to a hydrogen plasma in an RF driven ion source can substantially increase (up to a factor 4) the extracted and accelerated negative ion (H ) current (W. Kraus et al., Development of large radio frequency negative-ion sources for nuclear fusion, Rev. Sci. Instrum. 73(2) (2002)). Realizing such an increase in the filamented arc discharge negative ion sources used for neutral beam injection systems would have significant benefits. Unfortunately the reported studies of argon addition to filamented sources have not shown a similar gain, but so far these have been carried out with arc powers and plasma densities far from those typical of the plasma in the negative ion sources used on neutral beam injectors (N. Nishiura et al., Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002); N. Curran et al. , The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen, Plasma Scources, Sci. Technol. 9 (2000)). The Kamaboko III ion source operates at the pressure and plasma density close to those anticipated in the ion source proposed for the ITER neutral beam injectors. Measurements have been made of the plasma density, electron temperature and the negative ion yield as a function of the argon seeding rate. The plasma parameters are determined with a fast spatially scanning Langmuir probe system. The effect on the H yield is determined from the effect on the current extracted and accelerated from the source. Data will be presented for source filling pressures between 0.1 and 0.5 Pa of hydrogen, additions of argon from 0 to 30%, and a discharge power of 38 kW. All these data are collected in pure volume non-caesiated discharges. Some increase in the extracted H yield is measured for small percentage additions of argon, (0 /20%), but only at the highest H2 pressure used, 0.5 Pa. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Argon seeding; Kamaboko III; MANTIS test
1. Experimental set-up
* Corresponding author. Tel.: /33-4-4225-6293; fax: /33-44225-6233. E-mail address: [email protected] (D. Boilson).
The source used at the MANTIS test stand is a small-scale version of the source designed for the ITER neutral beam injection system called the Kamaboko III ion source. It is a 30 l quasicylindrical chamber machined of oxygen free copper, which forms the anode, with 12 filaments
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00166-2
610
D. Boilson et al. / Fusion Engineering and Design 66 /68 (2003) 609 /614
mounted on water cooled co-axial mounts (see Fig. 1). Electron confinement is enhanced by 16 magnetic line cusps created by SmCo permanent magnets (width /height /10 /20 mm2) arranged in longitudinally machined channels on the outside of the chamber. The cooling of the source is achieved by water lines brazed into channels on the outside of the source next to the columns of magnets. The source is directly attached to, but electrically insulated from, the accelerator. Columns of large (30 /30 /20 mm3 or 50 /30 /20 mm3, length by width by height) permanent magnets mounted inside the source flange produce a horizontal magnetic field across the front of the plasma grid. This field forms the ‘standard’ magnetic filter with a strength of 1200 G cm. The plasma grid is ‘actively cooled’ by water tubes attached to the plasma facing side of the grid. It is electrically biased with respect to the source to /5 V. The electrical connections for the source and the accelerator and their power supplies are shown in Fig. 2. The maximum extraction energy is 30 keV.
The plasma parameters, electron density, electron temperature, ion density, plasma potential and the floating potential were measured with a scientific systems fast injection Langmuir probe system [2]. The stroke length of this probe is 210 mm; the probe starting position is typically 5 mm back from the inner anode surface. Results presented in this paper are all taken at a position of 95 mm inside the source. A pressure range of 0.1 /0.5 Pa was investigated. A total of 0.5 Pa was the upper limit of source operation as it became impossible to hold the arc for higher gas pressures. The flow rate of argon was adjusted accurately by use of a needle valve and the source pressure was monitored by a Baratron capacitance manometer. The negative ion current is deduced from the accelerated current.
2. Results Plasma parameters were measured with the Langmuir probe in a hydrogen volume discharge
Fig. 1. Schematic off the KAMABOKO source.
D. Boilson et al. / Fusion Engineering and Design 66 /68 (2003) 609 /614
611
Fig. 2. Electrical schematic of MANTIS.
and are shown in Fig. 3(a) and (b); the variation in the electron density (ne), temperature (Te), and floating potential (Vf) are shown for a 26 kW discharge as a function of pressure and in a 0.32 Pa filling pressure discharge as a function of the power in the discharge. The ion density (ni) is shown rather than the electron density due to saturation of the I /V characteristic before electron current saturation was reached. The probe in this case was a tungsten wire of 0.5 mm diameter and 5 mm length. The ion density is seen to increase for a source filling pressure upto 0.15 Pa, after this it remains stable at 2.5 /1012 cm 3. It can be seen that the electron temperature decreases as the electron density increases and continues to decrease to approx. 5.3 eV at 0.55 Pa.
Stable operating conditions were established before the operation of the discharge with argon. The discharge behaves differently when operating with argon, and maintaining the arc with mixed gas discharges was found to be difficult. Fig. 4 shows the effect of the addition of Ar to the negative ion density with a discharge operated at either 0.3 or 0.5 Pa source filling pressure (H2 pressure measured prior to the discharge), with an input arc power of 38 kW. An increase of 7.5% in the accelerated negative ion current can be seen for additions of Ar up to 25% when operating at a source pressure of 0.3 Pa. However with higher percentage additions the density starts to decrease. In the case of the 0.5 Pa source filling pressure (H2), the increase in the negative ion density can be
612
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Fig. 3. (a, b) Plasma parameters in pure hydrogen discharge.
more clearly seen, with an increase of 36% being obtained with 26% of Ar. In order to try to understand the observed increase in the accelerated H current, the effect of the argon on the plasma parameters was investigated at 0.3 and 0.5 Pa filling pressure (H2). Fig. 5(a) and (b) show the measured electron temperature, density and floating potential when the added percentage of argon increased. In these cases, the probe tip had been decreased to a diameter of 0.3 mm, and the length to 2.5 mm, therefore allowing collection of the electron saturation current, and hence an accurate determination of the electron density. It can be seen that the effect of argon addition is to increase the electron density and to decrease the electron temperature. In Fig. 5(a) it can be seen that for a 0.3 Pa H2 discharge, seeding with 26% Ar, which produced an increase of 7.5% in the H current,
causes an electron temperature decrease from 5.2 to 4.6 eV, i.e. 11%, and an electron density increase from 2.5 /1012 to 3.1 /1012 cm 3, i.e. 23%. Fig. 5(b) shows the results with a 0.5 Pa filling pressure (H2) discharge. The effect of the argon causes the electron temperature to decrease from 4.3 to 3.8 eV and increases the electron density from 2.7 /1012 to 3.1 /1012 cm 3. Fig. 5(a) and (b) show that the value of the ion density, calculated from the Langmuir probe data decreases with increasing percentages of argon. This is in contradiction to the measured increase in the electron density. The reason is that the ion mass used in the calculation of the ion density is 1.24 amu for both the hydrogen, and hydrogen / argon discharges, which is certainly incorrect as in the latter case the ionisation of some of the Ar will increase the effective ion mass. (Note that the effective mass of 1.24 amu was chosen as it
D. Boilson et al. / Fusion Engineering and Design 66 /68 (2003) 609 /614
613
sis of these spectra is shown in Fig. 6. In all the cases shown, 0.1, 0.3, and 0.5 Pa H2, it can be seen that with the addition of Ar, the intensity of the argon line increases, as expected, but the intensity of the Ha lines decrease.
3. Discussion/conclusion
Fig. 4. Effect of argon on negative ion current in 0.3 and 0.5 Pa discharges.
corresponds to that calculated for a ratio of 70:20:10 for H:H2:H3, which is typical of the ion ratios extracted at high current densities from positive ion source without a magnetic filter, i.e. what one might expect upstream of the magnetic filter in the Kamaboko III source.) By the use of a 0.5 m spectrometer, emission spectra of the Ar and Ha lines were collected during the aforementioned discharges. The analy-
It has been found that with a source filling pressure of 0.5 Pa (H2), the effect of argon seeding of a filamented hydrogen arc discharge with plasma parameters in the range of interest for the ITER negative ion sources is to cause an increase in the accelerated H current of 36%, which is significantly below that observed with the RF source at IPP Garching. Little increase is observed at the ITER ion source design operating pressure (0.3 Pa of D2). The maximum accelerated H current density (deduced from the drain current) was 40 A/m2. The increase in the accelerated H current cannot be simply explained by an increase in the H density in the source created by the measured increase in the electron density, as suggested by Nishiura et al., [1], as the electron density increase is well below the measured H current increase. However, adding Ar also caused Te to fall, which could result in a reduced H destruction rate,
Fig. 5. (a, b) Effect of argon on plasma parameters in 0.3 and 0.5 Pa discharges.
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Fig. 6. Relative intensities of the Ha and Ar lines for different pressures of H2.
which, combined with the density increase, might explain the observed H current increase. It is worth noting that in some aspects the measured data differ somewhat from expectations. The ionisation rate constant for Ar is substantially higher (:/4 times) than that for H2 at an electron temperature of 10 eV, and it is almost certainly higher at the temperatures in the Kamaboko source. As the source has good electron confinement, this might not lead to an increase in plasma density, but the ion species mix should change substantially. The largest effective mass deduced from the ratio of the electron and ion saturation currents is :/1.8, when the gas mixture in the source consisted of 40% Ar and 60% H2, suggesting that the Ar made up only :/20% of the ions in the plasma. (The presence of H in the plasma was not considered here, but the characteristics measured in pure H2 suggest that this has little
effect on the probe current ratios under the conditions encountered during these experiments.) Further study is required for the understanding of the effect of argon on these discharges. It is proposed to investigate the effect of argon seeding in the presence of Cs. Also the use of a quantitative measurement of the negative ion densities would be of interest, either by use of the photo detachment probe technique or by use of the cavity ringdown spectroscopy.
References [1] N. Nishiura et al. , Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002). [2] M.B. Hopkins, W.G. Graham, Langmuir probe technique for plasma parameter measurement in a medium density discharge, Rev. Sci. Instrum. 57 (1986).