负氢离子源中的等离子诊断诊断

Plasma diagnostic tools for optimizing negative hydrogen ion sources U. Fantz, H. D. Falter, P. Franzen, E. Speth, R. Hemsworth et al. Citation: Rev. Sci. Instrum. 77, 03A516 (2006); doi: 10.1063/1.2165769 View online: http://dx.doi.org/10.1063/1.2165769 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v77/i3 Published by the American Institute of Physics. Additional information on Rev. Sci. Instrum. Journal Homepage: http://rsi.aip.org Journal Information: http://rsi.aip.org/about/about_the_journal Top downloads: http://rsi.aip.org/features/most_downloaded Information for Authors: http://rsi.aip.org/authors Downloaded 15 Mar 2012 to 218.199.90.49. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions g soci ul-le blin 2318 21 spec n th ydro nd a ITE utra sity how ourc e th tes 5769 Development of negative hydrogen ion sources for neu- tral beam heating systems of fusion devices such as ITER stripping and mutual neutralization. This means optimization demands knowledge of these quantities, and therefore simple, robust, and noninvasive diagnostic techniques. REVIEW OF SCIENTIFIC INSTRUMENTS 77, 03A516 �2006� requires an optimization of negative ion formation in low- pressure plasmas. Currently two types of discharges, an in- ductively coupled rf source and an arc source, are envisaged to meet the ITER requirements: extracted current densities of 20 mA/cm2 in D �28 mA/cm2 in H� over an extraction area of 2000 cm2 for a pulse length of 3600 s at a source pressure of 0.3 Pa. Under an extensive development program, the rf source at the IPP Garching achieved, and even exceeded, recently the extracted current densities in short-pulse operation,1,2 whereas the KAMABOKO-III arc source at CEA, Cadarache focuses on long-pulse operation.3,4 Both sources operate with cesium evaporation due to the fact that the negative ions are very efficiently produced by the surface effect, i.e., the interaction of hydrogen particles �atoms and ions� with a material of low work function: H, Hx + +surface e−→H−. The enhancement of negative ions is about one order of magnitude in comparison with a cesium- free operation when negative ions are formed by the volume effect, i.e., the dissociative attachment, a process which is therefore not discussed further in this article. Together with the boundary condition that the survival length of negative ions is in the centimeter range, optimization of negative ions is focused on optimizing the cesium layer on the plasma grid, reflected by the cesium balance in the source, atomic hydro- gen density, and positive ion density. The electron density ne II. DIAGNOSTIC METHOD The technique of optical emission spectroscopy �OES� is widely used as a noninvasive and powerful diagnostic tool which provides information on particle densities and temperatures.5 In general, line emission gives direct access to the population of the particle in its excited state, which de- pends on the plasma parameters. The coupling to the ground state or its ion state is provided by collisional radiative mod- els. The combination of such models with absolutely mea- sured line radiation is used to determine the following plasma parameters: ne, Te, Tgas, atomic and molecular hydro- gen densities, neutral and singly ionized cesium densities, and even densities of negative ions.6,7 III. EXPERIMENTAL RESULTS OES is applied to the rf sources at the IPP and KAMABOKO-III arc source at CEA. Collimator optics pro- vide parallel light �diameter �1 cm� in 40 �rf source� and 17 mm �arc source� distances to the grid, respectively, result- ing in line-of-sight averaged results. The optical windows are mounted at the source body, the line of sight has a length of �30 cm �width of the rf source body, length of the arc source�; the grid contributes �40% of the length of the line Plasma diagnostic tools for optimizin U. Fantz, H. D. Falter, P. Franzen, and E. Speth Max-Planck-Institut fuer Plasmaphysik, EURATOM As Germany R. Hemsworth CEA-Cadarache, EURATOM Association, F-13108 St. Pa D. Boilson Association EURATOM-DCU, PRL/NCST, Glasnevin, Du A. Krylov RRC Kurchatov Institute, 1 Kurchatov Square, Moscow 1 �Presented on 13 September 2005; published online The powerful diagnostic tool of optical emission parameters in negative hydrogen ion sources based o temperature, electron density, atomic-to-molecular h presented for two types of sources, a rf source a development for a neutral beam heating system of volume is obtained from cesium radiation: the Cs ne lower than the hydrogen density and the Cs ion den than the electron density in front of the grid. It is s useful for monitoring the cesium balance in the s densities are determined. In a well-conditioned sourc of magnitude as the electron density and correla American Institute of Physics. �DOI: 10.1063/1.216 I. INTRODUCTION 0034-6748/2006/77�3�/03A516/4/$23.00 77, 03A51 Downloaded 15 Mar 2012 to 218.199.90.49. Redistribution subject to AIP lic negative hydrogen ion sources ation, Boltzmannstrasse 2, 85748 Garching, z-Durance, France 13, Ireland 2, Russia March 2006� troscopy is used to measure the plasma e surface mechanism. Results for electron gen density ratio, and gas temperature are n arc source, which are currently under R. The amount of cesium in the plasma l density is five to ten orders of magnitude is two to three orders of magnitude lower n that monitoring of cesium lines is very e. From a line-ratio method negative ion e negative ion density is of the same order with extracted current densities. © 2006 � and electron temperature Te determine the losses by electron © 2006 American Institute of Physics6-1 ense or copyright; see http://rsi.aip.org/about/rights_and_permissions of sight for both sources. Typical magnetic filter field strength in this area is 0.4 mT m �rf source2� and 0.8 mT m �arc source4�. In the case of the arc source the same diagnos- tic flange is used for Langmuir probe measurements, measur- ing electron densities 17 mm in front of the grid center. Spectroscopic measurements were taken with two abso- lutely calibrated survey spectrometers �high and low spectral resolution, ��FWHM=20–35 pm and 1–1.8 nm, respec- tively� in hydrogen discharges at a pressure of 0.3 Pa with and without cesium evaporation, with a characteristic pulse length and power range: 6 s and 60–140 kW for the rf source and 30–400 s and 25–55 kW for the arc source. Cur- rent densities of 15–20 mA/cm2 are achieved typically at 110 and 45 kW, respectively. Cesium-free discharges result typically in 2 mA/cm2 at 0.3 Pa. Figure 1 shows examples of spectra recorded with a low-resolution survey spectrom- eter, which is also capable of recording time traces. The im- portant diagnostic lines of atomic hydrogen �Balmer lines�, molecular hydrogen, neutral Cs, and Cs+ are identified. The wavelength range of impurities �Cu, OH, N2, and CN� is indicated. In principle, the discharges are free of these impu- rities, except small amounts of oxygen �occasionally� as can be seen in the spectrum of the arc source. A. Plasma parameters Using a diagnostic gas, for example, argon �admixture of roughly 20%�, the plasma parameters ne and Te can be ob- tained from a line-ratio method and absolute intensity, respectively.5 The gas temperature �Tgas� is determined from the rotational structure of molecular radiation.5 The atomic- to-molecular density ratio is correlated with the intensity ra- tio H� line to molecular radiation �Fulcher transition, 600 -630 nm� via the ratio of the corresponding emission rate coefficients.5,6 As shown in Fig. 2, ne is much higher �almost by a factor of 10� in the arc source than in the rf source, whereas Te is roughly 1.5 eV lower. Probe measurements in the arc source result in reasonable agreement in densities, but give a 1 eV lower Te. This might indicate a non-Maxwellian electron energy distribution function. Both sources show FIG. 1. Examples of spectra with identification of important emission lines �H� and H� are saturated�. 03A516-2 Fantz et al. high gas temperatures �arc: 2000±300 K, rf: 1200±300 K�, almost independent of power. Due to the higher ne and Tgas, Downloaded 15 Mar 2012 to 218.199.90.49. Redistribution subject to AIP lic the degree of dissociation is much higher in the arc source. As expected for low-pressure plasmas, ne and H densities increase with power, whereas Te remains almost constant. B. Cesium and negative ions Analysis of cesium emission lines and results for the rf source are described in detail in an earlier article.7 In order to demonstrate the development of extracted current densities �jH− and je� and a correlation with the cesium line at 852 nm �neutral cesium�, Fig. 3 shows a sequence of discharges �rf source� starting with cesium evaporation. The Cs852 line increases steadily, whereas the plasma parameters �ne, Te, and H density� remain constant, indicated by a constant H� line. After 1 h of evaporation jH− increases, whereas the ex- tracted electron current decreases, clearly indicating the so- called cesium effect. This means the level of Cs intensity at constant power and pressure can be used to adjust the evapo- ration. Time traces of the Cs852 line and H� line during cesium evaporation in the arc source are shown in the upper part of Fig. 4. Again a steady increase from discharge to discharge is observed in the Cs signal, whereas H� shows the FIG. 2. Plasma parameters in the arc source �left part� and rf source �right part� determined by OES. The open symbols refer to Langmuir probe mea- surements. Rev. Sci. Instrum. 77, 03A516 �2006� FIG. 3. Temporal development of extracted current densities and spectro- scopic signals of the rf source after the start of Cs evaporation �in h�. ense or copyright; see http://rsi.aip.org/about/rights_and_permissions stable plasma conditions. With increasing pulse length the Cs signal seems to saturate, however, this behavior depends strongly on the evaporation rate which influences also the formation of negative ions. In general, it is observed that the evaporation rates needed for arc sources are much higher than those for the rf sources �roughly by a factor of 10�. This is partly due to the long-pulse operation and partly due to the evaporation of tungsten from the hot tungsten filaments ��2600 K�. Thus, a fresh layer of cesium will be buried under tungsten.8 The continuum emission of tungsten is clearly visible in the lower part of Fig. 1. However, tungsten emission lines are very weak, just allowing to get an upper estimate. In contrast to rf sources cesium ion lines are intense in the arc source �Fig. 1�, resulting typically in an ion-to- neutral ratio of 1000 �rf sources typically show values of 30�, which means more cesium is ionized because of the high ne. Since H� emission is influenced by the mutual neutralization process at density ratios H−/H�10−3, the line ratio H� /H� offers a method to obtain the negative ion density.6,9 For constant plasma parameters the line ratio correlates directly with the extracted current density as shown in the rf source.6 The lower part of Fig. 4 shows this direct correlation for a time trace of the arc source. The two experiments yield very similar negative ion densities in the observed plasma volume at comparable extracted current densities. IV. OPTIMIZATION OF NEGATIVE IONS The whole set of particle densities obtained in the two discharges close to the plasma grid are summarized in Fig. 5 for hydrogen discharges �0.3 Pa� at 110 �rf� and 45 kW �arc� which represent similar performance. Due to the higher gas temperature in the arc source �2000 K� as in the rf source �1200 K�, the neutral particle density differs for the same pressure. As already mentioned, ne and thus Hx + densities as FIG. 4. Time traces of spectroscopic signals �H� ,Cs� with Cs evaporation �top� and correlation of the line ratio H� /H� with the extracted current density in the arc source �bottom�. The pulse length varies from 50 to 140 s. 03A516-3 Plasma diagnostic tools for H sources well as H density are much higher in the arc than in the rf source, whereas Te is lower. Since the surface effect is the Downloaded 15 Mar 2012 to 218.199.90.49. Redistribution subject to AIP lic dominant formation mechanism for H−, low ne and Te are favorable for reducing losses via electron stripping and mu- tual neutralization. Lowering Te from 3.5 to 2 eV in the rf source would reduce the losses by a factor of �3, which are then determined by the mutual neutralization only.6 Losses are higher in the arc source than in the rf source due to the much higher ne. On the other hand, the higher Hx + and H densities produce more H− by the surface mechanism. Sys- tematic measurements show that enhancement of H− corre- late with the H density in the arc source.4 In balance, H− densities are similar for the two sources ��1017 m−3� as it is indeed measured at comparable conditions �Fig. 5�. Operat- ing in an almost Cs-free source results in a factor of 10 lower H− densities �light gray bars in Fig. 5�, reproducing exactly the difference in extracted current densities. In both sources cesium densities are orders of magnitude below hydrogen densities �Fig. 5�. Due to this small amount, changes in plasma parameters caused by cesium evaporation are not observed. In the arc source much more cesium is ionized than in the rf source and the total amount of cesium �neutrals plus ions� is roughly an order of magnitude higher. This matches with the higher evaporation rate used in the arc, which is needed because of the simultaneous evapora- tion of tungsten, as discussed in Sec. III B. The measured neutral tungsten density is ten times higher than the neutral cesium density. Since the ionization energy is also low �7.86 eV� and since Te in the plasma volume close to the tungsten filaments is around 5 eV, it is expected that most of the tungsten is also ionized. Comparable densities indicate that W and Cs evaporation rates are quite similar as sup- ported by examination of surface layers.8 It can be expected that cesium consumption can be considerably reduced avoid- ing poisoning by tungsten. Further reduction can be expected by optimizing the cesium distribution, monitored with OES. In summary, it was shown that OES is a very valuable tool for understanding and monitoring plasma dynamics, in par- ticular, the cesium balance, which is most important for re- liable and reproducible operation in long pulses. 1 P. Franzen, H. D. Falter, E. Speth, W. Kraus, M. Bandyopadhyay, A. Encheva, U. Fantz, Th. Franke, B. Heinemann, D. Holtum, C. Martens, P. McNeely, R. Riedl, A. Tanga, R. Wilhelm, Fusion Eng. Des. 74, 351 FIG. 5. Particle densities in the two types of negative ion sources �similar performance� measured by spectroscopic techniques. Rev. Sci. Instrum. 77, 03A516 �2006� �2005� 2 E. Speth, H. D. Falter, P. Franzen, U. Fantz, M. Bandyopadhyay, S. Christ, ense or copyright; see http://rsi.aip.org/about/rights_and_permissions A. Encheva, M. Fröschle, D. Holtum, B. Heinemann, W. Kraus, A. Lorenz, Ch. Martens, P. McNee-ly, S. Obermayer, R. Riedl, R. Süss, A. Tanga, R. Wilhelm, and D. Wünderlich, Nucl. Fusion �submitted�. 3 D. Boilson, H. P. L. de Esch, R. S. Hemsworth, M. Kashiwagi, P. Massmann, and L. Svensson, Rev. Sci. Instrum. 72, 1093 �2002�. 4 R. S. Hemsworth, D. Boilson, U. Fantz, L. Svensson, H. P. L. deEsch, A. Krylov, P. Massmann, and B. Zaniol, AIP Conference Proceedings 763 Subseries: Accelerators and Beams, edited by J. D. Sherman and Y. J. Belchenko, AIP, 2005. 5 U. Fantz, Contrib. Plasma Phys. 44, 508 �2004�. 6 U. Fantz, H. Falter, P. Franzen, D. Wünderlich, M. Berger, A. Lorenz, W. Kraus, P. McNeely, R. Riedl, and E. Speth, Nucl. Fusion �submitted�. 7 U. Fantz, M. Bandyopadhyay, H. D. Falter, P. Franzen, B. Heinemann, W. Kruas, P. McNeely, R. Riedl, E. Speth, A. Tanga, and R. Wilhelm, Fusion Eng. Des. 74, 299 �2005�. 8 A. Krylov, D. Boilson, U. Fantz, R. S. Hemsworth, O. Provitina, S. Pontremoli, and B. Zaniol, Nucl. Fusion �accepted�. 9 U. Fantz and D. Wünderlich, J. Phys. D: Appl. Phys. �to be submitted�. 03A516-4 Fantz et al. Rev. Sci. Instrum. 77, 03A516 �2006� Downloaded 15 Mar 2012 to 218.199.90.49. Redistribution subject to AIP lic ense or copyright; see http://rsi.aip.org/about/rights_and_permissions