Béchu, S.

Duval, B.

Fantz, U.

Furno, I.

Guittienne, P.

Howling, A.

Jacquier, R.

Karpushov, A.

Marini, C.

Simonin, A.

Wünderlich, D.

English

MPG.PuRe

Spectroscopic measurements of hydrogen dissociation degree and H−production in resonant antenna-generated helicon plasmasC. Marini1, B. P. Duval1, I. Furno1, A. A. Howling1, R. Jacquier1, A. N. Karpushov1, Ph.Guittienne2, U. Fantz3, D. Wünderlich3, S. Béchu4, and A. Simonin51EPFL, Swiss Plasma Center (SPC), CH-1015 Lausanne, Switzerland.2Helyssen, Route de la Louche 31, CH-1092 Belmont-sur-Lausanne, Switzerland.3Max-Planck-Institut fuer Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany.4LPSC, Université Grenoble-Alpes, CNRS/IN2P3, F-38026 Grenoble, France.5CEA, IRFM, F-13108 St Paul lez Durance, France.IntroductionA new generation of neutral beam (NB) systems will be required in fusion reactors, such asDEMO, able to deliver up to 50 MW of high neutral particle energy (0.8-1 MeV) with high wall-plug efficiency ('60%). Negative ion beams can access this performance, resulting in a strongresearch focus on negative ion production from surface or volumetric plasma sources. A novelhelicon plasma source, based on a resonant birdcage network antenna [1], is currently understudy at the Swiss Plasma Center (SPC), before installation on the Cybele negative ion source[2] at CEA-IRFM. This paper reports on passive spectroscopic measurements of the absoluteintensity of the Balmer Hα , Hβ and Hγ and the diagonal Fulcher-α lines. These are analysedusing the collisional radiative code YACORA [4] to estimate the hydrogen dissociation degreeand the H− density.Experimental setupFigure 1: 3D CAD drawing of RAID, with the inputoptics for spectroscopic measurements.Figure 1 shows a CAD drawing of the Res-onant Antenna Ion Device (RAID) togetherwith the input optics for the reported spectro-scopic measurements. RAID is a 1.8 m longlinear device, with a 40 cm diameter circularcross section and equipped with 6 magneticfield coils. The helicon antenna, able to de-liver up to 10 kW at 13.56 MHz, is installed at one end outside the main coil region. The coilcurrent is Icoil = 150 A , for a nominal field on axis of 150 G, while the current on the first coil43rd EPS Conference on Plasma Physics P4.110is reversed, Icoil1 = −30 A, to increase the B gradient. These experimental conditions B = 150G and neutral gas pressure p ≤ 0.3 Pa meet the specifications of the Cybele source [2]. Thespectroscopic setup consists of a field lens, an optical fibre bundle, a high throughput spectrom-eter and a detector. The input optics is a Navitar f/1.4 35mm lens with an image focal planeat 25 cm from the lens, i.e. the RAID axis. There are 19 fibres of numerical aperture 0.22 andwith a fused silica core of diameter 365 µm. The diameter of their image on the focal plane is2.9 mm, resulting in a sampling region ' 55 mm wide. To perform measurements at a relevantimpact parameter, up to 8 cm, two set of measurements with different viewing angle were re-quired. The viewline geometry was verified by back illuminating each fibre with a He/Ne laserand measuring the spot position on a reference anti-reflection graphite ring installed inside thevacuum vessel.6225 6230 6235 6240 6245 625001234567891011x 101522Q1 22Q2 22Q3 22Q43c1|2a1|P43c0|2a0|P73c1|2a1|Q94e1|2c4|Q1u1|2b7|R43b4|2a1|P5wavelength [A]spectral radiance [ph/s/sr/m2 /A]  2500 W3000 W3500 W5000 WFigure 2: H2 spectrum of Fulcher-α 22QN lines fordifferent RF powers.Collected light is analysed by a spectrom-eter, composed of two Nikon f/2 200 mmlenses and a holographic grating blazed at 400nm of size 120x140 mm2. The spectrometerinput slit width is set to 80 µm for a spectralbandpass of 0.9 Å at 615 nm sufficient to re-solve most of the Fulcher-α molecular lines.The detector is an Andor camera iXon Ultra897, with a back-illuminated 512x512 pixelsframe transfer sensor and an optional electronmultiplying EM readout register. The cameraintegration time and EM gain are adjusted at each scanning angle to maximise the counts andS/N ratio whilst avoiding saturation.Radiance and emissivity profilesMeasurements in both H2 and D2 were performed but in this paper, only H2 data is pre-sented. The light of each optical fibre is imaged to a region of interest (ROI) on the CCD, thephoton rate Rexp [ph/s] of each emitted line for each ROI is evaluated using a multi-gaussianfitting, that accounts for the experimental instrumental function. The absolute radiance is thendeduced by comparing the experimental rate Rexp with the photon rate Rcal measured with anabsolutely calibrated source at the same wavelength, using the same setup. This ensures correctcompensation for the system transmission and etendue. The radiance [ph/s/sr/m2] of a line isthen Lexp =RexpRcalLcal , where Lcal is the known radiance of the calibration source. An exampleof a molecular spectrum in the range of the 22QN (N from 1 to 4) lines is shown in Fig. 2,43rd EPS Conference on Plasma Physics P4.110Figure 3: Hβ and Hγ radiance profile for different RF powers.where the molecular lines are identified comparing their wavelength with Ref. [3]. Figure 3shows the resulting radiance profiles for the Balmer Hβ and Hγ lines respectively. The radi-ance of each line increases almost linearly with the input power, with constant profile shape.Figure 4: Hγ emissivity profile for different RF pow-ers.A minimum Fisher regularisation algorithm isthen applied to calculate the absolute emis-sivity profile for each emission line, in cylin-drical symmetry as experimentally observed.The correct convergence of the algorithm isverified by reconstructing the input radianceprofiles from the output emissivities. An errorestimate of the Abel-inverted profiles is per-formed using a Monte Carlo approach. Fig-ure 4 shows the case of Hγ line, similar fea-tures are observed for the other lines as well,in particular emissivity profiles are generally hollow and their shape is similar at different inputpowers.Analysis and resultsThe first 3 Balmer lines and the sum of the diagonal Fulcher-α lines profiles are interpretedhere using the collisional-radiative code YACORA [4]. As an example, the output ne and Teprofiles are compared in Fig. 5 with experimental profiles, measured with an RF-compensatedLangmuir probe (LP), at 3kW input power. The Te profile is in agreement while there is a dis-crepancy of about a factor 1.7 in the ne profile that is still under investigation. The gas transla-tional and vibrational temperatures can be estimated from the Fulcher lines and are respectivelyTtran = 900± 200 K and Tvib ≥ 6000 K. Figure 6 show the dissociation fraction fd = nH/nH243rd EPS Conference on Plasma Physics P4.1100 0.01 0.02 0.03 0.04 0.05 0.06012345678x 1017Radius [m]n e [m−3 ]  LPYacora0 0.01 0.02 0.03 0.04 0.05 0.061.522.533.544.555.5Radius [m]Te [eV]  LPYacoraFigure 5: Comparison of experimental ne and Te profiles (LP) with YACORA convergence values forinput power 3 kW.profile in blue and the ratio fneg = nH−/ne in red, for the 3kW case.0 0.01 0.02 0.03 0.04 0.05 0.060.50.60.70.80.91Radius [m]  00.10.20.30.40.5nH/nH2nH−/neFigure 6: Profiles of the ratios fd = nH/nH2 andfneg = nH−/ne from YACORA.The fd profile reaches interestingly high valuesfor use as a negative ion surface source, with adissociation degree α = fdfd+2 up to 33%. The ra-tio fneg is lower than 0.05 inside the plasma col-umn (radius < 2 cm), where the relatively highTe prevents a stable formation of H−. Interest-ingly, fneg increases at the edge of the plasmacolumn 3≤ r≤ 4 cm, resulting in the high valueof nH− ' 4 · 1016 m−3. In conclusion, the dis-sociation degree is fairly constant in the range25%-33% at a relatively low power of 3 kW. Thisperformances are promising for application of the helicon antenna as a high efficiency plasmasource for the Cybele NIB.This work has been carried out within the framework of the EUROfusion Consortium and has received fundingfrom the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views andopinions expressed herein do not necessarily reflect those of the European Commission.REFERENCES[1] Ph. Guittienne and E. Chevalier, J. Appl. Phys., 98, 083304 (2005)[2] A. Simonin et al., Nucl. Fusion 55, 123020 (2015)[3] G.H. Dieke, The hydrogen molecule wavelength tables of G. H. Dieke, Crosswhite (1972)[4] D. Wunderlich, S. Dietrich, U. Fantz, J. Quant. Spectros. Radia. Transfer 110, 62-71(2009)43rd EPS Conference on Plasma Physics P4.110

Spectroscopic measurements of hydrogen dissociation degree and H<sup>-</sup> production in resonant antenna-generated helicon plasmas

http://hdl.handle.net/21.11116/0000-0008-80FC-B

Spectroscopic measurements of hydrogen dissociation degree and H<sup>-</sup> production in resonant antenna-generated helicon plasmas

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe