Bacal, M.

Hatayama, A.

Kuppel, Sylvain

Matsushita, D.

English

Aberdeen University Research

Role of the plasma-electrode bias and the transverse magnetic field upon H- ion extraction in the negative ion sourceIntroduction / BackgroundSimulation modelFig. 3. 2D slab geometry.Influence of the PE biasFig. 4. Boundary conditions.Influence of electron diffusion      Using negative ion sources is a very promising way to produce the neutral beams required for future nuclear fusion reactors. In this study, we focus on the extraction physics of H- ions in negative ion sources, by using a PIC2D3V method to analyse the motion of charged particles self-consistently. It was shown experimentally that a peak of extracted H- current exists for an optimum bias voltage on the plasma electrode (PE), in the presence of a weak transverse magnetic field (see Fig. 1). The interpretation of these results requires to clarify the physical mechanisms involved in H- ion and electron extraction, which are closely linked. A parametric study also shows the influence of the electron diffusion across the magnetic field.IPEIep = 2 mTorr IH-plasma electrode bias VPE (V)Plasma electrode current I PE (mA) extracted electron current Ie (mA) 0      1       2      3       4      5      6       7      8       9     1005101520250.000.050.100.150.200.25200300400100extracted negative ion current IH-  (mA) Fig. 1. Experimental results in ECR driven Camembert III ion source with a "weak" magnetic field of 20G at maximum[1][1] P. Svarnas et al, IEEE TPS, 35, 1156 (2007)1858KEIO CALAMVS GLANDIO FORTIORConclusionS. Kuppel, D. Matsushita, A. Hatayama    (Keio University, Japan)M. Bacal                                                     (Ecole Polytechnique, France)Fig. 2. ECR driven negative ion source [1].PEPEEEIon source region (low B-field)LPE dPELylPELxExtractionregionExtractionaperturePEEEe-H-Source regione-H-H+H+e-H+= All the species (H+, H- and electrons)H- e-H+dφ/dy=0yxIH-~Ie 0 1 2 3 45 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5 0 0.1 0.2 0.3 0.4 0.5 0.6~ΦPENormalized negative ion extracted currentNormalized electron extracted current x10-3  x10-3~ 0 2 4 6 8 10 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5ΦPENormalized negative ion density dx = 25~~dx = 20~dx = 16~dx = 13~ 0 50 100 150 200 250 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5ΦPENormalized electron density~dx = 25~dx = 20~dx = 16~dx = 13~Fig. 5: Influence of the PE bias upon extracted H- and electron currents.  Fig. 6: Influence of the PE bias upon H- ion & electron density taken at 4 distances from the extraction aperture. 0  85 0 50       5       3     2.4       2     1.8     1.6     1.4       1xy~~PEPE 0 10 20 30 40 50~ΦDecrease of extracted H- current after the peak~ 0  85 0 50       5       3     2.4       2     1.8     1.6     1.4       1 0 10 20 30 40 50xy~~Φ~PEPEFig. 7: Contour plot of the potential profile for a) ΦPE=1.25, and b) ΦPE=2.5.~a) b)~Fig. 8: Spatial profile of the H- plasma current density, for a) ΦPE=1.25, and b) ΦPE=2.5.The local directions of the current density vectors are shown by arrows.~a) b) 40  85 0 50 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 xy~~JH-~PEPE(x10-3)  40  85 0 50xy~~JH-~PEPEThe mushroom-shaped plateau of potential widens and disappearswhen the PE bias overcomes the plasma potential,       The direction of the electric force changes.      The funnelling effect disappears, many H- ions are collected by the PE.The extracted H- ioncurrent has a peakfor an optimum bias,and decreases after.- Only half of the geometry is used as the calculation domain.xyPlasma Electrode (PE)Extraction Magnet (EM)magnetsECR driverBias voltage2.45GHzExtraction GridprobeTable 1. Numerical parameters.Table 2. System parameters.- The magnetic field satisfies the condition rLe<<Lx, rLH+,rLH->>LyThe extracted electroncurrent is significantlyreduced by the PE bias.The depletion of electron population with the PEbias continuously attracts H- ions in the extraction region. H- ions are accumulated at low bias in the extraction region, and their density increases. When the PE bias is high enough, H- ions areaccelerated towards the extraction channel. Their residence time and density decrease, while the extracted current rapidly increases. The extracted H- ion current has a peak for a optimum value of PE bias, and the extracted electron current is 0 0.2 0.4 0.6 0.8 1.0 1.2 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5With magnetic field, D3=D/4With magnetic field, D2=D/2With magnetic field, D1=DW/O magnetic field~ΦPENormalized extracted negative ion current x10-2Fig. 9: Time evolution of extracted H- ioncurrents, without PE bias.Fig. 10: Influence of the PE bias upon extracted H- currents.Electron diffusion across the magnetic field has beenconsidered through a 1D random-walk process in thex-direction. However, diffusive effect might have been overestimated, notably because the magnetic field is nottransverse everywhere.  We compare several cases, whereD is the reference diffusion coefficient used in the PEbias study.- Decreasing the electron diffusion coefficient across the B-field reduces the attractive force of H- ions in the extraction region, and the extracted current is reduced.- The mechanism leading tothe peak is however not affected, except in the casewithout magnetic field.significantly reduced. The underlying mechanism is the opening of the equipotential lines when the PE bias overcomes the plasma potential, leading H- ions to be collected by the PE instead of being extracted by the funnelling effect. Reducing the electron diffusion coefficient across the magnetic field does not affect this mechanism. 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00  20  40  60  80  100  120  140  160W/O B-field With B-field, D1=D x10-2Time iteration (x103)Normalized extracted negative ion currentWith B-field, D3=D/4 With B-field, D2=D/2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 (x10-3)

Role of the plasma-electrode bias and the transverse magnetic field upon H- ion extraction in the negative ion source

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Role of the plasma-electrode bias and the transverse magnetic field upon H- ion extraction in the negative ion source

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