Pressure and Magnetic Field in Long-Pulse DNB Steve Scott DNB Meeting March 28, 2005
Motivation • During beam-into-gas calibration shots, the relative populations of various n=3 states may be non-statistical. • This could affect the calibration of MSE, if we were looking at the sigma line (which we aren’t …) • Jill Foley (PPPL) has a collisional-radiative model that could compute the state populations. • She needs, as input, the gas pressure and magnetic field along the beamline.
P SOURCE Figure 3: Schematic of Alcator C-Mod Long-Pulse Diagnostic Neutral Beam. Alcator C-Mod Long-Pulse DNB P = P 1 (x) d 0 = 10.9* d 1 = 18.5 d 2 = 17.0 d 3 = 16.0 P TORUS P 2 d 4 ≈ 17.5 (7” = 17.8 cm) P = P 4 (x) Magnetic P 3 x Field R accel grid = 540.9 cm R=130.4 R ≈ 90 126.5 Tank-left Tank-right 5 68.5 P 3 P 2 gate L 1 = 100 valve L 2 = 58 L 3 = 57.5 P TORUS 50 50 Calori- plasma d 3 d 4 d 1 F-port d 0 P 1 (x) d 2 meter x accel Deflection d tank = 100 grid magnet L 4 = 195 V left L 5 = 40.4 V right 165 *745 circular aperatures 4mm diameter = same area as 10.9 cm circle
Component Torus Pressure (mTorr) 2.0 0.1 neutralizer 55% 84% left tank 6% 8% right tank 2% 2% calorimeter 24% 5% F-port 9% 0.6% torus 5% 0.3% Table 1: Percentages of gas encountered by beam particles in various beam components assuming a torus pressure of 2.0 mTorr or 0.1 mTorr. The source pressure is assumed to be 15 mTorr in both cases. Figure 1: Pressure and perpendicular magnetic field in the C-Mod DNB assuming a torus pressure of 2.0 mTorr. Vertical field taken from beam-into-gas calibration shot 1030521009. 2
Figure 2: Pressure and perpendicular magnetic field in the C-Mod DNB assuming a torus pressure of 0.1 mTorr. Vertical field taken from plasma shot 1040504021 ( I p = 1 . 0 MA). 3
Vertical field : The vertical field is computed for a particular beam-into-gas shot 1030521009, which had currents in the equilbrium coils ef3 = 2 kA abd ef4 = 5 kA. Note that the vertical field goes through zero at about R = 165 cm for this shot. Figure 4: Individual components of the magnetic field: deflection magnet (red), vertical field (green), and toroidal field (blue). The absolute value of the vertical field is shown. Calculation of Gas Pressure during Plasma Operation Alcator experiences a wide range of neutral pressure during plasma operation. As shown in Table 2, neutral pressures of 0.03 to 1.0 milliTorr are commonly observed. At the upper end of the range, the pressure distribution throughout the dnb beamline will be qualititatively 7
similar to that calculated above, and so the beam neutrals will pass through a considerable amount of gas outside the neutralizer cell. At the lower end of the range, most of the line-integrated gas pressure is confined to the neutralizer cell. There may be a weak scaling of the edge pressure with density (higher pressure being correlated with higher plasma density), but at any given plasma density there is a wide range of edge pressure. Pressure (mT) Percentage 0.01-0.03 3% 0.03-0.1 12% 0.1-0.3 42% 0.3-1 40% 1-3 3% 3 - 10 0.2% Table 2: Percentages of plasmas having various ranges of neutral gas pressure (as recorded at the midplane at G-port). Data are taken from the time of peak stored energy and are restricted to plasmas having line-integral densities less than 2 × 10 20 m − 2 . Figure 2 illustrates the calculated pressure and magnetic-field distribution through the beamline for conditions typical of a plasma with low edge pressure. The calculation was carried out assuming a torus pressure of 0.1 mTorr and B t = 5.2 Tesla. The vertical field was taken from shot 1040504021, which had a plasma current of 1.0 MA. 8
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