Pressure and Magnetic Field in Long-Pulse DNB Steve Scott DNB - - PowerPoint PPT Presentation

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.

x P1(x) P2 P3 Deflection magnet

x

P = P1(x)

Magnetic Field

Tank-left Tank-right gate valve

F-port

R=130.4 126.5 68.5 accel grid 165 50 50 Calori- meter

plasma

P2 P3 PTORUS PTORUS d1 d2 d3 d4 R ≈ 90 PSOURCE P = P4(x) L2 = 58 L3 = 57.5 L4 = 195 d0 = 10.9* d1 = 18.5 d2 = 17.0 d3 = 16.0 d4 ≈ 17.5 (7” = 17.8 cm) L5 = 40.4 L1 = 100 d0 *745 circular aperatures 4mm diameter = same area as 10.9 cm circle Vleft Vright dtank = 100 Raccel grid = 540.9 cm

Alcator C-Mod Long-Pulse DNB

Figure 3: Schematic of Alcator C-Mod Long-Pulse Diagnostic Neutral Beam. 5

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 (Ip = 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

• f 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

• f 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 × 1020m−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 Bt = 5.2 Tesla. The vertical field was taken from shot 1040504021, which had a plasma current of 1.0 MA. 8