A Motional Stark Effect Instrument to Measure q(r) on C-Mod

Transcription

1 1 A Motional Stark Effect Instrument to Measure q(r) on C-Mod Presented at the 42nd Annual Meeting of the American Physical Society Division of Plasma Physics, Quebec City, Can., Oct N. Bretz, Princeton Plasma Physics Laboratory, Princeton Univ. R. Bravenec, W. Rowan, N. Eisner, M. Sampsell, Fusion Research Center, Univ. of Texas at Austin, E. Marmar, H. Yuh, Plasma Science and Fusion Center, MIT A Motional Stark Effect (MSE) instrument to measure q(r) using the TEXT neutral beam on the Alcator C-Mod tokamak has been designed and installed. The neutral beam is on the midplane, aimed radially, and designed to operate in hydrogen at 50 kev. The MSE optics view the outer half of the plasma -0.3<(R-Ro)/a<1.05. This geometry results in a spatial resolution of 1 cm (edge) - 3 cm (center) from a beam apertured horizontally to 2 cm. Estimates of the signal levels indicate for Hα that statistical errors in the measurement of B θ /Bφ can be less than 0.2 deg. for neo<2.0x1014 cm-3 similar to other MSE instruments. The collection optics reside within the vacuum chamber reflecting and imaging the neutral beam through a vacuum window in the neutral beam port and are designed so that beam emission spectroscopy and MSE can be done simultaneously. The latest measurement results will be reported. Supported by US DOE Contract Nos. DE-AC02-76-CH and DE-FC02-99ER54512 and Grant No. DE-FG03-96ER

2 2 C-Mod MSE Requirements Measure Bp/Bt on Ro<R<[Ro+a] to an accuracy of about 0.2 deg. (similar to TFTR, DIII-D, etc.) with R 1 cm. Use TEXT neutral beam - perpendicular injection, - 50 kev, J(0) 20 ma/cm2, - beam dimensions 8 cm x 2 cm (width) Imaging system and detection scheme compatible with C-Mod environment. -Vacuum optics, shutters -External photo-elastic modulators, -Optics compatible with BES Operate over a range of plasma conditions to support L, H-Mode, and typical heating, ITB, and current drive schemes.

3 3 MSE/BES Vacuum Optics Enclosure

4 4 MSE/BES Image Dissector

5 C-Mod MSE Optics - Top View R o =66.5 cm a= cm 12.0 Dielectric Mirror - M3 Focusing Lens L3 Photo-Elastic Modulators 2 cm Focusing Lens L1 F Quartz Window Image of DNB & Field Lens, L2 Dielectric Mirrors M1, M2 TEXT DNB (2 cm x 8 cm) DNB Image (x 4.0 mag.) 5 G N. Bretz 10/13/00

6 C-Mod MSE Optics - Side View Dielectric Mirror M2 Field Pitch Image of DNB & Field Lens, L2 6.75" Conflat Dielectric Mirror & Lens, M3 & L3 4.2" quartz window Image of DNB from R = cm (x 4.0 mag.) 6 Dielectric Mirror M1 Focusing Lens, L1 & Shutter TEXT DNB (2 cm x 8 cm) F N. Bretz 10/16/00

7 7 Vacuum Optics PEM C-Mod Shield Wall F-Port DNB Shutter Control MSE Optics, F-Port, Photo-Elastic Modulators (PEM), Shield Wall

8 8 Dielectric Mirror, M2 Dielectric Mirror, M3 Imaging Lens L3 Field Lens, L2 Input Lens, L1 and Shutter Dielectric Mirror, M1 MSE Vacuum Optics, Cut-away View

9 Predicted Doppler & Stark Multiplet Shifts π+ σ Pol. V B xb Pol. V B xb B T = 5.3 T E B = 45 kev I B = 5 A λ[nm] 3 π 9 Hα 1 0 E B E B /2 E B /3 R R (cm) N. Bretz 10/13/00

10 Predicted Stark Spectrum Total Intensity B T = 5.3 T E B = 45 kev I B = 5 A R = MSE 76 cm I[A.U.] 0.5 Pol. V B B Pol. V B B π+ Bandpass Filter Function Hα [Suppressed] λ[nm] N. Bretz 10/13/00

11 B Hα,π+, B Brem (x10 13 photons/cm 2 /s/ster) Predicted H α, π+ and Brems. Brightness B Brem L-Mode R o λ Brems = 1.0 nm(π+) R (cm) B Hα,π+ E B E B /2 E B /3 Z eff = 1.0 N e (0) = 3.2x10 14 cm -3 T e (0) = 1.0 KeV J B0 = 20 ma/cm 2 H + :H + 2 :H3 + = 44:19:34 D B = 8.0 cm 11 N. Bretz 10/16/00

12 \ δ[b p /B t ](deg) Predicted (B p /B t ) for L-Mode Plasma R o N e (0) = 3.2x10 14 cm -3 T e (0) = 1.0 KeV E B = 45 KeV I B = 5 A λ Ha =1.0 nm (π + ) Z eff = T avg =10 ms A Ω = 2.0x10-2 cm 2 ster D B = 8 cm T optics = 0.1 QE PMT = R (cm) N. Bretz 11/16/00

13 13 Unmodulated MSE Signals R 86.9 cm 85.7 cm MSE Signal 84.4 cm 81.6 cm 78.6 cm 75.2 cm 73.4 cm 71.5 cm L-Mode n e dl = 1.25x10 16 cm -2 Z eff = 1.0 E B0 = 45 kev J B0 = 20 ma/cm 2 H + :H + 2 :H3 + = 44:19:34 D B = 8.0 cm 69.6 cm 67.5 cm Beam Voltage time N. Bretz 10/10/00

14 π+ Compare [B Hα /B Br ] Measured and Predicted 0.10 π+ [B H α /B Br ] meas π+ [B H α /B Br ] pred 0.05 R = cm L-Mode Z eff = 1.0 E B0 = 45 kev J B0 = 20 ma/cm 2 H + :H + 2 :H3 + = 44:19:34 D B = 8.0 cm n e dl [x10 16 cm -2 ] N. Bretz 10/23/00

15 15 MSE Conclusions The predicted statistical accuracy of the measurement of Bp/Bt or q(r) is limited by the DNB voltage and current and ne. Assuming 50 kev, J 0 (R+a) = 20 ma/cm, beam dimensions 8 (height) x 2 (width) cm, one has Low ne H-mode, ne(0) < 1.5x10 14 cm -3 δ[bp/bt] < 0.2 deg. [Ro<R<Ro+a] For ne(0) = 3.0x10 14 cm-3 H-mode 0.2<δ[Bp/Bt]<0.5 deg. [Ro<R<Ro+a] Using PMT detection, τavg 10 ms The measured ratio of beam induced MSE signals to Bremsstrahlung is more than ten times lower than our predictions. Actual q measurements will have low accuracy.