Evaluation of Anomalous Fast-Ion Losses in Alcator C-Mod S. D. Scott Princeton Plasma Physics Laboratory In collaboration with R. Granetz, D. Beals, C.

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Presentation transcript:

Evaluation of Anomalous Fast-Ion Losses in Alcator C-Mod S. D. Scott Princeton Plasma Physics Laboratory In collaboration with R. Granetz, D. Beals, C. Fiore, M. Greenwald MIT PLASMA Science and Fusion Center W. Rowan Fusion Research Center, University of Texas at Austin Alcator C-Mod Meeting November 10, 2003

Introduction Measured beam-target neutron rate during deuterium- DNB injection is about a factor of two lower than predicted by classical slowing down. Is this due to: Anomalous fast ion losses Neutron detector calibration error Anisotropic neutron emission rate Other diagnostic errors (Te, Zeff, Ne, impurity mix) Beam attenuation through edge plasma Errors in beam current measurement, full-energy fraction, or beam divergence.

Summary Measured beam-target neutron rate during deuterium- DNB injection is about a factor of two lower than predicted by classical slowing down. Is this due to: Anomalous fast ion lossesprobably not Neutron detector calibration error Anisotropic neutron emission rate Other diagnostic errorsnot from usual suspects Beam attenuation through edge plasmanot in all cases Errors in beam current measurement, possibly full-energy fraction or beam divergence.

C-Mod Diagnostic Beam 50 keV, ~4 amps Injected radially on midplane Diameter ~8 cm Classical fast-ion confinement is excellent. Collisionless orbits of 50 keV deuterons injected at 90 o, Z = 4 cm, R=75, 83 cm. Technique: compare beam-target dd neutron emission during D-DNB injection to classical calculations.

Plasma Conditions SHOTPlasmaLocked IpIp BtBt P RF N e (0)T e (0)T i (0)Z eff RegimeMode? 25Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeno Lmodeno Lmodeno Lmodeno Hmodeno Hmodeno MATeslaMW10^20keVKeV

Transp Beam and Neutron Calculations DNB Power (kW)Neutrons (10 12 /sec) SHOTPlasmaLockedPinjShineOrbitCXTRANSPMeasured RegimeMode? TOTALTXB-BB-TTotalBTRatio 25Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeyes Lmodeno Lmodeno Lmodeno Lmodeno Hmodeno Hmodeno

Calculated neutron rates from Global Model

Beam Deposition Profiles Range from Centrally-peaked to Edge-peaked

Transp Neutron Simulations (ordered by density) Shot 18 Neo 1.0 Ip 0.97 L-Mode locked Shot 25 Neo 0.5 Ip 0.98 L-Mode locked L-Mode locked Shot 23 Neo 1.0 Ip 0.97 Shot 24 Neo 1.3 Ip 0.97 L-Mode locked MEASURED THERMONUCLEAR BEAM-BEAM BEAM-TARGET TOTAL

Shot 17 Neo 1.3 Ip 0.98 L-Mode locked Shot 09 Neo 1.8 Ip 0.97 Shot 31 Neo 1.7 Ip 0.59 L-Mode Shot 11 Neo 1.7 Ip 0.97 L-Mode locked MEASURED THERMONUCLEAR BEAM-BEAM BEAM-TARGET TOTAL Transp Neutron Simulations (ordered by density)

Shot 15 Neo 4.0 Ip 0.98 H-Mode P rf = 2.3 Shot 32 Neo 1.8 Ip 0.38 L-Mode Shot 29 Neo 1.9 Ip 0.96 L-Mode Shot 30 Neo 3.0 Ip 0.96 H-Mode Transp Neutron Simulations (ordered by density)

Measured Beam-Target D-D Neutron Emission is a Factor ~2 less than Classical TRANSP Predictions for N eo > m -3 H-mode L-Mode Locked L-Mode

Nominal Analysis (black) Error Analysis Te +/- 10% Ne +/- 10% Zeff +/- 15% = 6, 20 edge attenuation TRANSP Error Analysis at Low Density L-mode Locked-mode Ip 0.97 Neo 1.0 Zeff 3.6 shot 23 Dominant uncertainty is composition of impurity mixture … Zeff is high, so dilution is significant. Measured neutron emission is less than predicted by TRANSP, but within diagnostic uncertainty. Attenuation of beam by edge gas pressure is a negligible effect. N h+d Z eff – 1 N e -1 =

TRANSP Error Analysis in Medium-Density L-Mode Nominal Analysis (black) Error Analysis Te +/- 10% Ne +/- 10% Zeff +/- 15% = 6, 20 edge attenuation MEASURED shot 17 L-mode Ip 0.98 Neo 1.3 Zeff 2.8 Measured neutron rate is just within measurement uncertainty. Dominant uncertainty is composition of impurity mix. Possible beam attenuation at plasma edge would more than reconcile measured neutron rate with classical predictions.

L-mode Locked-mode Ip 0.97 Neo 1.8 Zeff 1.6 Shot 09 TRANSP Error Analysis in Medium-Density, Locked L-Mode Nominal Analysis (black) Error Analysis Te +/- 10% Ne +/- 10% Zeff +/- 15% = 6, 20 edge attenuation Attenuation of beam by edge neutral gas might explain the low neutron emission.

L-mode Ip 0.38 Neo 1.8 Zeff 2.0 Shot 32 TRANSP Error Analysis in low-Ip, Medium Density L-mode Nominal Analysis (black) Error Analysis Te +/- 10% Ne +/- 10% Zeff +/- 15% = 6, 20 edge attenuation Excited-State Depo Measured neutron rate is anomalously low, outside of measurement uncertainties considered.

Nominal Analysis (black) Error Analysis Te +/- 10% Ne +/- 10% Zeff +/- 15% = 6, 20 edge attenuation H-mode Ip 0.96 Neo 3.0 Zeff 2.1 Shot 30 TRANSP Error Analysis in High Density H-mode The customary diagnostic uncertainties do not reconcile the measured neutron rate with TRANSP. Edge neutral pressure is low, so attenuation of beam by edge gas pressure is a negligible effect.

Measured Neutron Rate Implies Large Beam Ion Diffusivity D = 60 m 2 /s is best fit to magnitude of Rdd

Neutron Detector has ~0.1 ms Time Response 0.2 ms 0.15 ms 0.10 ms disruption

Scale Transp to Match Measured Rdd at end of DNB Pulse

Scaled D=0 Provides Best Match to Measured Rdd following beam turn-off D=60 is poor match to measured decay rate.

Scaled D=0 Also Provides Best Match to Measured Rdd at Beam Turn-on D=60 is poor match to measured rise time.

DNB Current Typically Varies during Pulse 18% Time (sec) Amps

MSE signal strength is nonlinear function of I DNB 26% 32% 18% 35%  I DNB = 18% Suggests problem with beam current or full-energy fraction or beam divergence.

Summary Measured beam-target neutron rate during deuterium- DNB injection is about a factor of two lower than predicted by classical slowing down. Is this due to: Anomalous fast ion lossesprobably not Neutron detector calibration error Anisotropic neutron emission rate Other diagnostic errorsnot from usual suspects Beam attenuation through edge plasmanot in all cases Errors in beam current measurement, possibly full-energy fraction or beam divergence.

Conclusions Both a simplified global analysis and standard TRANSP analysis show that the measured d-d beam-target neutron emission is a factor ~2 less than expected from classical fast ion confinement and thermalization. In some -- but not all – plasmas, the discrepancy is within diagnostic uncertainty. Error analysis is ongoing. An enormous fast-ion diffusivity (60-80 m 2 /s) is required to match the measured neutron emission rate. The time dependence of the neutron rate does not match simulations with such high diffusivities.

Measured Neutron Rate Implies Large Beam Ion Diffusivity D = m 2 /s is best fit to magnitude of Rdd

Scale Transp-Predicted Neutron Rate to match Neutrons at end of beams

High Edge Neutral Pressure is Observed in Some High Density Plasmas

High Edge Neutral Pressure May Reduce Expected Neutron Emission by 20-40% in Some Plasmas Range of neutral pressure in various plasmas Edge Pressures and expected attenuation are small at low- to moderate density

Estimated Beam Attenuation through Plasma Edge (full energy component)

Beam Shine-Through Fraction Beam Shine-Thru is Small Except at the Lowest Density Power loss (kw) P DNB ~ 135 kW Beam Shine-Through Power

Classical Beam Orbit Losses are Less Than 15% Except at Highest Densities Power loss (kw) P DNB ~ 135 kW

File: neutrons nov10.ppt Date:November 10, 2003