Investigation of causes for the discrepancy between the measured and modeled helium emissions using a gas puff imaging diagnostic S. Baek, J. Terry, D. P. Stotler*, D. Brunner, B. LaBombard MIT Plasma Science and Fusion Center *Princeton Plasma Physics Laboratory 59th Annual Meeting of the Division of Plasma Physics Milwaukee, Wisconsin October 23, 2017 1/11
Goal: Validate a neutral transport code for a quantitative study of neutral transport using a gas-puff imaging (GPI) diagnostic GPI measurements are useful in studying plasma fluctuations at the tokamak boundary. Theodorsen CO4.00012, Kube YI2.00003 The absolute brightness profile comparison with DEGAS 2* is useful for: Code validation (first time with helium) GPI design We compare the profiles in the D2- and he- puff experiments, and attribute the discrepancy observed in the he-puff case to: Gas puff perturbations Plasma fluctuations 2-D Helium I line brightness (587 nm) profile Z LCFS R *Stotler, http://w3.pppl.gov/degas2/ 2/11
3/11 A gas puff imaging diagnostic on Alcator C-Mod Side View of C-Mod Tokamak A nozzle that puffs deuterium molecules or helium atoms at the low-field-side. A optics system that images the emission with a rectangular field of view (4 cm x 4 cm). Both the gas puff flow rate and detector response are absolutely calibrated.
4/11 A good agreement is found between the experimental and modeled D α brightness profiles. (n e /n G = 0.2) Both the peak brightness and the profile shape are well-matched. In line with the previous study on NSTX [Cao, FST (2009)].
The experimental helium peak brightness is lower by a factor of three than the modeled peak brightness. (n e /n G = 0.2) (n e /n G = 0.7) The profile shape is not reproduced at higher densities. The shifts in the input n e & T e profiles did not resolve the discrepancy. A better match is found with the decrease in the input T e. 5/11
The injected helium neutrals remain cold. Deuterium atoms have a relatively high energy. Dissociation ( ~ 3 ev) from the injected D 2 molecules Charge exchange Helium atoms Weak elastic scattering with D + Remain cold (~0.026 ev) with a short mean free path Incoming Neutral Flux Helium neutrals have a weaker penetration, and they are less dispersive in space than deuterium neutrals [Stotler JNM (2007), Cao FST (2009)] Labombard, KN1D, PSFC/RR-01-3 (2001) 6/11
7/11 Electron cooling can occur via neutral ionizations. Gas-puff cooling might cause a drop in T e on the flux tubes that intersect the gas cloud. DEGAS 2 Result The SOL ne & Te profile measurement location is not on the same flux tube. Cooling rate (kw/m 2 ) along the toroidal direction can be estimated from DEGAS 2. (n e /n G = 0.7)
8/11 The cooling effect can be significant in the far SOL. The cooling rate (q,loss ) can be compared with the heat flux required to sustain the measured T e. (n e /n G = 0.7) q 200 kw/m 2 for T u = 10 ev q k 0 T u 7 2 /(3.5 L) where k 0 = 2000 [W/m/eV 7/2 ] & L 10 m for C-Mod q,loss > q for Te 10 ev Consistent with the model analysis (i.e., the need for the drop in T e )
9/11 Another possible cause is the presence of plasma fluctuations. Time series of the measured n e & T e T e n e S ion ( n e, T e ) is used in the model But, S ion ( n e, T e ) S ion (n e, T e ) Modeling studies [Marandet AIP 2017] show: Increase in the effective ionization rate in the presence of the T e fluctuation. Neutrals with a short m.f.p. are more susceptible to the time-dependent effect. Effective ionization rate can experimentally be evaluated using the high-resolution SOL data.
Effective ionization rate can be increased up to 50% at high density. The effective emission rate coefficient is 10% lower than that evaluated from <n e > & <T e >. With the effective ionization rate of S ion, eff = 1.5 S ion, the neutral density is found to decrease by ~15% in a simple 1D model. The effective emission decreases by 25% at high density. Helium-puff experiments 10/11
11/11 Summary In the GPI experiment, the measured helium brightness peak is found to be lower by a factor of than the modeled brightness peak. In the low T e region, the gas-puff cooling may be significant, consistent with the model analysis. At a high density, plasma fluctuations can partially be responsible for the discrepancy (~25%). Implications in the helium GPI data anaysis: Gas puff perturbation effects may need to be considered. Helium neutral transport may be subject to plasma fluctuation effects.