Reduction of Turbulence and Transport in the Alcator C-Mod Tokamak by Dilution of Deuterium Ions with Nitrogen and Neon Injection M. Porkolab, P. C. Ennever, S. G. Baek, E. M. Edlund, J. Hughes, J. E. Rice, J. C. Rost and the Alcator C-Mod Team, MIT PSFC; G. M. Staebler and J. Candy, General Atomics; M. L. Reinke, ORNL EPS 2017, 44 th Conference on Plasma Physics, Belfast, Northern Ireland, June 26-30, 2017 Poster P5-176 * Supported by US DOE awards DE-FG02-94-ER54235 and DE-FC02-99-ER54512 1
Motivation for Dilution Experiments on C-Mod Controlled injection of Nitrogen or Neon for improving divertor heat load performance during core-edge physics integration may have beneficial impact on core transport which mitigates (even cancels) radiative power loss from the plasma core Additional benefit of medium Zi impurity injection to machines with ITER Like Wall (ILW ) (eg, tungsten) is to control high Z impurity influx Neon is preferred for reactor (ITER) applications due to tritium inventory control (no ammonia formation as with nitrogen) How to mitigate edge pedestal pressure reduction needs further investigation (c/f JET) Seeding induced change in intrinsic rotation was seemingly unrelated to a change in electron collisionality - appears to be related to n D q 95 - ion collisionality? 2
Background and Motivation - Recent dilution experiments in C-Mod were motivated by earlier experiments (2010-2012), confirmed by TGLF modeling (Staebler and Candy) that reducing n D /n e by up to 20 % by assuming a few % Zi = 8 impurity species reduced core ion energy transport (Porkolab et al, PPCF 54, 124029 (2012)) - Controlled Nitrogen seeding experiments in Alcator C-Mod in 2014 (Ennever, Porkolab, et al, Phys. Plasmas 22, 072507 (2015), ibid 23, 082509 (2016) ) and more recently by Neon (Porkolab et al, IAEA FEC 2016) further diluted the plasma, resulting in a reduction of ion energy transport beyond the residual values and also reduced density fluctuations (electron energy transport was changed to a lesser extent) - Reduced ion transport was correlated with reduced turbulence as measured by Phase Contrast Imaging (PCI) in the region r/a 0.6-0.8; turbulence is absent for r/a < 0.5 due to weak gradients (but sawtoothing at r/a < 0.3) - Nonlinear GYRO simulations in regions where turbulence dominates transport agreed with experimental energy fluxes and density fluctuation measurements turbulence reduction was due either to changes in critical ion temp gradient (LOC regime) or changes in stiffness (SOC regime) - Global energy confinement did not change since improved ion energy confinement was countered by increased core radiation 3
Ohmic plasmas display two characteristic energy confinement regimes: Linear Ohmic Confinement (LOC) and Saturated Ohmic Confinement (SOC) Figure from P. Ennever, M. Porkolab, J. Candy, et al, Phys. Plasmas 22, 072507 (2015). - The determining parameter is n e q 95 - Safety factor q 95 (B T /B p )(a/r) - At low densities, the energy confinement time is linearly related to density: linear ohmic confinement (LOC) - At high densities, the energy confinement time saturates and is independent of density: saturated ohmic confinement (SOC) - LOC and SOC plasmas also have different intrinsic rotation directions 4
Assuming that plasmas were diluted by a medium Z i impurity species (Z i = 8), TGLF predicted reduced ion energy transport in agreement with experiment Electron Diffusivity Ion Diffusivity Impurity Assumed to be Oxygen (Z imp =8) Ion Fraction (n D /n e ) Z eff 0.93 1.5 0.84 2.1 *0.76 *2.7 0.64 3.5 *Experimental Value A similar scan assuming Z imp = 40 showed no effect of Z eff on energy transport From: Porkolab, M., et al. Plasma Physics and Controlled Fusion 54.12 (2012): 124029. 5
To determine the physics of dilution, ohmic C-Mod plasmas were seeded with nitrogen at a range of I P and n e values while n e was held fixed in time using a cryopump - q 95 was changed by changing I p, B T = 5.4 T for all plasmas - Changing I p changed P OH, T e, T i - Seeding increased Z eff by about 1 - Seeding decreased n D /n e by 10-20% 6
Adding nitrogen decreased the concentrations of both intrinsic impurities (meaning it was more than a modification of impurity source) 7
Nitrogen seeding reduced the ion energy flux and increase the ion temperature gradient at r/a = 0.8, indicating a change in the turbulent energy transport q 95 = 3.9, SOC, n D /n e = 0.95 (unseeded), 0.84 (seeded) 8
The turbulence was measured with phase contrast imaging (PCI), an absolutely calibrated diagnostic that measures line-integrated density fluctuations - The PCI system on C-Mod images density fluctuations along 32 chords onto a LN 2 cooled photoconductive HgCdTe detector - PCI signal V ñ e dl ; only sensitive to waves moving perpendicular to chords - Line-integration means that fluctuations from the core and edge, top and bottom, are added together - Measures k R of turbulence, which is approximately k Θ in the core - Sensitive to waves with f < 1.5 MHz and 0.5 cm -1 < k R < 30 cm -1 PCI Chords 9
In the high-current (q 95 = 3.4) plasmas, a high-frequency feature of the measured PCI spectrum was reduced by nitrogen seeding 10
Nitrogen seeding significantly reduced high frequency (f > 150 khz) PCI spectral feature in both LOC and SOC q 95 = 3.4 cases ~ 11
Radially localized reflectometer channels measuring density fluctuations in the region of 0.8 r/a 0.9 showed a similar decrease in high-f fluctuations to PCI 12
The synthetic PCI amplitude from local GYRO simulations of the q 95 = 3.4 plasmas at r/a = 0.85 agrees with the experimental PCI within uncertainties 13
At r/a > 0.75 the ion turbulence level is well above marginal stability, and Q i >> Q GB ; importantly, seeding with nitrogen reduced both turbulence and heat flux Reflectometer shows moderate decrease in fluctuations with seeding Reflectometer shows large decrease in fluctuations with seeding 14
Nitrogen seeding did not substantially affect the total plasma energy confinement time q 95 Unseeded Seeded 4.2 3.9 3.4 15
Summary of Nitrogen Seeding Experiments - Experiments on Alcator C-Mod demonstrated that nitrogen seeding reduced ion energy transport and density fluctuations at r/a = 0.85 where the turbulence dominates ion heat transport - Local nonlinear GYRO simulations were able to simultaneously match the turbulent energy flux and density fluctuation amplitudes - Although linear GYRO predicted reduction of ETG growth rates by dilution, nonlinear multiscale (ETG- and ITG-scale) simulations are unavailable at this time - Change in intrinsic rotation was related to n D q 95, seemingly unrelated to ν eff 16
Seeding with neon also decreased the concentrations of intrinsic impurities 17
Neon seeding diluted more than nitrogen seeding (larger change in Z eff ) 18
Both nitrogen and neon seeding substantially increased the neutron rate (T i increased by about 10%) 19
Change in ohmic power with seeding is negligible and cannot explain the increase in neutron rate 20
Neither nitrogen nor neon seeding improved global energy confinement 21
Effect of dilution on toroidal velocity seemed consistent between neon and nitrogen seeded discharges - both cases were dependent on n D q 95 22
Summary - Nitrogen and neon seeding both showed net improvement in ion transport (reduced turbulence) and fusion performance (increased neutron rate ) - Overall, neon and nitrogen show similar effects on transport and turbulence: - Ion transport was reduced - Similar reduction in turbulence - Ion temperature gradients with nitrogen seeding shown to increase - Although not discussed here, similar intrinsic rotation behavior - The overall global energy confinement did not change because of increased radiation losses; - Dilution will be present in future fusion devices such as ITER, due to both helium ash buildup and extrinsic impurity seeding that will be necessary to reduce divertor heat loads 23