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Title: Studies of local electron heat transport on TFTR

Abstract

The anomalously fast relaxation of the perturbations to the electron temperature profile caused by a sawtooth crash has been studied extensively on TFTR. We will show that on a short timescale the heat pulse is not simply diffusive as has been generally assumed, but that modeling of the heat pulse requires a transient enhancement in {chi}{sub e} following the sawtooth crash. It will be shown that the time-dependent enhancement in {chi}{sub e} predicted by non-linear thermal transport models, i.e., incremental {chi} models or the Rebut-Lallia-Watkins transport model, is much smaller than that required to explain the anomalies in the heat pulse propagation.

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Princeton Univ., NJ (United States). Plasma Physics Lab.
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
10183507
Report Number(s):
PPPL-2898
ON: DE93040429; TRN: 93:020125
DOE Contract Number:
AC02-76CH03073
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 16 Aug 1993
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; TFTR TOKAMAK; CHARGED-PARTICLE TRANSPORT; HEAT TRANSFER; ELECTRON TEMPERATURE; SAWTOOTH OSCILLATIONS; HEAT FLUX; THERMAL DIFFUSION; 700330; PLASMA KINETICS, TRANSPORT, AND IMPURITIES

Citation Formats

Fredrickson, E.D., Chang, Z.Y., Janos, A., McGuire, K.M., Scott, S., and Taylor, G. Studies of local electron heat transport on TFTR. United States: N. p., 1993. Web. doi:10.2172/10183507.
Fredrickson, E.D., Chang, Z.Y., Janos, A., McGuire, K.M., Scott, S., & Taylor, G. Studies of local electron heat transport on TFTR. United States. doi:10.2172/10183507.
Fredrickson, E.D., Chang, Z.Y., Janos, A., McGuire, K.M., Scott, S., and Taylor, G. Mon . "Studies of local electron heat transport on TFTR". United States. doi:10.2172/10183507. https://www.osti.gov/servlets/purl/10183507.
@article{osti_10183507,
title = {Studies of local electron heat transport on TFTR},
author = {Fredrickson, E.D. and Chang, Z.Y. and Janos, A. and McGuire, K.M. and Scott, S. and Taylor, G.},
abstractNote = {The anomalously fast relaxation of the perturbations to the electron temperature profile caused by a sawtooth crash has been studied extensively on TFTR. We will show that on a short timescale the heat pulse is not simply diffusive as has been generally assumed, but that modeling of the heat pulse requires a transient enhancement in {chi}{sub e} following the sawtooth crash. It will be shown that the time-dependent enhancement in {chi}{sub e} predicted by non-linear thermal transport models, i.e., incremental {chi} models or the Rebut-Lallia-Watkins transport model, is much smaller than that required to explain the anomalies in the heat pulse propagation.},
doi = {10.2172/10183507},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Aug 16 00:00:00 EDT 1993},
month = {Mon Aug 16 00:00:00 EDT 1993}
}

Technical Report:

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  • Results from helium, iron, and electron transport on TFTR in L-mode and Supershot deuterium plasmas with the same toroidal field, plasma current, and neutral beam heating power are presented. They are compared to results from thermal transport analysis based on power balance. Particle diffusivities and thermal conductivities are radially hollow and larger than neoclassical values, except possibly near the magnetic axis. The ion channel dominates over the electron channel in both particle and thermal diffusion. A peaked helium profile, supported by inward convection that is stronger than predicted by neoclassical theory, is measured in the Supershot The helium profile shapemore » is consistent with predictions from quasilinear electrostatic drift-wave theory. While the perturbative particle diffusion coefficients of all three species are similar in the Supershot, differences are found in the L-Mode. Quasilinear theory calculations of the ratios of impurity diffusivities are in good accord with measurements. Theory estimates indicate that the ion heat flux should be larger than the electron heat flux, consistent with power balance analysis. However, theoretical values of the ratio of the ion to electron heat flux can be more than a factor of three larger than experimental values. A correlation between helium diffusion and ion thermal transport is observed and has favorable implications for sustained ignition of a tokamak fusion reactor.« less
  • Results from helium, iron, and electron transport on TFTR in L-mode and Supershot deuterium plasmas with the same toroidal field, plasma current, and neutral beam heating power are presented. They are compared to results from thermal transport analysis based on power balance. Particle diffusivities and thermal conductivities are radially hollow and larger than neoclassical values, except possibly near the magnetic axis. The ion channel dominates over the electron channel in both particle and thermal diffusion. A peaked helium profile, supported by inward convection that is stronger than predicted by neoclassical theory, is measured in the Supershot The helium profile shapemore » is consistent with predictions from quasilinear electrostatic drift-wave theory. While the perturbative particle diffusion coefficients of all three species are similar in the Supershot, differences are found in the L-Mode. Quasilinear theory calculations of the ratios of impurity diffusivities are in good accord with measurements. Theory estimates indicate that the ion heat flux should be larger than the electron heat flux, consistent with power balance analysis. However, theoretical values of the ratio of the ion to electron heat flux can be more than a factor of three larger than experimental values. A correlation between helium diffusion and ion thermal transport is observed and has favorable implications for sustained ignition of a tokamak fusion reactor.« less
  • A database of 71 distinct and randomly collected cold pulse cases from TFTR is analyzed. Observations show a striking parameter regime cutoff for the presence of nonlocal transient transport and coincident MHD (1/1-mode) activity as well as for changes in the radial speed of the nonlocal transport effect and changes in the sawtooth period. A nontrivial link is demonstrated between electron heat transport and MHD properties through observation of a common cutoff in the parameter n{sub e}(0)/T{sub e}(0){sup 1/2} and a common threshold in injection size for radial speed and sawtooth period changes. Auxiliary heating (via energetic neutral beams) destroysmore » whatever process is responsible for the nonlocal transport effect, unless the discharge contains significant amounts of injected tritium. These observations are preliminary, but they represent important circumstantial evidence for mysterious propagation of changes in some MHD-related phenomenon as being responsible for a large fraction of electron heat transport. This propagation is then probably a function of n{sub e}(0)/T{sub e}(0){sup 1/2}, ion mass, and possibly beam power. An analysis of Ohmic cases shows that the cutoff in n{sub e}(0)/T{sub e}{sup 1/2} indicates the nonlocal transport effects may occur when the electrons are collisionally thermally decoupled from the ions.« less
  • The roles of turbulence stabilization by sheared E x B flow and Shafranov-shift gradients are examined for TFTR. Enhanced Reverse-Shear plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E x B and Shafranov shiftmore » effects was performed by varying the E x B shear through changes in the toroidal velocity with nearly-steady-state pressure profiles. Transport and fluctuation levels increase only when E x B shearing rates are driven below a critical value that is comparable to the fastest linear growth rates of the dominant instabilities. While a turbulence suppression criterion that involves the ratio of shearing to linear growth rates is in accord with many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was varied. The forward transition into the ERS regime has also been examined in strongly rotating plasmas. The power threshold is higher with unidirectional injection than with balanced injection.« less
  • Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core comparedmore » to that in the rest of the supershot plasma.« less