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1.
Phys Rev Lett ; 132(6): 065107, 2024 Feb 09.
Article in English | MEDLINE | ID: mdl-38394599

ABSTRACT

The perturbed ion temperature and toroidal flow were measured in rotating neoclassical tearing modes (NTM) in a tokamak for the first time. These toroidally and radially resolved profiles were obtained by impurity ion spectroscopy in a 2,1 NTM in DIII-D. In agreement with drift-kinetic simulations, the electron temperature profile is flat, while the ion temperature gradient is restored across the magnetic island O point in the presence of fast ions; the perturbed flow has minima in the O points and maxima at the X points. These measurements provide the first confirmation of the theoretically expected ion temperature and flow response to a magnetic island needed to predict the NTM onset threshold scaling for ITER and other future tokamaks.

2.
Phys Rev Lett ; 114(10): 105002, 2015 Mar 13.
Article in English | MEDLINE | ID: mdl-25815938

ABSTRACT

Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.

3.
Phys Rev Lett ; 108(24): 245003, 2012 Jun 15.
Article in English | MEDLINE | ID: mdl-23004282

ABSTRACT

Predictions are developed for gradients and profiles of the electron density and temperature in tokamak H-mode pedestals that are in transport quasiequilibrium. They are based on assuming paleoclassical processes provide the irreducible minimum radial plasma transport and dominate in the steep gradient regions of pedestals. The predictions agree (within a factor of about two) with properties of a number of pedestal experimental results.

5.
Phys Rev Lett ; 106(22): 225002, 2011 Jun 03.
Article in English | MEDLINE | ID: mdl-21702606

ABSTRACT

Observation of a theoretically predicted peak in the neoclassical toroidal viscosity (NTV) force as a function of toroidal plasma rotation rate Ω is reported. The NTV was generated by applying n=3 magnetic fields from internal coils to low Ω plasmas produced with nearly balanced neutral beam injection. Locally, the peak corresponds to a toroidal rotation rate Ω(0) where the radial electric field E(r) is near zero as determined by radial ion force balance.

6.
Phys Rev Lett ; 102(24): 245005, 2009 Jun 19.
Article in English | MEDLINE | ID: mdl-19659019

ABSTRACT

Anomalously fast equilibration of the electron distribution function to a Maxwellian in gas-discharge plasmas with low temperature and pressure, i.e., Langmuir's paradox, may be explained by electron scattering via an instability-enhanced collective response and hence fluctuations arising from convective ion-acoustic instabilities near the discharge boundaries.

7.
Phys Rev Lett ; 103(20): 205002, 2009 Nov 13.
Article in English | MEDLINE | ID: mdl-20365986

ABSTRACT

A generalized Lenard-Balescu theory that accounts for instability-enhanced collective responses is used to calculate the collisional friction between ion species in the plasma-boundary transition region (presheath). Ion-ion streaming instabilities are shown to cause such a strong frictional force that the relative flow speed between ion species cannot significantly exceed the critical threshold value (DeltaV(c)) at which instability onset occurs. When combined with the Bohm criterion, this condition uniquely determines the flow speed of each ion species at the plasma-sheath boundary. For cold ions, DeltaV(c) --> 0 and each ion species leaves the plasma at a common system sound speed c(s).

8.
Phys Rev Lett ; 99(6): 065001, 2007 Aug 10.
Article in English | MEDLINE | ID: mdl-17930835

ABSTRACT

A model for field-error penetration is developed that includes nonresonant as well as the usual resonant field-error effects. The nonresonant components cause a neoclassical toroidal viscous torque that keeps the plasma rotating at a rate comparable to the ion diamagnetic frequency. The new theory is used to examine resonant error-field penetration threshold scaling in Ohmic tokamak plasmas. Compared to previous theoretical results, we find the plasma is less susceptible to error-field penetration and locking, by a factor that depends on the nonresonant error-field amplitude.

9.
Phys Rev Lett ; 94(5): 055002, 2005 Feb 11.
Article in English | MEDLINE | ID: mdl-15783654

ABSTRACT

It is hypothesized that radial electron heat transport in magnetically confined toroidal plasmas results from paleoclassical Coulomb collision processes (parallel electron heat conduction and magnetic field diffusion). In such plasmas the electron temperature is equilibrated along magnetic field lines a long length L (>> poloidal periodicity length piR0q), which is the minimum of the electron collision length and an effective field line length. Thus, diffusing field lines induce a radial electron heat diffusivity M identical with L/(piR0q) approximately 10>>1 times the magnetic field diffusivity eta/mu0 approximately nue(c/omegap)2.

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