The physics of transport barrier formation in the PBX-M H-mode

Abstract

Measurements of edge profiles, turbulence, and turbulentdriven transport were made inside the last-closed flux surface (LCFS) and in the scrape-off layer (SOL) of PBX-M L-mode and H-mode plasmas using a fast reciprocating Langmuir probe diagnostic. Direct measurements of the potential profile confirm the generation of a strong inward radial electric field (Er --100 V/cm) just inside the LCFS in Hmode. Density and potential fluctuation levels are reduced at the L-H transition, resulting in significantly lower turbulent transport. The edge density gradient then steepens in response to the reduced cross-field transport. After a stable H-mode edge equilibrium is formed, the fluctuation amplitudes recover back to nearly L-mode values. The poloidal wavenumber spectrum is narrowed in the transport barrier, and the density and potential fluctuations are decorrelated. The reduction in turbulent transport occurs

Edge turbulence measurements in toroidal fusion devices

“…Changes were seen at the L-H transition in DIII-D in the intermittency [137], bispectral coupling [162] and nonlinear dynamics [163]. Langmuir probe measurements have also seen a change in the phase and correlation coefficient between the density and potential fluctuations, which enters into the turbulent transport rate [164,165]. A difference between the long-time period correlations in L-mode and H-mode was seen in MAST [71].…”

Edge turbulence and SOL transport in tokamaks

“…A velocity shear layer was identified [28] in the boundary of tokamaks and stellarators by measuring the phase velocity of the turbulence in the poloidal direction [28,93], which changes sign from the electron diamagnetic drift direction inside the last closed flux surface to the ion diamagnetic drift direction in the SOL [94]. The shear layer is correlated with the reduction of turbulence [95], effect that has been verified in many devices [45,[96][97][98][99] and can be responsible for the formation of an edge transport barrier resulting on a spontaneous transition from low (L-mode) to high (H-mode) [100] confinement, a bifurcation process. However, the process was only fully verified when H-mode-like behavior was obtained after a velocity sheared region was introduced artificially via an externally applied radial electric field [101,102].…”

“…A reduction in T e fluctuations was associated with the shear flow in biased Hmodes in TEXTOR [29] and changes have been seen in the phase and correlation coefficient between the density and potential fluctuations, leading to a reduction in the turbulent transport rate [99,109]. Finally, changes were seen at the L-H transition in DIII-D in the intermittency [45], bi-spectral coupling [110] and nonlinear dynamics [111], indicating they are all interconnected.…”

Transport by intermittency in the boundary of the DIII-D tokamak

“…Since ⌫ r has been characterized extensively in many devices [35][36][37][38] and in particular, DIII-D, 39 the Texas Experimental Tokamak 40 ͑TEXT͒, [41][42] and the Torus Experiment for Technology Oriented Research 43 ͑TEXTOR͒ 44,45 it can therefore be stated that the general properties of the intermittency, such as its response to sheared electric fields that decorrelate turbulent transport resulting in reduced transport and enhanced confinement conditions, have been indirectly characterized.…”