Toroidal and poloidal momentum transport studies in JET

Abstract

This paper reports on the recent studies of toroidal and poloidal momentum transport in JET. The ratio of the global energy confinement time to the momentum confinement is found to be close to τ E /τ φ =1 except for the low density discharges where the ratio is τ E /τ φ =2-3. On the other hand, local transport analysis of tens of discharges shows that the ratio of the local effective momentum diffusivity to the ion heat diffusivity is χ φ /χ i 0.1-0.4 rather than unity, as expected from the global confinement times and used in ITER predictions. The apparent discrepancy in the global and local momentum versus ion heat transport is explained by the fact that momentum confinement within edge pedestal is worse than that of the ion heat and thus, momentum pedestal is weaker than that of ion temperature. Another observation is that while the T i has a threshold in R/L Ti and profiles are stiff, the gradient in v φ increases with increasing torque and no threshold is found. Predictive transport simulations also confirm that χ φ /χ i 0.1-0.4 reproduce the core toroidal velocity profiles well. Concerning poloidal velocities on JET, the experimental measurements show that the carbon poloidal velocity can be an order of magnitude above the neo-classical estimate within the ITB. This significantly affects the calculated radial electric field and therefore, the E×B flow shear used for example in transport simulations. The Weiland model reproduces the onset, location and strength of the ITB well when the experimental poloidal rotation is used while it does not predict an ITB using the neo-classical poloidal velocity. The most plausible explanation for the generation of the anomalous poloidal velocity is the turbulence driven flow through the Reynold's stress. Both TRB and CUTIE turbulence codes show the existence of an anomalous poloidal velocity, being significantly larger than the neo-classical values. And similarly to experiments, the poloidal velocity profiles peak in the vicinity of the ITB and is caused by flow due to the Reynold's stress.

Comparison of Poloidal Velocity Meassurements to Neoclassical Theory on the National Spherical Torus Experiment

“…These behaviors give some confidence in the veracity of the poloidal velocity measurements. The measured poloidal velocity profiles on NSTX are equal to or lower in magnitude than expected from neoclassical calculations from NCLASS and GTC-Neo, in contrast to measurements on JET and DIII-D. [2][3][4] Differential changes in measured mean poloidal velocity are seen to be a factor of 2.6-3.3 lower than changes in NCLASS and GTC-Neo. There is no indication of an inconsistency with the direction of the measured poloidal velocity compared to neoclassical expectations, as was observed on DIII-D. 4 These results are consistent with measurements on MAST with showed that, within experimental error, the measured poloidal flow was similar to neoclassical values in magnitude and direction.…”

“…Some recent measurements of poloidal velocity differ by an order of magnitude with neoclassical predictions: on JET in the region of an internal transport barrier (ITB) 2,3 , and on DIII-D during H-mode and quiescent H-mode discharges 4 , where the direction of the poloidal flow also differed from the neoclassical expectation. Measurements of poloidal flow on JT-60U…”

On steady poloidal and toroidal flows in tokamak plasmas

“…16 As noted previously, rapid poloidal rotation has been observed to be correlated with internal transport barriers in several tokamaks, including JET. 10,28 The highest poloidal velocities recorded in this device, 75Ϯ 20 km s −1 ͑in the ion diamagnetic direction͒, were measured inside a transport barrier in shot number 58094; immediately outside the barrier, the poloidal velocity measurement was consistent with zero. Between these two radial locations, respectively, at R Ӎ 3.34 m and R Ӎ 3.46 m, the ion temperature was observed to drop from about 20 to about 7 keV, while the electron density dropped by approximately a factor of two, the local safety factor q being close to 2 at both locations.…”

Physics Mechanisms of Toroidal Rotation Profile and Properties of Momentum Transport in JT-60U

“…1. The worldwide progress in understanding the physics of momentum transport and rotation has been made both experimentally [9][10][11][12][13][14][15][16][17][18][19][20] and theoretically [21][22][23][24][25]. As for the elements determining V t , the characteristics of momentum transport, which consists of diffusive (toroidal momentum diffusivity, χ φ ) and non-diffusive (convection velocity, V conv ) terms [10,14,18], the external momentum input, author's e-mail: yoshida.maiko@jaea.go.jp and the intrinsic rotation [15,17,19,23] have been reported individually.…”