Impurity toroidal rotation has been observed in the centre of Alcator C-Mod ion cyclotron range of frequencies (ICRF) heated plasmas, from the Doppler shifts of argon X ray lines. Rotation velocities greater than 1.2 × 107 cm/s (ω = 200 krad/s) in the co-current direction have been observed in H mode discharges that had no direct momentum input. There is a correlation between the increase in the central impurity rotation velocity and the increase in the plasma stored energy (confinement enhancement), induced by ICRF heating, although other factors may be at play. The toroidal rotation velocity is highest near the magnetic axis, and decreases with increasing minor radius. A radial electric field of 300 V/cm at r/a = 0.3 has been inferred from the force balance equation. The direction of the rotation changes when the plasma current direction is reversed, remaining co-current. Impurity toroidal rotation in ICRF heated plasmas is in the direction opposite to the rotation in ohmic L mode plasmas; co-current rotation has also been observed during purely ohmic H modes. When the ICRF heating is turned off, the toroidal rotation decays with a characteristic time of order 50 ms, similar to the energy confinement time, and much shorter than the calculated neoclassical momentum damping time.

https://iopscience.iop.org/article/10.1088/0029-5515/38/1/306/pdf

Observations of central toroidal rotation in ICRF heated Alcator C-Mod plasmas

Recent progress in the area of ICRF physics is examined and summarized. A very brief summary of simplified theoretical predictions is given. Illustrative examples of experimental results in the area of ICRF heating and fast-wave current drive are given. ICRF heating is found to be highly effective in heating plasmas to high temperatures in modern tokamaks. Fast-wave current drive has been demonstrated in several experiments and the results are in good agreement with theory. Mode-conversion heating and current drive should be effective for profile control, but experimental results are still preliminary.

/pdf/recent-progress-in-icrf-physics-1di3ldwcol.pdf

Recent progress in ICRF physics

A novel rotatable two-strap antenna has been installed in the current drive experiment upgrade (CDX-U) [T. Jones, Ph.D. thesis, Princeton University (1995)] in order to investigate high-harmonic fast wave coupling, propagation, and electron heating as a function of strap angle and strap phasing in a spherical torus plasma. Radio-frequency-driven sheath effects are found to fit antenna loading trends at very low power and become negligible above a few kilowatts. At sufficiently high power, the measured coupling efficiency as a function of strap angle is found to agree favorably with cold plasma wave theory. Far-forward microwave scattering from wave-induced density fluctuations in the plasma core tracks the predicted fast wave loading as the antenna is rotated. Signs of electron heating during rf power injection have been observed in CDX-U with central Thomson scattering, impurity ion spectroscopy, and Langmuir probes. While these initial results appear promising, damping of the fast wave on thermal ions a...

High-harmonic fast magnetosonic wave coupling, propagation, and heating in a spherical torus plasma

When the density at the ion Bernstein wave (IBW) antenna is relatively low, mode transformation of the electron plasma wave to the IBW is sensitive to the density gradient scale length, and hence to ponderomotive effects. A second-order nonlinear ordinary differential equation that describes mode transformation at the lower-hybrid layer, including self-consistent ponderomotive density profile modification, is solved for the rf electrostatic potential in front of the IBW antenna, for the particular case of heating just below the second harmonic of the deuterium cyclotron frequency. The complex antenna impedance and a local reflectivity are calculated, assuming vacuum within the antenna box. These calculations reveal diminished antenna coupling to the IBW with increasing ponderomotive density expulsion, as compared to the linear prediction. The ponderomotive force increases the density gradient in the edge plasma, thus enhancing reflection and lowering the loading resistance. The model also describes the di...

Role of ponderomotive density expulsion in ion Bernstein wave coupling to the core plasma

Core plasma rotation of both L-mode and H-mode discharges with ion cyclotron range of frequency (ICRF) minority heating (MH) scheme was measured with a tangential X-ray imaging crystal spectrometer on EAST (Experimental Advanced Superconducting Tokamak). Co-current central impurity toroidal rotation change was observed in ICRF-heated L- and H-mode plasmas. Rotation increment as high as 30 km/s was generated at ~1.7 MW ICRF power. Scaling results showed similar trend as the Rice scaling but with significant scattering, especially in L-mode plasmas. We varied the plasma current, toroidal field and magnetic configuration individually to study their effect on L-mode plasma rotation, while keeping the other major plasma parameters and heating unchanged during the scanning. It was found that larger plasma current could induce plasma rotation more efficiently. A scan of the toroidal magnetic field indicated that the largest rotation was obtained for on-axis ICRF heating. A comparison between lower-single-null (LSN) and double-null (DN) configurations showed that LSN discharges rendered a larger rotation change for the same power input and plasma parameters.

Observation of Central Toroidal Rotation Induced by ICRF on EAST

Ion Bernstein wave (IBW) antennas are known to have substantial localized effects on the plasma edge. To allow better understanding and measurement of these effects, the TFTR edge reflectometer has been relocated to the new IBW antenna. This move was facilitated by the incorporation of a diagnostic access tube in the IBW antenna identical to the original diagnostic tube in the fast-wave (FW) antenna. This allowed the reflectometer launcher to simply be moved from the old FW antenna to the new IBW antenna. Only a moderate extension of the waveguide transmission line was required to reconnect the reflectometer to the launcher in its new location. Edge density profile modification during IBW experiments has been observed. Results from IBW experiments will be presented and contrasted to the edge density modifications previously observed during FW heating experiments.

https://digital.library.unt.edu/ark:/67531/metadc677251/m2/1/high_res_d/485978.pdf

Measurements of edge density profile modifications during IBW on TFTR

ICH‐induced toroidal plasma rotation was initially observed from Doppler shift measurements of the FeXXV Kα line using the TFTR horizontal crystal spectrometer for discharges with ion cyclotron heating of 2 MW RF power and without any neutral beam power in a D(H) plasma. The magnitude of plasma rotation depended on the IC resonance location. The observed rotations were in the counter‐plasma‐current direction and of the order of 5×104 m/s. Simple estimates relate the induced rotation to loss of ions from the plasma core. Later experimental results, as recorded by the charge exchange recombination spectrometer during beam blips at the end of the ICH pulse, with up to 6 MW of RF power in a 4He(H) plasma provided the radial profile of rotation.

ICH-induced plasma rotation on TFTR

Effects of Heating Profile on Energy Transport in Neutral Beam Heated TFTR Plasmas Y. Takase, R.J. Goldston,* M. Bell,* M. Bitter,* C.E. Bush,* A. Cavallo,* P.H. Diamond,t P.C. Efthimion,* A. Ehrhardt,* R. Fonck,* E.D. Fredrickson,* B. Grek,* G.W. Hammett,* R.J. Hawryluk,* D. Heifetz,* H. Hendel,* K.W. Hill,* B. Howell,* K. Jaehnig,* D.W. Johnson,* R. Knize,* D. Mansfield,* D.C. McCune,* K. McGuire,* E. Nieschmidt,* H. Park,* A. Ramsey,* M.H. Redi,* G. Schilling,* J. Schivell,* S.D. Scott,* S. Sesnic,* W. Tang,* G. Taylor,* M.C. Zarnstorff* Plasma Fusion Center Massachusetts Institute of Technology Cambridge, MA 02139 September, 1991

Effects of heating profile on energy transport in neutral beam heated TFTR plasmas

Note: Proc. AIP Conference, No. 403, pp 301-304 (1997) Reference CRPP-CONF-1997-053 Record created on 2008-05-13, modified on 2017-05-12

/pdf/role-of-plasma-edge-in-the-direct-launch-ion-bernstein-wave-u9javhndys.pdf

Role of plasma edge in the direct launch Ion Bernstein Wave experiment in TFTR

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Measurements of edge density profile modifications during IBW on TFTR

ICH-induced plasma rotation on TFTR

Effects of heating profile on energy transport in neutral beam heated TFTR plasmas

Role of plasma edge in the direct launch Ion Bernstein Wave experiment in TFTR