TY - JOUR
T1 - Ion-cyclotron range of frequencies in the scrape-off-layer
T2 - Fine structure radial electric fields
AU - Cziegler, I.
AU - Terry, J. L.
AU - Wukitch, S. J.
AU - Garrett, M. L.
AU - Lau, C.
AU - Lin, Y.
PY - 2012/10
Y1 - 2012/10
N2 - Gas-puff-imaging techniques are utilized to detect radial electric field structures in the scrape-off-layer (SOL) of the Alcator C-Mod tokamak via the observation of poloidal motion of advected fluctuations. When the diagnostic's field of view is magnetically connected to the ion-cyclotron range of frequencies (ICRF) active antennas, large (up to 8kms 1) poloidal velocities are observed in a radial region encompassing both field lines terminating on and those passing in front of the antennas. The radial electric field switches sign indicating a peak in the potential profile corresponding to the transition from piercing to passing field lines. The electric field extends a few centimeters into the SOL and its local magnitude is of order E r20-30kVm 1. The corresponding plasma potentials scale as the square root of RF power; the poloidal structure is peaked when the field lines are connected to the top and bottom of the antenna. This structure is consistent with the presence of potential structures arising as a consequence of sheath rectification of the RF waves. The most striking result, however, is that the radial penetration λ of the potential structures is an order of magnitude larger than the basic theoretical expectation (λ10δ e, where δ e is the skin depth). This substantial broadening is expected to have a strong impact on RF impurity physics. A (weak) power dependence observed in the width of the poloidal velocity features is explained as a competition between the RF induced and the background potential gradients.
AB - Gas-puff-imaging techniques are utilized to detect radial electric field structures in the scrape-off-layer (SOL) of the Alcator C-Mod tokamak via the observation of poloidal motion of advected fluctuations. When the diagnostic's field of view is magnetically connected to the ion-cyclotron range of frequencies (ICRF) active antennas, large (up to 8kms 1) poloidal velocities are observed in a radial region encompassing both field lines terminating on and those passing in front of the antennas. The radial electric field switches sign indicating a peak in the potential profile corresponding to the transition from piercing to passing field lines. The electric field extends a few centimeters into the SOL and its local magnitude is of order E r20-30kVm 1. The corresponding plasma potentials scale as the square root of RF power; the poloidal structure is peaked when the field lines are connected to the top and bottom of the antenna. This structure is consistent with the presence of potential structures arising as a consequence of sheath rectification of the RF waves. The most striking result, however, is that the radial penetration λ of the potential structures is an order of magnitude larger than the basic theoretical expectation (λ10δ e, where δ e is the skin depth). This substantial broadening is expected to have a strong impact on RF impurity physics. A (weak) power dependence observed in the width of the poloidal velocity features is explained as a competition between the RF induced and the background potential gradients.
UR - http://www.scopus.com/inward/record.url?scp=84866348890&partnerID=8YFLogxK
U2 - 10.1088/0741-3335/54/10/105019
DO - 10.1088/0741-3335/54/10/105019
M3 - Article
AN - SCOPUS:84866348890
SN - 0741-3335
VL - 54
JO - Plasma Physics and Controlled Fusion
JF - Plasma Physics and Controlled Fusion
IS - 10
M1 - 105019
ER -