Speaker
Description
See the full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P1.1045.pdf
Detailed properties of the 3D structure of (1/1) internal kinks can be accessed using a modern suite of spectroscopic imaging diagnostics. A combination of high time- and space-resolved 1D measurements of electron density and temperature fluctuations together with local 2D soft x-ray (SXR) radiated power density profiles can shed light on the spatial structure of the ideal internal kink eigenfunction during sawtooth-free (1,1) fishbone bursts and early phase of longlived saturated modes. Particularly useful are the SXR reconstruction routines based on a Bessel formalism to extract the lowest order component and fit the data with a Bessel function of order one which can constrain the ideal-kink eigenfunction. SXR signatures of the (1,1) and midplane J1(lr) fit for an ideal kink. solution (y1~J1). Soft x-ray (SXR) emissivity profiles of electron fishbones in Alcator C-Mod [1,2] show for example that the mode forms and grows as a small amplitude (1/1) kink with a nearly circular cross section. The perturbation has a characteristic eigenfunction of the form J1(lr)cosq which is adequate for the 1/1 internal kink in a cylindrical plasma; lrq=13.83 is the first zero of J1(x) and rq=1 is the location of q=1 which in this case is 5 cm away from the magnetic axis (rq=1/a~20%). In comparison, the inversion radius is at 2.5 cm. Combining this functional form with data from a two-color interferometer is possible to infer an electron density perturbation of the order of 4% which peaks at the same location of the SXR intensity. An additional constraint can be obtained using a 1D ECE radiometer which also provides additional evidence that LHCD fast electrons are connected to the mode onset and evolution. Radiation at the second harmonic of the electron cyclotron frequency produced by fast electrons in the central region are often detected in a frequency channel assigned at the low-field edge nearly 20 cm away from the magnetic axis and only while the mode is active. This apparent mismatch in the location of the ECE emission is due to the strong downshift of electron gyro-frequency introduced by relativistic effects. Nonetheless, assuming a 1/R dependence for the toroidal magnetic field and a perturbation of the ECE signals that maximizes/minimizes at a midplane position at opposite sides of the kink (e.g. ±rk), it is thus possible to obtain a simple solution of the downshift profile leading to rk=1.53 cm; not surprisingly, rk is also at the peak of the J1(lr) perturbation inside the q=1 radius. These high resolution tools are also being used to study the early phase of long-lived saturated (1,1) kink modes before their resistive phase [3-6].
Work supported by the US DOE under contract DEAC05-00OR22725 at ORNL and DE-AC02-09CH11466 at PPPL.
[1] L. Delgado-Aparicio, et al., PoP, 22, 050701, (2015)
[2] L. Sugiyama, et al., 25, 082120, (2018)
[3] L. Delgado-Aparicio, et al., PRL, 110, 065006, (2013)
[4] L. Delgado-Aparicio, et al., NF, 53, 043019, (2013)
[5] A. Wingen, et al., NF, 58, 036004, (2018)
[6] A. Wingen, et al., PoP, 58, 022501, (2019).
