"Study of internal magnetic field via polarimetry in tokamaks," by Jie Zhang (UCLA)

Friday, April 5, 2013 - 1:00pm to 2:00pm
Plasma Seminars

Plasma Seminar

Friday, April 5, 2013
Location: PAB 4-330

Speaker: Jie Zhang (UCLA)


Title: "Study of internal magnetic field via polarimetry in tokamaks"



Polarimetry is a powerful diagnostic technique to probe plasma equilibria and magnetic
fluctuations in fusion plasmas. In high beta plasma devices such as NSTX, these
measurements are important for understanding the stability, structure and anomalous
transport induced by electromagnetic fluctuations. A 288 GHz polarimeter prototype
operating along the major radius has been developed and is being tested on DIII-D prior
to deployment on NSTX-U. The system launches a rotating linearly polarized beam and
detects phase shifts related to polarization changes due to the plasma. To improve phase
resolution, quasi-optical isolation is used to minimize multi-path feedback effects. In the
equilibrium experiment on DIII-D, the measured phase responses reasonably agree with
theory predictions over a wide range of plasma conditions. The high level agreement
verifies both the system hardware and the synthetic diagnostic code. Polarimetry
calculations based on the output of microtearing mode simulations indicate the feasibility
for this polarimeter to detect the magnetic fluctuations associated with microtearing
modes. Various coherent modes are also observed in measured spectra, demonstrating the
potential for internal magnetic fluctuation measurements via polarimetry in tokamaks.