Description
Microwave reflectometry is routinely used in magnetic confinement devices to measure both the electron density profile and its turbulent fluctuations. Over the past decades, continuous wave (CW) methods, measuring the amplitude and phase of the reflected beam, have become the standard. Pulsed methods, directly measuring the time of flight of short (~ns) probing pulses, were sporadically attempted, with limited success due to hardware requirements and pulse dispersion concerns.
Extensive studies of CW methods over the years have revealed issues in measuring the amplitude δn and radial correlation length lx of the turbulent density fluctuations. The latter is overestimated at low turbulence amplitudes due to small angle scattering effects [1]. while at high amplitudes both δn and lx are underestimated. Worse, determining which of these regimes applies in experiment is impossible without prior knowledge of δn and lx [2].
Thanks to advancements in hardware, the pulsed concept has now experienced a breakthrough with the deployment of a Short Pulse Reflectometer (SPR) system on the TCV tokamak [3]. SPR features a pulse repetition rate of up to 50 MHz, digital pulse acquisition and negligible pulse dispersion, making it suitable for fast measurements of the density profile. This work focuses on the development of the method for measuring lx with SPR. In particular, theoretical analysis, validated with full-wave simulations using the CUWA code, demonstrates the method’s ability to overcome the issues limiting CW methods, making it a highly attractive alternative to conventional techniques.
[1] Gusakov E.Z. and Kosolapova N.V. 2011 Plasma Phys. Control. Fusion 53 045012
[2] Blanco E. and Estrada T. 2008 Plasma Phys. Control. Fusion 50 095011
[3] Molina Cabrera P.A., Labit B., Coda S. and Porte L. 2019 Rev. Sci. Instrum. 90 123501