Design of a variable frequency comb reflectometer system for the ASDEX Upgrade tokamak

Abstract

Comb reflectometers offer the advantage of measuring at several radial positions in the plasma simultaneously. This allows the investigation of fast timescales during L-H transitions, I-phases, I-mode bursts, transients during heat wave propagation, etc. A drawback of many present-day systems is that they use a fixed frequency difference between the probing frequencies. Hence, although the central probing frequency can be varied, usually the probing frequency difference is fixed. The new design presented in this work uses an advanced microwave generation and detection scheme, which allows for arbitrary probing frequencies and probing frequency separations.

Microwave diagnostics damage by parametric decay instabilities during electron cyclotron resonance heating in ASDEX Upgrade

“…In these cases, the PDIs may even lead to conversion of a significant fraction (up to 80% according to current theoretical work [24,25]) of the pump wave power to daughter wave power, altering the ECRH characteristics from those expected based on linear theories; this point has been demonstrated experimentally in low-temperature plasma filament experiments [26]. Here, we report the first observations of damage to the standard ECE system [2] and a Doppler reflectometer [5], caused by PDIs involving trapped primary daughter waves, during thirdharmonic X-mode ECRH at the ASDEX Upgrade tokamak. We further investigate the possibility of such PDIs occurring at the ITER tokamak, along with the microwave diagnostics which may be affected by their occurrence.…”

“…Microwaves are an essential tool in magnetic confinement fusion research due to the strong plasma−wave interactions facilitated by the electron plasma and cyclotron frequencies both being on the order of microwave frequencies in the typical schemes. Well-known uses of microwaves in magnetic confinement fusion research include electron cyclotron resonance heating (ECRH) and current drive [1], as well as electron cyclotron emission (ECE) [2], reflectometry [3][4][5][6], and collective Thomson scattering (CTS) [7] diagnostics. Regular models of plasma−microwave interactions for the above applications only consider effects with a linear dependence on the microwave power, as the frequencies and group velocities of the microwaves are usually considered too large to permit significant nonlinear coupling, unless free-electron maser sources are employed [8].…”

Application of Dual Frequency Comb Method as an Approach to Improve the Performance of Multi-Frequency Simultaneous Radiation Doppler Radar for High Temperature Plasma Diagnostics

“…Since electromagnetic waves in the millimeter-wave range can be used in relation to the electron plasma frequency and the electron cyclotron frequency range, the system is expected to be used as a stable and robust measurement method, and is also expected to be used in future nuclear burning fusion reactors. In fusion plasma research, this Doppler radar is called a Doppler reflectometer or a Doppler back-scattering and has been applied to various experimental devices around the world, such as helical/stellarators (Wendelstein 7-AS [1,2], 7-X [3], TJ-II [4], LHD [5][6][7][8]), tokamaks (Tuman-3M [9], ASDEX Upgrade [10][11][12][13], Tore Supra [14,15], DIII-D [16,17], JT-60U [18], MAST [19], JET [20], HL-2A [21], TCV [22], EAST [23][24][25]), and linear machines (C-2 FRC [26], GAMMA-10 [27]). In particular, in recent years, systems capable of simultaneous multi-frequency observation have been developed with the aim of understanding the instantaneous spatial structure of turbulence [6][7][8]13,17,[21][22][23][24][25].…”

“…In fusion plasma research, this Doppler radar is called a Doppler reflectometer or a Doppler back-scattering and has been applied to various experimental devices around the world, such as helical/stellarators (Wendelstein 7-AS [1,2], 7-X [3], TJ-II [4], LHD [5][6][7][8]), tokamaks (Tuman-3M [9], ASDEX Upgrade [10][11][12][13], Tore Supra [14,15], DIII-D [16,17], JT-60U [18], MAST [19], JET [20], HL-2A [21], TCV [22], EAST [23][24][25]), and linear machines (C-2 FRC [26], GAMMA-10 [27]). In particular, in recent years, systems capable of simultaneous multi-frequency observation have been developed with the aim of understanding the instantaneous spatial structure of turbulence [6][7][8]13,17,[21][22][23][24][25]. When observing turbulence in torus plasmas with this Doppler reflectometer, an ordinary or extraordinary wave is injected into the magnetic confined plasma, and the back-scattered wave from the vicinity of the cut-off position corresponding to the injecting frequency is observed, so the measurement position can be changed by changing the frequency.…”

“…In other words, a simple measurement system using a single source can extract a broad frequency component and achieve high spatial and temporal resolution at the same time. A frequency comb is a signal with periodic peak frequency components and utilizes oscillation from a single source such as a non-linear transmission line (NLTL) or step recovery diode (SRD), as shown in Figure 2, or comb-like frequency component radiation using a multiplier [13,[21][22][23][24][25].…”