Active Measurement of Plasma Parameters in Over-Dense MFE Reactor Plasma Using High Frequency Beat-Wave Resonantly Generated Thermal Bernstein Waves
UC Davis was funded to explore the combination of beat-wave excitation and laser scattering to make completely non-invasive determination of local plasma conditions. This involves the use of two high frequency mm-wave sources of differing frequencies (the pump waves) to excite plasma waves at the difference of the frequencies, which is known as the beat frequency. The difference frequency or the frequency of the beat wave is chosen so that we produce plasma waves whose fluctuating electron density is a sensitive function of plasma parameters at the location where the source waves overlap. This fluctuation can be detected from outside the plasma, by measuring the deflection of a probe beam directed at the overlap region between the two pump waves (i.e., collective scattering). This work is an extension of mm-wave frequencies of a 10 GHz beat-wave experiment conducted on the DDT toroidal device (see Fig. 1) in a magnetized plasma where the excited Langmuir wave is replaced by the magnetized upper hybrid wave. Here, two high power (> 1 kW) ~94 GHz pump sources will be employed to match the upper hybrid resonance. The primary envisioned experiment is to drive the electron Bernstein wave (EBW) in the interior of a magnetized plasma. The EBW is a hot-plasma electrostatic wave, which cannot propagate in vacuum. Roots of the EBW dispersion relation occur at frequencies slightly higher than harmonics of the electron cyclotron frequency. Use of the EBW has particular advantages for measuring plasma magnetic field in plasmas with relatively high density and low magnetic field, as is the case in spherical tokamaks. This is the “over dense” condition of future efficient high beta magnetic confinement fusion reactors such as the spherical tokamak (STs). The overarching goal of the proposed project is to demonstrate beat wave EBW excitation in low density and temperature parameters that are directly scalable to over-dense spherical tokamaks such as NSTX or MAST, where measurement of magnetic field by electron cyclotron emission (ECE) is experimentally complex, and requires models of plasma profiles in order to interpret results. In such experiments, availability of point measurements of magnetic field strength provided by a beat-wave diagnostic would serve as a powerful independent check on the ECE results, serving to validate the assumed profiles and wave-propagation models used. The first step was to assemble/modify the target chamber in which the beat-wave plasma experiments will take place. Here, the target chamber of the CTIX compact-toroid (spheromak) injector was chosen. At the time that the proposal was submitted, the aim was to conduct the experiments in the CTIX experimental facilities located at the Livermore campus of UC Davis using microwave/mm-wave equipment brought from the main campus., However, by the time DoE funds for this project were awarded, the CTIX experimental facilities located at the Livermore campus of UC Davis were in the process of being shut down due to the closure of the Livermore campus by Lawrence Livermore National Laboratory (LLNL). Consequently, the equipment needed for this project was crated up and shipped from Livermore to Davis as part of the laboratory shutdown after space on campus was secured and College of Engineering approval secured. Once it arrived in Davis on December 20, 2018, work on the project began in earnest with the initial efforts directed toward developing a new plasma target source. Because of the shutdown of the CTIX facility, the scope of the project thus had to be reduced due to the abbreviated amount of time allocated to the project once the shutdown/move had been completed. Chamber modifications have been completed, including the fabrication and installation of a 38 GHz interferometer, as described in Sec. 3. The thermionic plasma source has been tested and characterized. The magnetic field required for the EBW experiments will be generated by permanent magnets mounted on plates positioned above and below the chamber. Magnetic simulations have been performed, and show that a 3-ring magnet arrangement on each plate, with a plate separation of 50 cm, is required to achieve the ~100 G uniform magnetic field in the interaction region that we feel is necessary to ensure good EBW coupling. We proved unable to successfully revive either of Prof. Luhmann’s two high power (≥20 kW) 94 GHz millimeter-wave sources after their extended time in storage and decided to postpone further efforts and concentrate on slightly lower power sources. Consequently, we were successful in reviving a ~2 kW extended klystron amplifier (EIK), which together with a ~2 kW extended interaction oscillator (EIO) will provide the two pump sources required for the project. Safe operation of the EIO requires extensive modifications to one of Prof. Luhmann’s pulse modulators - these modulator modifications are ongoing, which has unfortunately delayed the planned beat-wave experiments. Details of this work are provided in Sec. 3. Even though project funding has now ended, we still plan to proceed to complete the EIO pulse modulator modifications, install the two ~94 GHz pump wave sources on the target chamber, and conduct unmagnetized tests of the beat-wave concept which involve coupling to unmagnetized electron plasma waves rather than magnetized EBWs. Should these experiments prove successful, we plan to submit a follow-up proposal for additional funding which would allow us to undertake the magnetized EBW experiments.