Detection of lower hybrid waves at the plasma edge of a diverted tokamak

[摘要] In this thesis, two experimental investigations are presented in an attempt to understand the loss of lower hybrid current drive (LHCD) efficiency in reactor-relevant, high-density plasmas on the diverted Alcator C-Mod tokamak. While the previous work has identified that edge interactions, such as collisional absorption and excessive up-shift of the parallel refractive index due to full-wave effects, could potentially explain the observed loss of LHCD efficiency in a wide range of line-averaged densities, these simulations still over-predict the fast electron population generated by LHCD above line-averaged densities of 1 x 10²⁰ m- ³. It is critical to identify remaining mechanisms in order to demonstrate advanced tokamak operation at reactor-relevant densities. The first investigation is to perform microwave backscattering experiments to detect electrostatic lower hybrid (LH) waves in the scrape-off layer (SOL), where a significant amount of the injected LH power may be deposited due to a number edge loss mechanisms. An existing ordinary-mode (O-mode) reflectometer system has been modified to measure the backscattered O-mode wave as a result of Bragg backscattering of the incident O-mode wave off the LH wave. The detection of LH waves in a region that is not magnetically connected to the launcher implies a weak single pass absorption of LH waves in high density plasmas. The observed spectral width of the backscattered signals indicates the presence of non-linear effects on the propagation of LH waves, but no experimental evidence is found to confirm whether the non-linear mechanism that is responsible for the observed spectral broadening is responsible for the observed loss of LHCD efficiency. The second investigation is to examine the change in LH frequency spectra due to density-dependent non-linear effects, such as parametric decay instability (PDI) above the line-averaged density of 1 x 10²⁰ m-³, using the probes installed on the launcher, outer divertor, and inner wall. Ion cyclotron PDI is found to be excited above line-averaged densities of 1 x 10²⁰ m -³, suggesting that ion cyclotron PDI may be a remaining mechanism to understand the loss of LHCD efficiency. Ion cyclotron PDI is observed to be excited not only at the low-field-side edge but also at the high-field-side (HFS) edge of Alcator C-Mod, further corroborating that LH waves are weakly absorbed on their first pass. Evidence of pump depletion is found with the onset of ion cyclotron PDI at the HFS edge in lower-single-null plasmas. However, no apparent pump depletion is seen when the magnetic geometry is switched to an upper-single-null. Moreover, ion cyclotron PDI is excited at the LFS edge in this case. Thus, the role of the observed ion cyclotron PDI on the loss of LHCD efficiency needs further experimental investigation to be conclusive due to the different ion cyclotron PDI strength and excitation location, depending on magnetic configurations. A summary of the new findings of this thesis is as follows: First measurements of PDI below the classical threshold (wo/wLH(0) ~~ 2). First ever observation of PDI on HFS of a tokamak and its relationship to being in a multi-pass damping regime. ;; Advancement of PDI as a candidate mechanism for the LHCD density limit.

[发布机构] Massachusetts Institute of Technology