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Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2023)

Diagnosis of the fast ion population, which is necessary for heating but can drive dangerous instabilities, is crucial in achieving and maintaining a burning plasma regime. Present fast ion diagnostic methods tend to rely on delicate components unlikely to survive in a reactor environment, motivating exploration into alternative diagnostic methods. One candidate, the subject of this thesis, entails measurement of collective ion cyclotron emission (ICE) spectra via robust magnetic pickup loops integrated with the first wall, making it potentially compatible with ITER and other reactor-relevant devices [116, 69]. ICE, distinct from thermal electron cyclotron emission, manifests as narrowband peaks at harmonics of the ion cyclotron frequency $f_{ci}$ and is generally localized near the core or the edge of the plasma. These modes have been observed to be sensitive to the fast ion population, be they introduced by auxiliary heating systems or fusion products. However, the connection between observed ICE spectra and the fast ion population has not been sufficiently established, meriting diagnostic upgrades and dedicated experiments.

This work aims to characterize ICE mode structure and fine-scale phenomena in L- and H-mode plasmas on DIII-D using the ICE diagnostic, which was recently upgraded to include additional channels and consequently new measurement capabilities. In addition to basic frequency and amplitude information, measurements of mode polarization at the plasma edge, amplitude as measured at the centerpost and outer wall of the machine, and toroidal mode number are now possible and will be demonstrated through this work. Additionally, mode sensitivity to the vacuum region, plasma shape, and particularly the fast ion distribution is explored. The detailed mode structure measurements and phenomenology presented in this work can be used to test and validate theoretical models en route to developing a more complete physical understanding of ICE, so that it can be leveraged as a passive fast ion diagnostic in future burning plasmas.

    Cover page: Ion cyclotron emission on the DIII-D tokamak
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2021)
    The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties for potential medical applications of electron acceleration. In the near-critical density regime, electrons are accelerated by the ponderomotive force followed by the electron sheath formation, resulting in a flow of bulk electrons. This behavior represents a qualitatively distinct regime from that of low-density LWFA. A quantitative entropy index for differentiating these regimes is proposed. The dependence of accelerated electron energy on laser amplitude is also examined. For the majority of this study, the laser propagates along the axis of the target of carbon nanotubes in a 1D geometry. After the fundamental high-density physics is established, an alternative, 2D scheme of laser acceleration of electrons using carbon nanotubes is considered.
      Cover page: High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to NanotubesCreative Commons 'BY' version 4.0 license
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