Visualization of energetic electron loss channels of electron cyclotron heating (ECH) plasma in ring trap 1 (RT-1)...

An x-ray CCD camera was installed on RT-1, a dipole magnetic field device with a magnetically levitated superconducting (SC) coil. The diagnostic was used to investigate the loss channels of energetic electrons in electron cyclotron heating plasmas by imaging bremsstrahlung x rays generated at material surfaces. The camera views a region including the SC coil case and the coil elevator structures and has sensitivity to photons in the energy range of ∼1-12 keV. It can, therefore, visualize locations where energetic electrons, which produce a major fraction of the plasma pressure, are lost to metallic surfaces. Initial measurements were performed under mechanically supported, partially levitated, and fully levitated coil conditions. In the mechanically supported case, intense localized x-ray emission was observed from the support structure located in the relatively low-field region, indicating that this structure is the dominant energetic-electron loss surface. Magnetic levitation strongly reduced the localized emission and revealed a weaker ring-shaped loss region near the inner surface of the SC coil. Test particle orbit calculations were consistent with the observed x-ray emission patterns and qualitatively reproduced the upward shift of the energetic electron loss locations under the partially levitated condition. These observations demonstrate that energetic-electron losses in mechanically supported dipole plasmas are strongly influenced by the magnetic connectivity between the ECR region and exposed support structures. The results highlight the importance of considering energetic-electron orbit losses, in addition to mechanical constraints, when designing coil-support structures for higher-field dipole devices.