Speaker
Description
Runaway Electrons (RE) are one of the main particle populations in tokamaks that must be carefully controlled and understood. Future devices will need to operate either without disruptions or with reliable mitigation strategies. Disruptions are characterised by abrupt losses of plasma confinement, during which strong electric fields can be generated. These fields may accelerate electrons into the "runaway" regime, where collisional friction is no longer sufficient to slow them down. REs can impact the tokamak wall, potentially causing severe damage.
These phenomena can be indirectly detected through gamma-ray diagnostics. In particular, REs interacting with plasma ions emit intense Bremsstrahlung (Bs) radiation in the Hard X-Ray (HXR) energy range from some keV to several MeV. At JET, Bs emission has been observed during dedicated experiments using various gamma-ray diagnostics, including the Gamma Camera and both tangential and vertical gamma-ray spectrometers, based on the Lanthanum Bromide scintillator. Through these measurements, information on the RE energy distribution and RE emissivity profile can be obtained.
In this study, we analyse the spectral features of Bs emission when different Lines of Sight (LoSs) in RE experiments are employed. Previous studies typically assumed a strongly co-passing RE distribution, with negligible population at pitch angles different from zero. Based on theoretical considerations, we show that the slope of the HXR spectrum can vary when comparing measurements along different LoSs and if the particles are not only strongly co-passing. This effect may help explain experimental observations at JET, where different Bs spectral slopes are measured using gamma-ray spectrometers with different observation angles relative to the magnetic field.
