Is the guiding-center radiation model applicable to runaway electron dynamics?

For energetic runaway electrons in tokamaks, synchrotron radiation is the main mechanism of energy loss and an important means for diagnosis. It has been revealed that the lowest order guiding-center assumption no longer holds for energetic runaway electron dynamics in complex tokamak fields. Accordingly, an ensuing question is whether the synchrotron radiation laws derived from the guiding-center model still apply to runaway electrons. In this paper, we conduct a detailed study of the errors of guiding-center radiation model (GCRM) under different conditions, thereby providing a basis for its applicable conditions. Numerical calculations show that GCRM aligns closely with the exact synchrotron radiation formula for charged particles in the non-relativistic region. However, in the relativistic region, the discrepancy increases with the pitch angle. Although the violation of guiding-center assumption becomes more pronounced with the increase of relativistic factors, the relative error of the guiding-center radiation rule does not increase with the indicator of the guiding-center deviation, i.e. the relative variations of the background magnetic field and magnetic moment within one gyro-period. Instead, the relative error δ decreases as the increase of and . However, the pitch angle clearly has a significant impact on the accuracy of the guiding-center radiation law. The strength of the background magnetic field significantly affects the validation of GCRM, particularly at small pitch angles. A weaker magnetic field results in a greater discrepancy. In the parameter space spanned by energy and pitch angle, the relative error δ of total synchrotron radiation power calculated from GCRM reveals a ‘nose-like’ structure, suggesting a surprisingly non-monotonic dependence on these variables. For runaway electrons experimentally measured in several tokamaks, the relative error of the GCRM varies from 86% to 100%.