An energy limit of hot electrons in a bumpy torus

Numerical estimation shows that an upper energy limit of electrons confined in the Nagoya Bumpy Torus, NBT-1M, exists under electron-cyclotron heating (ECH). This energy limit is determined from the power balance between the microwave input and the losses due to synchrotron radiation and Coulomb dra g by bulk electrons: when the maximum harmonic number of ECH corresponding to the energy limit plays an important role. The calculation result of the orbit of the particle reveals us that mirror-trapped electrons can have ene rgies of 10 MeV, while electrons passing through linked mirrors cannot be c onfined if their energies are above 3 MeV for the magnetic current of 4.2 k A. Successive ECH to a higher harmonic resonance with an injected microwave accelerates electrons, especially mirror-trapped ones, to the energy limit . However, the resonant heating efficiency estimated from the wave damping decreases as the harmonic number n(H) increases, and also the total power l oss increases as heated electrons become more energetic. The resultant powe r balance occurs at the electron energy of 4 MeV, to which resonances up to n(H) similar or equal to 10 contribute. This numerical result agrees with the observed maximum electron energy of 4 MeV.