The dynamics of relativistic runaway electrons in tokamak plasmas is analyzed using a test particle description that includes acceleration in the toroidal electric field, collisions with the plasma particles, and deceleration due to synchrotron radiation losses. The region of momentum space in which electron runaway takes place is determined. It is found that relativistic and synchrotron radiation effects lead to a critical electric field $E_R{\mathrm{>(kT}}_e{\mathrm{/m}}_e{c}^2{\mathrm{)E}}_D,$ below which no runaways are generated. In addition, the trajectories of the test electrons in momentum space show a stable equilibrium point that sets a limit on the energy that the runaway electrons can reach. Analytical expressions are given for this energy limit as a function of the toroidal electric field and plasma parameters. The dominant radiative mechanisms limiting the runaway electron energy are identified in the whole range of electric field values.

The dynamics of high energy (up to 20 MeV) runaway electrons in the Frascati Tokamak Upgrade (FTU) is investigated using a gamma-ray spectrometer system which detects photons produced when runaway electrons interact with the plasma facing components [B. Esposito et al., Nucl. Instrum. Methods 476, 522 (2002)]. Runaway electrons are usually generated during the plasma current ramp-up, accelerated to MeV energies, and contained stably during the whole discharge time, which lasts for more than one second. This time is long enough for them to reach the limiting energy that results from the balance between acceleration in the electric field, collisions with the plasma particles and synchrotron radiation losses. The maximum energy inferred from the gamma spectra is shown to be in agreement with the runaway limiting energy predicted by a test particle description of the runaway dynamics [J. R. Martı́n-Solı́s et al., Phys. Plasmas 5, 2370 (1998)]. It is found that the runaway energy behavior during the discharge is determined by the time evolution of the plasma parameters only (mainly $E_{\parallel },$ $n_e$ and $Z_{\mathrm{eff}})$ and that the synchrotron radiation losses associated with the electron gyromotion around the magnetic field lines can explain the measured limiting runaway energy in FTU.

Recent results from an ITPA joint experiment to study the onset, growth, and decay of relativistic electrons (REs) indicate that loss mechanisms other than collisional damping may play a dominant role in the dynamics of the RE population, even during the quiescent I_p flattop. Understanding the physics of RE growth and mitigation is motivated by the theoretical prediction that disruptions of full-current (15 MA) ITER discharges could generate up to 10 MA of REs with 10–20 MeV energies. The ITPA MHD group is conducting a joint experiment to measure the RE detection threshold conditions on a number of tokamaks under quasi-steady-state conditions in which V_loop, n_e, and REs can be well-diagnosed and compared to collisional theory. Data from DIII-D, C-Mod, FTU, KSTAR, and TEXTOR have been obtained so far, and the consensus to date is that the threshold E-field is significantly higher than predicted by relativistic collisional theory, or conversely, the density required to damp REs is significantly less than predicted, which could have significant implications for RE mitigation on ITER.

Quasistationary operation has been achieved on the Joint European Torus tokamak in internal-transport-barrier (ITB) scenarios, with the discharge time limited only by plant constraints. Full current drive was obtained over all the high performance phase by using lower hybrid current drive. For the first time feedback control on the total pressure and on the electron temperature profile was implemented by using, respectively, the neutral beams and the ion-cyclotron waves. Although impurity accumulation could be a problem in steady state ITBs, these experiments bring some elements to answer to it.