Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confi ned in toroidal tokamak devices requires the development of effi cient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing signifi cant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

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 ER>(kTe/mec2)ED, 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∥, ne and Zeff) 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 Ip 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 Vloop, ne, 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.

Abstract. At JET the dynamics of Internal Transport Barriers (ITBs) has been explored by trying to decouple the effects of heating on one hand and torque on the other with the ultimate objective of identifying the minimum torque required for the formation of transport barriers. The experiments shed light on the physics behind the initial trigger for ITBs, which often shows to be linked to the shape of the q profile and magnetic shear, while the further development was influenced by the strength of the rotational shear. In discharges with a small amount of rotational shear ITBs were triggered, which suggest that the rotational shear is not the dominant factor in the triggering process. However, the subsequent growth of the barrier was limited if the rotational shear was too low at the time of triggering. This growth phase may be highly non-linear, with several possible positive feedback loops, such as the increases of the toroidal and poloidal component of the rotational shear caused by the ITB itself.