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Status of research toward the ITER disruption mitigation system

An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given. The ITER DMS is based on massive injection of impurities, in order to radiate the plasma stored energy and mitigate the potentially damaging effects of disruptions. The design of this system will be extremely challenging due to many physics and engineering constraints such as limitations on port access and the amount and species of injected impurities. Additionally, many physics questions relevant to the design of the ITER disruption mitigation system remain unsolved such as the mechanisms for mixing and assimilation of injected impurities during the rapid shutdown and the mechanisms for the subsequent formation and dissipation of runaway electron current.

Runaway electron drift orbits in magnetostatic perturbed fields

Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To mitigate the disruption and suppress the runaway beam the application of resonant magnetic perturbations has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the orbit losses for various initial energies and magnetic perturbation magnitudes. In the simulations we use a TEXTOR-like configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including synchrotron radiation and collisions. The results indicate that runaway electrons are well confined in the core of the device, but the onset time of runaway losses closer to the edge is dependent on the magnetic perturbation level and thereby can affect the maximum runaway current. However, the runaway current damping rate is not sensitive to the magnetic perturbation level, in agreement with experimental observations.

Destabilization of magnetosonic-whistler waves by a relativistic runaway beam

Magnetosonic-whistler waves may be destabilized by runaway electrons both in fusion and astrophysical plasmas. A linear instability growth rate of these waves in the presence of a runaway avalanche is calculated both perturbatively and by numerical solution of the full dispersion equation. The local threshold of the instability depends on the fraction of runaways, the magnetic field, and the temperature of the background plasma. The quasilinear analysis shows that the main result of the instability is the scattering of the electrons in pitch-angle. It appears possible that this instability could explain why the number of runaway electrons generated in tokamak disruptions depends on the strength of the magnetic field.

Runaway electron generation in a cooling plasma

The usual calculation of Dreicer [Phys. Rev. 115, 238 (1959); 117, 329 (1960)] generation of runaway electrons assumes that the plasma is in a steady state. In a tokamak disruption this is not necessarily true since the plasma cools down quickly and the collision time for electrons at the runaway threshold energy can be comparable to the cooling time. The electron distribution function then acquires a high-energy tail which can easily be converted to a burst of runaways by the rising electric field. This process is investigated and simple criteria for its importance are derived. If no rapid losses of fast electrons occur, this can be a more important source of runaway electrons than ordinary Dreicer generation in tokamak disruptions.

Nonlinear neoclassical transport in a rotating impure plasma with large gradients

The theory of neoclassical transport in an impure toroidal plasma is extended to allow for larger pressure and temperature gradients and faster toroidal rotation than are usually considered. Under these conditions, the density of heavy impurities is not constant on flux surfaces, and the neoclassical transport becomes a nonlinear function of the gradients. Rapid toroidal rotation increases the transport, which can significantly exceed the conventional Pfirsch–Schluter value if the impurity Mach number is of order unity. In a plasma with steep density or temperature profile, the transport is severely reduced and can even be a nonmonotonic function of the gradients. Finally, in the presence of both rapid toroidal rotation and steep gradients, the transport becomes sensitive to the geometry of the magnetic equilibrium. For instance, in a single-null diverted magnetic field the ion particle flux is typically inward if the ion drift is toward the X-point and changes direction if the toroidal field is reversed.

Runaway electron losses caused by resonant magnetic perturbations in ITER

Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To suppress the runaway beam the application of resonant magnetic perturbations (RMPs) has been suggested. In this work we investigate the effect of RMPs on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energies and different magnetic perturbation configurations. In the simulations we model the ITER RMP configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including a time-dependent electric field, radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where the normalized perturbation amplitude δB/B is larger than 0.1% in a properly chosen perturbation geometry. This applies to the region outside the radius corresponding to the normalized toroidal flux ψ = 0.5.

Magnetic field threshold for runaway generation in tokamak disruptions

Experimental observations show that there is a magnetic field threshold for runaway electron generation in tokamak disruptions. In this work, two possible reasons for this threshold are studied. The first possible explanation for these observations is that the runaway beam excites whistler waves that scatter the electrons in velocity space prevents the beam from growing. The growth rates of the most unstable whistler waves are inversely proportional to the magnetic field strength. Taking into account the collisional and convective damping of the waves it is possible to derive a magnetic field threshold below which no runaways are expected. The second possible explanation is the magnetic field dependence of the criterion for substantial runaway production obtained by calculating how many runaway electrons can be produced before the induced toroidal electric field diffuses out of the plasma. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected to give rise to substantial runaway population, the whistler waves can stop the runaway formation below a certain magnetic field unless the postdisruption temperature is very low.

Influence of the radio frequency ponderomotive force on anomalous impurity transport in tokamaks

Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor −∇nz∕nz is only moderately affected by the rf fields.Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor −∇nz∕nz is only moderately affected by the rf fields.

Synchrotron radiation from a runaway electron distribution in tokamaks

Analysis of the synchrotron radiation emitted by runaway electrons in a fusion plasma presents a direct route to knowledge of the particle momenta and pitch-angles of the runaway electron population, through the strong dependence of the synchrotron spectrum on these parameters. Information about the runaway density and its spatial distribution, as well as the time evolution of the above quantities can also be deduced. Previously, synchrotron spectra have been interpreted under the assumption that all runaways have the same energy and pitch-angle [1]. In practice, however, runaway electrons have a wide range of energies and pitch-angles, and that influences the synchrotron radiation spectrum. In this paper we present the synchrotron radiation spectra for typical avalanching runaway electron distributions. We compare the characteristics of the spectra obtained for a distribution of electrons to the one based on the emission of electrons having the same energy and pitch-angle. We examine the eects on the spectrum of including or neglecting magnetic field curvature in the calculation and analyse the sensitivity of the resulting spectrum to perturbations to the runaway distribution. We also discuss the implications for the deduced runaway parameters.

Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zero-flux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating.