H. Smith P. Helander L.-G. Eriksson T. Fülöp
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 …
PublishedP. Helander R. J. Akers L.-G. Eriksson
It is well known that when neutral beams inject ions into trapped orbits in a tokamak, the transfer of momentum between the beam and the plasma occurs through the torque exerted by a radial return current. It is shown that this implies that the angular momentum transferred to the plasma can be larger than the angular momentum of the beam, if the in…
PublishedP. Helander R. J. Akers
The collisional interaction between neutral-beam ions and bulk plasma electrons leads to convective transport of particles and energy similar to the well-known Ware pinch. These transport fluxes are calculated, and it is found that the particle flux is outward when the neutral beams are in the same direction as the plasma current and inward otherwi…
PublishedR.K. Galloway A.L. Mackinnon E.P. Kontar P. Helander
Finite thermal velocity modifications to electron slowing-down rates may be important for the deduction of solar flare total electron energy. Here we treat both slowing-down and velocity diffusion of electrons in the corona at flare temperatures, for the case of a simple, spatially homogeneous source. Including velocity diffusion yields a consisten…
PublishedL.-G. Eriksson P. Helander F. Andersson D. Anderson M. Lisak
Self-consistent modeling of the evolution of the plasma current during disruptions in large tokamaks is presented, taking into account both the generation of runaway electrons and their backreaction on the electric field. It is found that the current profile changes dramatically, so that the postdisruption current carried by runaway electrons is mu…
PublishedP. Helander H. Smith T. Fülöp L.-G. Eriksson
The distribution function of suprathermal electrons in a slowly cooling plasma is calculated by an asymptotic expansion in the cooling rate divided by the collision frequency. Since the collision frequency decreases with increasing velocity, a high-energy tail forms in the electron distribution function as the bulk population cools down. Under cert…
PublishedP. Helander D. J.Ward
It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about 10 14 positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-li…
PublishedD. Hughes M. Paczuski R.O. Dendy P. Helander K.G. McClements
A model for the solar coronal magnetic field is proposed where multiple directed loops evolve in space and time. Loops injected at small scales are anchored by footpoints of opposite polarity moving randomly on a surface. Nearby footpoints of the same polarity aggregate, and loops can reconnect when they collide. This may trigger a cascade of furth…
PublishedP. Helander T. Fülöp Peter J. Catto
The transport of angular momentum due to neutral atoms in the tokamak edge is calculated and shown to be sensitive to the poloidal location of the neutrals. In the absence of external momentum sources, the edge plasma is predicted to rotate spontaneously in the opposite direction to the plasma current, at a speed proportional to the radial ion temp…
PublishedP. Helander L.-G. Eriksson R. J. Akers C. Byrom C.G. Gimblett M. R. Tournianski
Spontaneous acceleration of ions to suprathermal energies is observed during magnetic reconnection in the Mega-Ampere Spherical Tokamak (MAST). A high-energy tail is observed in the iondistribution function following each internal reconnection event in Ohmic discharges. This phenomenon is explained in terms of runaway ion acceleration in the electr…
Published