A negative current in the scrape-off region beyond the separatrix of a
diverted tokamak can improve its MHD stability. By utilizing a strong
thermionic emitter a current of up to 25 % of the plasma current can
be driven in the scrape-off region. If the thermionic current is emitt
ed at only one end of the open field lines, a current will flow in the
absence of a potential bias. The heat to maintain the electron emissi
on that drives the scrape-off current would be supplied by the plasma
outflow and no additional power is needed.