Free electron masers for tokamak applications

Electron cyclotron heating (ECH) has been proposed as the primary heating and current drive source for tokamaks. Other uses of ECH include profile control and stabilization of m = 1 and m = 2 oscillations. The stabilization may be accomplished by steady heating to alter the current profile or by the more efficient alternative of driving current within the m = 1 or m = 2 islands with synchronous ECH. This could result in disruption avoidance, better energy confinement, increased β/sub c/, and higher maximum ohmic heating power density. Advantages of ECH include wave penetration at full density, precise power absorption zones, avoidance of alpha resonances, and simple launching structures. The technology of the free electron maser (FEM) has advanced to where it is now an attractive source on ECH power at reasonable cost. To adapt the FEM concept to a high power, tunable, CW device, TRW has developed a concept that is based on two key ideas: first, the use of a high-quality (low energy spread and low emittance) electron beam of moderate current (< 50 A) and a large-amplitude wiggler ensures high-gain operation with good efficiency. Second, the small-diameter, cylindrical spent beam has a relatively low energy spread making a highly efficient current and energy recovery system feasible. An FEM experiment using a 3-MeV, 2-A electron beam has demonstrated an energy recovery efficiency of 99%. To advance the FEM for fusion and other applications, TRW is in the process of building a device based on the principles just described