FUNCTIONAL DEPENDENCE OF THE LOWER-HYBRID POWER ABSORPTION-COEFFICIENT IN JET

The fraction of the coupled lower hybrid (LH) power absorbed in divert or plasmas in JET has been determined experimentally with a method uti lizing the time derivative of the total stored energy (plasma and magn etic). This method can account for the power absorbed inside a normali zed Bur coordinate psi approximate to 0.7. The experimental LH absorpt ion coefficient reaches 100% at low plasma densities, (n) over bar(e) < 1.0 x 10(19) m(-3), and decreases to 25% at (n) over bar(e) > 3.5 x 10(19) m(-3). The LH wave accessibility to the plasma core has been fo und to play an important role in determining the power absorption and the radial deposition profile. The decreasing absorption is correlated with a gradual shift of the LH power deposition profile, as determine d by the hard x-ray profiles, towards the plasma periphery. Similar be haviour is found in ray-tracing + Fokker-Planck code calculations. The frequency spectrum of the LH pump wave, as determined by a probe outs ide the tokamak vessel, broadens strongly as the wave accessibility is reduced and the absorption drops.