Abstract
The fraction of the coupled lower hybrid (LH) power absorbed in divertor plasmas in JET has been determined experimentally with a method utilizing the time derivative of the total stored energy (plasma and magnetic). This method can account for the power absorbed inside a normalized flux coordinate . The experimental LH absorption coefficient reaches 100% at low plasma densities, , and decreases to 25% at . The LH wave accessibility to the plasma core has been found 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 determined by the hard x-ray profiles, towards the plasma periphery. Similar behaviour is found in ray-tracing + Fokker - Planck code calculations. The frequency spectrum of the LH pump wave, as determined by a probe outside the tokamak vessel, broadens strongly as the wave accessibility is reduced and the absorption drops.
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