Measured and predicted beam attenuation in neutral beam drift ducts for tokamaks

Experimental evidence obtained with the Princeton Large Torus, PLT, neutral beam injectors at both the Oak Ridge National Laboratory and at Princeton has indicated that the transport of neutral beams through conductance limited drift ducts can induce exponentiating beam loss and subsequent beam blocking. Evidence indicates that beam loss on the drift duct wall causes thermally induced outgassing of absorbed hydrogen gas. Application of such a model will be shown to yield predictions which agree both in magnitude and time dependence with the observed results. Further application of such modeling to Doublet III at General Atomic Company will also be discussed