On the possibility of measuring the q-profile in dense plasmas by means of molecular hydrogen beams

A method of measuring the q-profile in a plasma is analyzed in detail for its applicability to fusion plasmas. Radially injected neutral hydrogen molecules will be ionized in the plasma and will rotate according to the local magnetic field. After dissociation the information on the velocity of the molecular ion at the time of dissociation can be carried by the neutral atom to a neutral particle analyzer. This paper shows how this information can be used to calculate the local field. The production and penetration of molecular beams are discussed as well as the requirements to be met by the beam and analyzer. Also discussed are the possibility and influence of neutral atom beams and their possible relevance to the same type of measurement. It is shown that the Shafranov shift can be determined by suitable choices of the injection and detection geometries. The method seems to be especially suited to measuring the time variation of local q-values. Although the penetration of the particles depends on the plasma conditions, the quantity measured by the method discussed only depends on the local field ratio. It is shown that the method could also be applied in large plasma experiments the size of JET, and that an accuracy in measuring q of better than or about 10% may be achieved.