Magnetic field measurement techniques with heavy ion beam probes

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Abstract
Spatially (0.1 cm3) and temporally (1 μs) resolved magnetic field measurement techniques using a heavy ion beam probe as a test particle source are described. The measurement of both steady‐state and time‐varying fields is discussed. The plasma flux function can be determined by measuring the toroidal velocity of the beam ion in an axisymmetric device, because the canonical angular momentum of a particle, Pφ=qψ+MνφR, is conserved in an axisymmetric system. Corrections due to nonaxisymmetry can be significant in tokamaks and must be taken into account for the current profile and fluctuation measurements. The requirements and design of a toroidal velocity detector are discussed. The signals expected in experiments using the Texas Experimental Tokamak (TEXT) heavy ion beam probe with a velocity detector have been calculated, and they are at least two orders of magnitude higher than the amplifier noise for dc measurements of poloidal and ergodic magnetic limiter fields and for sawtooth and MHD oscillations. Low‐level turbulence is expected to produce signals below the noise level.

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