Instrumentation for the joint European torus motional Stark effect diagnostic

A motional Stark effect magnetic field pitch angle diagnostic has been implemented on the joint European torus (JET) tokamak. The instrumentation designed following the study by Hawkes et al. (these proceedings) is described. D_α emission from the Octant 4 neutral beams is collected by optics which transport the plasma image outside the vacuum vessel and through a pair of photoelastic modulators (PEMs) and a linear polarizer. The light is fiber-optically coupled to interference filter spectrometers, which incorporate a remotely controlled filter tilting mechanism. This allows the center wavelength of the filter bandpass to be tuned over a range sufficient for observation of the σ and π lines of the Stark spectrum emitted by the full- and half-energy components of the beam, providing flexibility to make measurements with a variety of beam configurations. The detectors are low-noise avalanche photodiode modules. Fast digital signal processing techniques are used to extract the Fourier components of the signal at the PEM first and second harmonic frequencies. Analysis of these signals will yield the magnetic field pitch angle, which will be used as a constraint on EFIT equilibrium reconstruction modeling to obtain the q(r) profile. The system has 25 spatial channels covering the outer-half of a JET plasma with spatial resolution of 0.03-0.07 m per channel with ∼0.05 m channel-to-channel separation. Time resolution is expected to be 1-10 ms.