Deflectometer Experiments

A simple method of obtaining the line-averaged density of a plasma can be made by measuring the deflection of a laser beam as it passes through a gradient in the local index of refraction, which is determined by the local electron density of the plasma as explained by the first image below. This method of density profile measurement uses short coherence length diode lasers and point detectors, and places far less requirements on the laser quality and alignment procedures then laser interferometers. In particular, the main advantage a deflection diagnostic is that it is especially sensitive to local gradients of index of refraction; thus it is useful for detection of plasma shock fronts created during the deceleration phase of a colliding SCT. We are planning to deploy an array of deflectometers on the CTIX device to resolve plasma shocks. In these upcoming experiments an accelerated SCT plasma will collide with a flat conducting plate and is expected to produce a backward traveling compression shock wave or a localized step in plasma density. The results and analysis of this set of experiments will provide the ground-work for the Ph.D. dissertation of a UC Davis graduate student (Samuel Brockington).deflect1

 

 

 

 

 

 

Angle of laser deflection is determined by line integrated density gradient, and is inversely proportional to plasma critical density. This implies that for a probe laser in the optical wavelengths, deflection angles will be small.

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Radial (A) and Axial (B) laser deflection signals for a high density SCT match closely to predicted signals for a compact toroid with a radially parabolic and axially Gaussian density profile. Predicted SCT pulse width was shorter than observed because the model assumed constant velocity, yet for this shot the SCT was slowing down due to mass accumulation during an accelerator gas puff experiment.

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