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Plasma density gradient measurement using laser deflection
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View: Figures


Image of FIG. 1.
FIG. 1.

Mechanism of laser deflection can be developed from a beam model consisting of two parallel rays passing through a plasma of dielectric function, . The angle of deflection is the angle between the axis and a surface of constant phase.

Image of FIG. 2.
FIG. 2.

(Color online). Four channel photodiode array can be used to detect both axial and radial deflection of the laser spot.

Image of FIG. 3.
FIG. 3.

(Color online). CTIX is a coaxial plasma rail gun. The deflection assembly was mounted perpendicular to CTIX at the port. A heterodyne interferometer was mounted at the port. Axial magnetic field data was gathered with magnetic field probes at those same locations.

Image of FIG. 4.
FIG. 4.

(Color online). Typical CTIX deflection installation.

Image of FIG. 5.
FIG. 5.

(Color online). Radial deflection as a function of the radial displacement of the laser chord was plotted for several aspect ratios . Radial deflection is the sum of two competing effects: the deflection created by the plasma and the refraction of the cylindrical geometry of the plasma. A SCT is expected to cause of deflection with the present laser chord positions. Laser chord placement is heavily constrained by CTIX port positioning.

Image of FIG. 6.
FIG. 6.

(Color online). Each element of the photodiode array had a custom preamplifier constructed for it. These preamplifiers separated fast ac deflection signals and slower dc signals. It was necessary to use a phase compensated transimpedance amplifier for the first stage of each channel. Amplifier response is largely limited by first stage input capacitance.

Image of FIG. 7.
FIG. 7.

(Color online). Bode plot of transimpedance gain of photodiode preamplifier channels. Channel gain must be matched as closely as possible to avoid having attenuation signals appear as deflection signals.

Image of FIG. 8.
FIG. 8.

(Color online). Radial deflection at was compared to axial magnetic field at the same location. A peak deflection signal on the order of would correspond to a SCT peak density of .

Image of FIG. 9.
FIG. 9.

(Color online). Deflection measurements taken at the port were compared with density measurements taken by a Heterodyne interferometer mounted at the port. Using the parabolic SCT density profile assumption described in the expected signals section, the peak deflection of would correspond to a peak density of , which is close to the interferometer’s predicted peak density of . Axial magnetic field data, and for each position is shown for reference.

Image of FIG. 10.
FIG. 10.

(Color online). To confirm that detected signals were not artifacts of unbalanced gain between channels, radial deflection was measured with the photodiode array mounted in normal and rotated 90° orientations. Thus any true deflection of the laser beam would survive the change in coordinate systems.

Image of FIG. 11.
FIG. 11.

(Color online). Common mode attenuation at was compared to radial deflection at the same position. Attenuation was found to occur after the peak of radial deflection and persisted longer than the deflection signal.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Plasma density gradient measurement using laser deflection