LIF Techniques
(Fluorescence, Ion Distributions, Spatial Convection)

Laser induced fluorescence operates in a manner similar to common observations of Doppler-shifted changes in frequency when a police car siren speeds by while one waits at the side of a road. Light of a particular frequency, w_r , may excite a stationary plasma ion to emit a photon. If the ion is moving at some velocity, v, it will not be excited to emit a photon unless the exciting light (typically from a laser) at frequency, w_l, is changed enough to account for the Doppler shift of the ion. In other words, the laser, with wavenumber, k, will only be able to induce photon emission from the ion when w_l - k · v = w_r . By scanning the laser frequency (see figure at right), ions of all velocities can be fluoresced. For the example shown, singly-ionized argon is excited by 611.5 nm laser light and a 460.9 nm fluorescent photon is emitted and then detected.

Indeed, by collecting the emitted photon flux as a function of laser frequency, the ion distribution function in the direction of k can be found. A scan of laser frequency through the Doppler-shifted velocities of an argon plasma is shown in the figure at left. The induced fluorescence yields the ion distribution. That is, the x-axis is related to the ion speed in the direction of k, so a graph of f_i(v) can be calculated from these data. By changing the angle of k ion distributions in any direction may be measured. Sets of these ion distributions can be converted into the multi-dimensional ion distributions via optical tomography (see Ion Conics).

By calibrating the laser frequency an absolute measure of ion velocity can be obtained. Laser frequency can be calibrated via an iodine cell. Lacking an iodine cell, running the laser through the plasma in opposing directions will yield v = 0 also. With an absolute velocity scale, LIF will give the ion fluid drift speeds as well as all other moments of the distribution function. For the example shown, the plasma typically has vertical drift speeds ranging from 100 - 2000 m/s, depending on plasma parameters. Argon ion temperatures commonly range from about 0.1 - 0.5 eV.

Much of the physics described here is covered in Lab Publications.

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