Page 3 - First fast-ion D-alpha (FIDA) measurements and simulations on C-2U

11E526-3
Deepak K. Gupta
Rev. Sci. Instrum. 87, 11E526 (2016)
FIG. 3. (a) Radial view with a laser as an external illumination source. (b) Polarization fraction at two Hanle field sensitivities. H ⇠ 1 at Hanle- field, BH.
measurement of FRC center location and shape, based on the internal magnetic structure. With time varying signals this will provide valuable information about the n = 1 wobble mode and n = 2 rotational instabilities.
C. Radial view
Axial views in a linear FRC plasma machine are not always, or at least easily, available. Many times a preferred (or only) option for diagnostics is observing the signal from radial views. With external illumination incident from the radial direction, observation views perpendicular to the incident radi- ation direction in the same radial plane may be chosen (Fig. 3). In this arrangement, magnetic field is directed perpendicular to the scattering (or observation) plane. For this situation, instead of Eq. (1) the fractional linear polarization is given by the relation6
1+2H2
pL = 1+6H2. (2)
Notice here that the range of change in polarization is only two- thirds, compared to the axial views observation cases (Eq. (1)). Moreover, no change in the direction of linear polarization will occur.
Asymmetric self-illumination may also provide Hanle ef- fect signal with the radial view. The signal will look similar to the case of an external source outside the null radius, however inside the null radius the situation will change, and one needs to take into account the full geometrical e↵ect to estimate the signal. In spite of these di↵erences, the fraction of linear polarization will be maximum at the null-locations.
In principle, with the radial view, FRC length can also be measured by measuring the distance between the location of polarized signal from the two X-point cusps along the length of the FRC. However, this needs to be further explored due to the small area of the X-point and its motion along the length with shrinking FRC.
D. Azimuthal and vector field measurement
It is believed that a small azimuthal magnetic field (B✓) may be present in C-2/C-2U FRCs. Knowledge of this azimuthal field is also highly desirable. By using two resonance radiation-lines with di↵erent sensitivities (H ⇠ 1 and H 1), vector components of the field can also be estimated.7 This may allow the measurement of both axial and azimuthal magnetic fields simultaneously.
IV. CONCLUSION
The new non-perturbative diagnostic based on the Hanle e↵ect, presented here, can measure the low magnetic field in a laboratory plasma. The diagnostic can provide the position and shape of the zero axial-magnetic-field (axial-field null) location in an FRC. External illumination technique provides a better control; however, transition line needs to be identified and a corresponding illuminating source (or laser) must be available. The self-illumination technique is simplest to imple- ment only if signal level is su ciently strong. Further explo- ration of both technique is underway. The diagnostic is also applicable to most laboratory plasmas, where measurements of low magnetic-field are desirable, such as magnetic cusp and X-point in tokamak divertor.
ACKNOWLEDGMENTS
Author thanks TAE shareholders for their support and trust, and all fellow TAE sta↵ for their dedication, excellent work, and extra e↵orts.
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