Page 4 - Measuring dynamic fast ion spatial profiles with fusion protons in the Madison Symmetric Torus

10I104-4 Magee et al.
Rev. Sci. Instrum. 89, 10I104 (2018)
FIG.5. Equilibriumfastiondensityprofilemeasuredinnon-reversedplasma. Time histories of count rates for each aiming angle (left) and average profile from t = 31 to t = 35 ms (right) are plotted. The solid black line on the right shows the expected signal profile based on a Shafranov-shifted, Gaussian fast ion density profile with σ = 9 cm. Bounds of ±2 cm are shown with the dashed-dotted lines.
a forward modeled signal (black line) found by orbit tracing through a fast ion density profile with σ = 9 cm and 5 cm Shafranov shift, as described above. The dashed-dotted lines show the effect on the signal of widening or narrowing the fast ion density by 2 cm. This result agrees well with the one obtained from measurements made in similar plasma conditions with a collimated neutron detector. The neu- tron data predict a Gaussian fast ion density profile with σ = 8 cm.13
C. Dynamic profiles
In order to measure dynamic profiles, we turn to standard reversed plasmas which are punctuated by discrete bursts of magnetic reconnection due to multiple, coupled tearing modes. These sawtooth crashes occur semi-periodically throughout the plasma discharge. To uncover the effect on the fusion emission profile, a large number of similar sawtooth crash events are averaged together (40-50 events per measurement location) by aligning the events in time. To enhance the time resolution of the measurement, the signals from indi- vidual shots are added together first and the pulses of the summed signal are counted, rather than counting the pulses first and averaging the count rates, as was done in the F = 0 plasmas.
Plotted in the left frame of Fig. 6 are the event-averaged time histories from each measurement location (see Fig. 4 for color code). The time average profile is plotted in the right frame (the time windows for the “before,” “during,” and “after” lines are, respectively, −0.6 < t < −0.1, −0.1 < t < 0.1, and 0.1 < t < 0.6 ms). It can be seen that at the time of the crash, the core fast ion density drops by about 30% and the edge increases by a similar amount. The redistribution is fast, happening on time scales of 100 μs or less.
FIG. 6. Dynamic fast ion density profile measured during the course of a global tearing mode burst. The horizontal axis is the impact parameter of the proton orbit to the geometric axis of the machine.
V. SUMMARY
A fusion proton detector has been developed for measur- ing fast ion profile dynamics. A steerable collimator actuated with a linear feedthrough allows a continuous scanning of the line of sight through the plasma so that the fusion emis- sion profile can be built up over several 10 s of discharges. The fast ion density profile can be reconstructed from the measured emission profile by tracing 3 MeV proton orbits through the reconstructed magnetic field. The results of the measurement indicate that the profile remains constant dur- ing non-reversed quiescent plasmas but flattens during global tearing mode activity.
ACKNOWLEDGMENTS
We thank our shareholders for their support and trust and all fellow TAE staff for their dedication, excellent work, and extra efforts. We also thank the MST team for their generous support, ingenuity, and creativity in both the data collection and analysis phases of the project.
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