Page 4 - A combined mmwave and CO2 interferometer on the C-2W Jet plasma

Nucl. Fusion 58 (2018) 082011 2.0
1.5 1.0
0.5
0.0
5 6 7 t (ms) 8 9 10
R.M. Magee et al
fA (MHz) f (MHz)
2.0
1.5
1.0
0.5
0.0
0 10 20
30r (cm)40
50 60
70
In summary, we present an experimental characterization of a high frequency mode observed in the decaying phase of an FRC. The mode frequency scales with the edge Alfvén velocity and co-propagates with the injected beam ions. Azimuthal mode numbers n = 2, 3, and 4 are observed and they typically have a cyclic behavior in which they alternate dominance. Reconstruction of the plasma density profile indi- cates that the modes reside in the mid-radius region of the open field line plasma.
Finally, we note that CAEs in STs can have serious con- sequences, as there is strong experimental evidence that they are responsible for a degradation of plasma electron thermal confinement [10]. However, this is not a concern for the beam driven FRC. The observed modes, even if they can be posi- tively identified as CAEs, occur only late in time, after the FRC has significantly decayed. During the flat top of sustained C-2U shots, no high frequency Alfvén activity is observed.
Acknowledgments
We thank our investors for their support of TAE Technologies and the TAE and Budker teams for their contributions to this project.
References
[1] Ivanov A.A. and Prikhodko V.V. 2013 Plasma Phys. Control. Fusion 55 063001
[2] Turner W.C. et al 1979 Nucl. Fusion 19 1011–28
[3] Binderbauer M.W. et al 2015 Phys. Plasmas 22 056110 [4] Thompson M.C. et al 2012 Bull. Am. Phys. Soc. 57
[5] Roche T. et al 2016 Rev. Sci. Instrum. 87 11D409
[6] Deng B.H. et al 2016 Rev. Sci. Instrum. 87 11E125
[7] Thompson M.C. et al 2012 Rev. Sci. Instrum. 83 10D709 [8] Heidbrink W.W. 2008 Phys. Plasmas 15 055501
[9] Simonen T.C. et al 1983 Phys. Rev. Lett. 50 1668–71
[10] Gorolenkov N. et al 2002 Nucl. Fusion 42 977–85
[11] Binderbauer M.W. et al 2010 Phys. Rev. Lett. 105 045003 [12] Gorolenkov N. and Cheng C. 1995 Nucl. Fusion 35 1743 [13] Nardone C. 1992 Plasma Phys. Control. Fusion 34 1447 [14] Armstrong W.T. et al 1981 Phys. Fluids 24 2068
Figure 6. (top) Magnetic spectrogram showing four bands of Alfvén mode activity: n = 4 at the highest frequency with red overlay, n = 3 at mid frequency with orange overlay, and n = 2 at lowest frequency with blue overlay. (bottom) The radial profile of the m = 0 Alfvén frequency from 5 < t < 7 ms (solid), from
6 < t < 9.5 ms (dashed), and from 9 < t < 10 ms (dash–dot). Radial location of each mode is determined by matching the measured frequency to the point on the curve, illustrated with diamonds of color corresponding to the overlays in the top plot.
r = 57 cm. We therefore hypothesize that the n = 4 exists in this far outboard region of the plasma.
Because we currently lack internal magnetic field meas- urements, we cannot quantify the accuracy of the recon- struction. A lower limit on the uncertainty in the calculated Alfvén continuum is 7%, given by the variation in the density measurement.
The argument above is not rigorous. Because the magnetic field goes to zero inside the FRC while the plasma density does not, and the plasma density goes to zero at the plasma edge while ∥B∥ does not, any frequency can be matched. But the modes are found to reside mid-radius, and not at either extrema, so this analysis does serve as an additional piece of evidence, along with the onset criterion, that the modes are excited by an interaction between the fast ions and the open field line plasma.
4
5. Conclusion