Abstract
Spectral properties of coherent waves in an argon plasma column are examined using fluctuation data from fast imaging. Visible light from ArII line emission is collected at high frame rates using a highspeed digital camera. A crossspectral phase technique allows direct visualization of dominant phase structures as a function of frequency, as well as identification of azimuthal asymmetries present in the system. Experimental dispersion estimates are constructed from imaging data alone. Driftlike waves are identified by comparison with theoretical dispersion curves, and a tentative match of a lowfrequency spectral feature to KelvinHelmholtzdriven waves is presented. Imaging measurements are consistent with previous results, and provide noninvasive, singleshot measurements across the entire plasma crosssection. Implications of the measured spectral properties for imaging measurements of mode dynamics are explored.
The authors gratefully acknowledge support from the Center for Momentum Transport and Flow Organization, funded by the United States Department of Energy (award number DESC0001966).
I. INTRODUCTION
II. EXPERIMENT DESCRIPTION
III. IMAGING DETAILS
IV. CROSSSPECTRALDENSITY PHASE MAPPING
V. DISPERSION ESTIMATES FROM INTENSITY
VI. IMPLICATIONS FOR DYNAMICS
VII. CONCLUSIONS
Key Topics
 Plasma fluctuations
 16.0
 Dispersion
 14.0
 Dispersion relations
 12.0
 Plasma diagnostics
 11.0
 Coherence
 10.0
Figures
Time average profiles for plasma parameters. (a) Is and electron temperature profiles. The black solid line is a Gaussian fit to the data (gray dots). (b) Measured plasma potential and resulting azimuthal velocity.
Time average profiles for plasma parameters. (a) Is and electron temperature profiles. The black solid line is a Gaussian fit to the data (gray dots). (b) Measured plasma potential and resulting azimuthal velocity.
(a) Raw image example (single frame). White corresponds to zero intensity. (b) Normalized radial profiles of mean I 488 (solid black), RMS I 488 fluctuation amplitude (dotted black), and mean Is (solid gray).
(a) Raw image example (single frame). White corresponds to zero intensity. (b) Normalized radial profiles of mean I 488 (solid black), RMS I 488 fluctuation amplitude (dotted black), and mean Is (solid gray).
Comparison of (a) Is and (b) I 488 frequency spectra at various radii. The innermost radial trace is not shifted, while each successive radial trace is shifted downward by a factor of ten to allow all traces to be visible.
Comparison of (a) Is and (b) I 488 frequency spectra at various radii. The innermost radial trace is not shifted, while each successive radial trace is shifted downward by a factor of ten to allow all traces to be visible.
The average coherence spectrum of the plasma column compared with the average power spectrum. Error bars in the crossspectral power are statistical uncertainty in the discrete CSD estimate. Note that the power spectrum is plotted on a logarithmic scale. (a)–(d) are the four most prominent peaks, corresponding to the phase maps in Fig. 5 .
The average coherence spectrum of the plasma column compared with the average power spectrum. Error bars in the crossspectral power are statistical uncertainty in the discrete CSD estimate. Note that the power spectrum is plotted on a logarithmic scale. (a)–(d) are the four most prominent peaks, corresponding to the phase maps in Fig. 5 .
Sample phase maps for the four most prominent coherent modes. Vertical bars in Fig. 4 highlight the frequency component corresponding to each map (a)–(d).
Sample phase maps for the four most prominent coherent modes. Vertical bars in Fig. 4 highlight the frequency component corresponding to each map (a)–(d).
Standarddeviation maps [Eq. (3) ] corresponding to Fig. 5 . Note that particularly asymmetric parts of the phase maps seem to coincide with high uncertainty.
(a) Timeaverage frame. (b) RMS frame. (c) Azimuthal variation at for timeaverage (mean) I 488, RMS I 488, and the radial gradient (negative) of I 488. Intensity is normalized to its maximum value in the image sequence.
(a) Timeaverage frame. (b) RMS frame. (c) Azimuthal variation at for timeaverage (mean) I 488, RMS I 488, and the radial gradient (negative) of I 488. Intensity is normalized to its maximum value in the image sequence.
Normalized spectral density andmeasured dispersion relations at from (a) 2D autospectral density of image data, (b) average of 32 twopoint estimates from pixel data, (c) twopoint floatingpotential data (probe), and (d) twopoint pixel data. The color scale for each plot indicates the power relative to the total spectral power at the given radius. White circles represent the maxima of the kspectra for frequencies where the total spectral power is more than 1% above background. The dashed line in (a) represents the raw theoretical expectation [Eq. (6) ] calculated from the profiles in Fig. ( 1 ). The hashed region in (a) indicates likely bounds for Dopplershifted theory curves based on the profile of Fig. 1(b) . The solid line in each plot is the theoretical expectation including the bestfit Doppler shift for (a).
Normalized spectral density andmeasured dispersion relations at from (a) 2D autospectral density of image data, (b) average of 32 twopoint estimates from pixel data, (c) twopoint floatingpotential data (probe), and (d) twopoint pixel data. The color scale for each plot indicates the power relative to the total spectral power at the given radius. White circles represent the maxima of the kspectra for frequencies where the total spectral power is more than 1% above background. The dashed line in (a) represents the raw theoretical expectation [Eq. (6) ] calculated from the profiles in Fig. ( 1 ). The hashed region in (a) indicates likely bounds for Dopplershifted theory curves based on the profile of Fig. 1(b) . The solid line in each plot is the theoretical expectation including the bestfit Doppler shift for (a).
Comparison of dispersion estimates at various radii: (a) , (b) , and (c) . The solid line is the theoretical curve from the Ellis model including the Doppler shift from the bestfit velocity. Best fit values for are , , . White circles represent the maxima of the kspectra for frequencies where the total spectral power is more than 1% above background. The dashed line in (c) is the KelvinHelmholtz dispersion curve calculated from Eq. (7) .
Comparison of dispersion estimates at various radii: (a) , (b) , and (c) . The solid line is the theoretical curve from the Ellis model including the Doppler shift from the bestfit velocity. Best fit values for are , , . White circles represent the maxima of the kspectra for frequencies where the total spectral power is more than 1% above background. The dashed line in (c) is the KelvinHelmholtz dispersion curve calculated from Eq. (7) .
Frequency spectra of image sequences filtered in mode number, demonstrating shared frequency content. Shaded regions (a)–(c) correspond to the frequency pass bands used for the filtered sequences in Fig. 11 .
Frequency spectra of image sequences filtered in mode number, demonstrating shared frequency content. Shaded regions (a)–(c) correspond to the frequency pass bands used for the filtered sequences in Fig. 11 .
Time evolution of m = 2 image component with various frequency filters applied. The rows (a,b,c) correspond to the passbands around the peaks distinguished in Fig. 10 . The color scale for each mode is normalized to the maximum intensity value in the entire image sequence.
Time evolution of m = 2 image component with various frequency filters applied. The rows (a,b,c) correspond to the passbands around the peaks distinguished in Fig. 10 . The color scale for each mode is normalized to the maximum intensity value in the entire image sequence.
Article metrics loading...
Full text loading...
Most read this month
Most cited this month










Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets
Stephen P. Hatchett, Curtis G. Brown, Thomas E. Cowan, Eugene A. Henry, Joy S. Johnson, Michael H. Key, Jeffrey A. Koch, A. Bruce Langdon, Barbara F. Lasinski, Richard W. Lee, Andrew J. Mackinnon, Deanna M. Pennington, Michael D. Perry, Thomas W. Phillips, Markus Roth, T. Craig Sangster, Mike S. Singh, Richard A. Snavely, Mark A. Stoyer, Scott C. Wilks and Kazuhito Yasuike

Commenting has been disabled for this content