Experimental test of resonant absorption theory. Final report, January 1, 1978-December 31, 1979 Page: 4 of 17
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EE'ERIMENTAL TEST OF RESONANT ABSORPTION,, THEORY .
This experimental- research has probed the nature of resonant absorption
(BA) of laser light by laser-produced plasmas. The plasmas were created by
. optical breakdown of a shockftont produced in an electrothermal shock tube.
-This procedure allows the density structure of .the plasma, and in particular.
the orientation of the plasma critical-density surface, to be reproducibly
formed from one shot to the next, Thus, for the first time1 RA has been
controllably and reproducibly studied in isolation from other plasma physics.
The angular distribution of fast electrons emitted by RA and "wavebreaking"
has been studied, and it is observed that the emission is directed in a
narrow cone centered on the shockfront density-gradient vector, in agreement
with the theory of wavebreaking. This unidirectional emission allows the
construction of a simple magnetic-focussing electron spectrometer, which
yields the "temperature", Th of the suprathermal tail of the electron
energy spectrum. For 500 psee FWaO laser pulses with 1k2 116 W _ Wjfcm2
it is found that Th scales with laser intensity I as Th a 1-, where
6 - .32 .03. This is in agreement with predictions of the theory of
profile modification by the ponderomotive force. For much shorter laser
pulses (20 psec risetime, 100 to 200 psee FWHM)., the scaling exponent 6 is
found to be 6.- .1 1 .1, in disagreement with theory. This observation
seems to be due to the -dominance of a transient regime in which most elee-
trons are emitted in a "chirped" monoenergetic pulse of 1 to 2 psee duration.
In this modal the observed electron energy spectrum is due to the integrated
temporal variation of the energy of the electron burst. A numerical calcu-
lation of this effect yields the same scaling as seen experimentally.
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Yablonovitch, E. Experimental test of resonant absorption theory. Final report, January 1, 1978-December 31, 1979, report, May 1, 1979; United States. (digital.library.unt.edu/ark:/67531/metadc1053080/m1/4/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.