Table Of ContentLaser Interaction
and Related Plasma Phenomena
Laser Interaction
andR elated Plasma Phenotnena
Proceedings of the First Workshop, held at Rensselaer Polytechnic Institute,
Hartlord Graduate Center, East Windsor Hili, Connecticut, June 9-13, 1969
Editedby
HelmutJ. Schwarz
Professor of Physics
Rensselaer Polylechnie Institute
and
Heinrich Hora
Adjunct Associate Professor of Physics
Rensselaer Polylechnie Institute
and
Institut für Plasmaphysik
(Max Planck Society)
Garching, Germany
~
SPRINGER SCIENCE+BUSINESS MEDIA, LLC • 1971
ADVISORY BOARD
N. BLOEMBERGEN P. HARTECK
Harvard University Rensselaer Polytechnic Institute
J. M. DAWSON A. SCHLÜTER
Princeton University Institut für Plasmaphysik
(Max Planck Society) and
University of Munich
Library of Congress Catalog Card Number 79-135851
ISBN 978-1-4684-0903-1 ISBN 978-1-4684-0901-7 (eBook)
DOI 10.1007/978-1-4684-0901-7
© 1971 Springer Science+Business Media New York
Originally published by Plenum Press, New York in 1971
Softcover reprint of the hardcover 1st edition 1971
All rights reserved
No part of thls publication may be reproduced in any form
without written permission from the p'ublisher
FOREWORD
N. G. Basov
P. N. Lebedev Physical Institute of the
Academy of Sciences of the U.S.S.R.
Noscow, U.S.S.R.
The remarkable properties of laser radiation, namely
high degree of spatial and time coherence at high power
levels, open new possibilities for the use of light energy
to radically change the thermodynamic state of matter over
a wide range of temperatures. Existing lasers provide
maximum speed of introducing energy into matter, signifi
cantly exceeding the level attainable by means of electric
discharge devices. Lasers, therefore, represent an
outstanding instrument for experimental investigations
in the field of plasma physics, including thermonuclear
physics, thermophysics, spectroscopy, technology, etc.
It is important to note that lasers are used successfully
not only as a powerful means of acting on matter, but
also for registering and diagnosing e.i., high-speed
photography, interferometry, holography, temperature
measurements and determining optical constants.
The use of lasers in such investigations has led to
the creation of a new field of experimental and theor
etical physics including problems related to the inter
action of powerful electromagnetic waves and matter and
high-temperature gas dynamics. vlork in this field over
the past few years has made it possible to establish the
basic laws of the physical and gas dynamical processes
accompanying the action of laser radiation on matter.
Conducted investigations have made it possible to out
line the basic directions of the applications of lasers
in high-temperature and plasma physics and evaluate the
possibility of their use in the solution of problems of
controlled thermonuclear reaction.
At the present time, intensive investiqations on
heating of plasma by laser radiation are under way. It
can be saiQ that here, even when comparatively laser
l~~
v
vi FOREWORD
energies of the order of tens of joules are used, there
have been attained temperatures of the 03der of 2 kev
at plasma densities of about 1023 per em and the first
thermonuclear reactions have been registered. The rapid
development of laser technology and the use of magnetic
methods or containing plasma will lead in. the coming
years to a significant increase in the temperature of
heated plasma.
By means of lasers, under laboratory conditions
powe,ful shock waves, the velocity of which reaches
5.10 em/sec are created in a gas. The "hardening"
effect of the ionization state during expansion of laser
plasma in a vacuum has been observed. Laser plasma
constitutes also a source of multiply-charged ions. In
particular, the spectrum of ions having an effective
charge of about 20 has been registered. Spectroscopic
investigations of laser plasma have led to the identifi
cation of a number of new lines.
In the field of relatively lmv temneratures, the
thermodynamic parameters of matter, especially those
difficult to sublimate, are being studied successfully.
Such investigations are laying the basis for the use of
lasers in various technological processes.
Lasers have found ~"ide apnlication in the technology
of producing thin films, which has made nossible their
use in micro-electronics. It has become possible to
produce micro-condensers ,,,ith a specific capacitance of
2.5 \IF/cm2•
The use of laser breakdo',m to commute high energies
has led to the construction of commutators without
triggering electrodes and to the solution of the problem
of synchronizing a large number of discharges with a
high degree of accuracy and small operation time.
Methods of high-speed photography in scientific
investigations have significantly changed. The use of
light delays makes it possible to correlate the instant
of photogra?hing relative to the investigated process
with an accuracy of 1 picosec.
11.11 these problems, as well as other no less inter
esting ones, are reflected in the program of the present
conference.
Even a brief enumeration of the main directions of
investigation of high-temperature and nlasma nhenomena
and the obtained results indicate that a ne~'l field of
interesting and promising physical investigations has
developed.
PREFACE
Since the development of high power lasers, entirely new fields of
physics have opened up, as pointed out by Academician Basov in the
Foreword. The heating, evaporation and ionization of matter by laser
energy offered new technologies. Research on plasmas entered regions
where much higher densities, temperatures and pressures were en
countered than before. Developments give reasonable hope of success
for heating laser produced plasmas up to conditions of nuclear fusion.
There is a very strong desire for both beginners and experts to ob
tain an introduction or an orientation as to what is going on in these
fields; therefore, the necessity for a review on this subject. However,
the very rapid advancement in this field makes it almost impossible to
get a review in the same way as one obtains from any monography. In
most cases, a monography is the last stone in the building of a settled
f.ield of research. Therefore, the starting point for presenting a review
on laser interaction and related plasma phenomena was to organize a
Workshop where the presentation of reviewing talks were discussed with
a group of physicists familiar with the field or highly interested in
starting work in the field which due to their training and experience is
easily accessible to them.
Starting with historical remarks on induced* emission of radiation
and on fusion reactions from the viewpoint of the present state of the
art, we could elaborate on picosecond pulses and could supplement the
papers on terawatt laser pulses with presentations on low laser inten
sities in several technical applications of an essentially new kind.
Work in laser produced plasma is heterogeneous. To warn every be
ginner from trusting any special model too much, we could demonstrate
the complexness of the results obtained by different and differing
theoretical and experimental groups. This pronounced analytical (or
better analyzing) view should help to reach a higher level of a syn
thesis.
It was decided not to incorporate the problems of laser produced
gas breakdown in this first Workshop, even though these problems are
*Einstein preferred the word tlerzwungenetl emission.
vii
viii PREFACE
not understood in detail and for all conditions. There is little effect
of the ionization mechanism in gases on the interpretation of the high
intensity interaction process. In gas breakdown by ruby or neodymium
glass lasers, the threshold field strength of the light is close to
107 V fcm, depending somewhat on the gas p:pessure and the laser
pulse length, ~hereas the primary ionizati on process in solids starts
at fields of 10 V fcm and therefore, cannot be studied in analogy to
that in gases. To understand laser interaction with matter, including
the creation of plasma, studies are preferably done in vacuum. The
presence of gas is only desirable when applying shock wave diagnostics.
The laser field strengths at the conditions discussed here are withi~
103 to 1010 Vfcm corresponding to intensities of 104 to 1017 Wfcm •
The first section of these Proceedings contains a general introduc
tion to the Workshop by p. Harteck, and a review of A. J. DeMaria
on the technique of picosecond laser pulses. To emphasize the in
terest in the aspects of nuclear fusion, P. Harteck presented histori
cal comments on the spirit of the time when Einstein deduced the stim
ulated emission process, and when the first observations of simulated
emission of radiation from a plasma (Ladenburg and Kopfermann 1928)
were reported. Harteck's discovery of tritium at Rutherford's Lab
oratory in 1934 is one of the basic processes for nuclear fusion.
Therefore, his remarks on handling tritium were of special interest.
He connected physicochemical aspects of lasers and remarks on the
CO2 laser mechanism with his present work.
The second section deals with the interaction of low energy laser
radiation with material. Interest in the low energy field is still active
for evaporation of such complex materials as strontium-titanate by
lasers without chemical decomposition (H. Schwarz). The contribution
of R. E. Honig is of a classical type and contains mainly early work of
laser interaction with solids at low intensities. It also presents the
method of mass spectroscopy, a technique which is very important for
all experiments and diagnostics of laser produced plasma.
The scattering of laser radiation presented by S. C. Wait supplies
some features of processes of high intensity interactions, since there
is an influence of the reflectivity of light at· surfaces due to stimulated
scattering or some emission of stimulated scattering in self focusing
in a plasma. Laser triggered switches are reviewed by A. H. Guenther
in cooperation with R. Bettis with the latest results of reaching a syn
chronization of 10-10 seconds at megavolt gaps. The explanation of
many experimental details may be helpful for applications and some un
explained effects will be of interest for further research in the entire
field.
The third section contains experimental work on laser produced
plasmas. At the beginning of the work on laser irradiation of plasma
PREFACE ix
forITled froITl solid targets in vaCUUITl, laser intensities of a few ITlega
watts created ions which ITloved against the laser light with energies of
keV. This effect, first observed by W. 1. Linlor, =ay be called the
Linlor-effect. Up to this date, no satisfactory theory exists for all the
details of this effect. Linlor presented SOITle unpublished results of
his earlier ITleasureITlents which are still valuable for future theoreti
cal work. In H. Schwarz1s contribution experiITlental ITlethods of study
ing plasITla produced by lasers froITl COITlpact solid targets (thick or
thin foils) are given and ITlany results are discussed for several ele
ITlents with and without the influences of ITlagnetic fields. Also experi
ITlental ITlethods and results involving solid hydrogen foils are given by
S. Witkowski. The results of the experiITlental work on plasITlas pro
duced by lasers froITl cOITlpact solids perforITled at the Lebedev Institute,
Moscow, are reported extensively by G. V. Sklizkov. He devoted
special attention to diagnostics and to the recoITlbination or freezing of
the ionization state of the expanding plasITla after the laser interaction.
To avoid the lateral interaction of the plaSITla with the rest of the tar
get in the case of cOITlpact targets, ITlany experiITlents have been per
forITled to heat free, nearly spherical targets, suspended in vaCUUITl in
the focus of the laser beaITl. S. Witkowski1s contribution describes
the different ITlethods used and H. Hora reviews the experiITlental re
sults.
The fourth section covers theoretical work of laser produced plas
ITla based on special assuITlptions of very short durations and low therITlal
conductivity of the irradiated ITlaterial where shock waves are created.
Pioneeri.ng work in this field was perfor~ed by A. Caruso and O.
Krokhin. A. Caruso presented a review including his recent ITlodel in
volving laser pulses of picosecond length. J. W. Shearer showed
nUITlerical calculations perforITled in cooperation with W. S. Barnes for
the analysis of ITleasureITlents of Basov e. a. with terawatt-picosecond
laser pulses. These calculations were a first step in considering the
conditions of electrons separately froITl those of ions. If the probleITl
of space charge effects is neglected, energy is transferred by trans
port processes in regions of higher density, while at lower densities
shock waves can influence the ion teITlperature.
These proceedings also contain a contribution of S. Witkowski which
was not discussed at the Workshop; it deals with the original analysis
of Caruso1s ITlethods and reports on p. Mulser1s nUITlerical calculations
of the shock wave ITlechanisITl. It is shown that this nUITlerical ITlodel
fits also the cases where good therITlal conductivity of the target, e. g.
hydrogen with ITletallic properties at higher densities, prevents the
developlnent of shock waves.
Another group of presentations deals with the self-siITlilarity ITlodel
as it was first developed by J. M. Dawson. If a fast enough distribu
tion of the incident laser energy within the target can be assuITled, this
x PREFACE
ITlOdel can be applied. The self-siITlilarity is given by conservation of
profiles of density and velocity of the plasITla. Dawson's presentation
contains his first and later developITlents of the ITlodel with respect to
nonspherical targets. Further details of the absorption process in the
irradiated plasITla surface are discussed. Another generalization of
the self-siITlilarity ITlodel is given by H. Hora and applied to experi
ITlental results. H. Hora also gives a review on the nonli.near effects
resulting in forces due to direct interaction of the light with the plasITla.
These forces will doITlinate with dense plasITlas if the radiation inten
sities are higher than 1014 W /CITl2• Self-focusing in plasITla at power
levels as low as 1 MW indicates the involveITlent of the nonlinear pro
cesses in ITlany experiITlents not yet explainable. The high energy ions
(Linlor effect) ITlay be due to the nonlinear acceleration while self
focusing occurs.
The fifth section of the Workshop was devoted to therITlonuclear
fusion. H. Hora reported on cOITlputer-optiITlized conditions to reach
ine rtially confined exotherITl las er pg-0duced fusion plasITlas of D-T
ITlixtures; here laser energies of 10 joules are necessary. A confine
ITlent by nonlinear effects of the radiation pressure is considered and the
measureITlents of Basov et al on the first detection of fusion-neutrons
are discussed. The contribution of F. Floux describes in detail the ex
periITlental and theoretical nuclear fusion work of the French group.
The results of neutron detection were not available at the tiITle of the
Workshop and the ITleasureITlent cOITlpleted a few weeks later are pre
sented in an appendix of F. Floux's presentation.
The after dinner address of N. BloeITlbergen printed at the end of
these proceedings presents a general view on nonlinear optical probleITls
involving lasers.
The proceedings cOITlprise a collection of individual reviews in the
authors' terminology. The papers contain only SOITle additional reITlarks
as a product of the ITlany discussions.
We wish to gratefully acknowledge the cooperation of all participants
of the Workshop who ITlade these proceedings an integrated introduction
and review.
Our gratitude goes also to the following Adviso rs of the Workshop
for their counsel: Prof. N. BloeITlbergen (Harvard U'niversity), Prof.
J. M. Dawson (Princeton University), Prof. P. Harteck (RPI) and
Prof. A. SchlUter (Max-Planck Society and University of Munich). We
are grateful for support of the Rensselaer Polytechnic Institute repre
sented by President R. G. FolsOITl, Vice President and Provost C. O.
Dohrenwend, Vice President and Dean of the Hartford Graduate Center
of RPI W. C. Stoker, and Dean M. H. Jordan of RPI for his welcoITling
