Investigation of the Compression of Magnetized Plasma and Magnetic Flux

103 Pages·2019·4.345 MB·English

by  Dimitry Mikitchuk

Springer Theses
Recognizing Outstanding Ph.D. Research
Dimitry Mikitchuk
Investigation of
the Compression of
Magnetized Plasma
and Magnetic Flux
Springer Theses
Recognizing Outstanding Ph.D. Research
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Dimitry Mikitchuk
Investigation
of the Compression
of Magnetized Plasma
and Magnetic Flux
Doctoral Thesis accepted by
the Weizmann Institute of Science, Rehovot,
Israel
123
Author Supervisor
Dr. Dimitry Mikitchuk Prof. YitzhakMaron
DepartmentofPhysicsofComplexSystems Faculty of Physics
WeizmannInstitute ofScience WeizmannInstitute ofScience
Rehovot, Israel Rehovot, Israel
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-20854-7 ISBN978-3-030-20855-4 (eBook)
https://doi.org/10.1007/978-3-030-20855-4
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’
Supervisor s Foreword
It is my pleasure to introduce Dr. Dimitry Mikitchuk’s Ph.D. research for publi-
cationintheSpringerThesisseries.Dr.MikitchukwasawardedhisPh.D.fromthe
Weizmann Institute of Science in January 2017 for the research presented in this
book. The main subject of his experimental study is the investigation of the
compression of magnetized plasma and magnetic field by plasma implosion. This
subject is relevant to the Magnetized Liner Inertial Fusion and likely to astro-
physical plasmas, such as sunspots or other astrophysical objects where the mag-
netic flux is frozen in an imploding plasma. Here, the magnetized plasma and
magnetic flux compression are achieved by using a Z-pinch configuration with
preembedded axial magnetic field. A pulsed axial current is driven through the
plasma column generating an azimuthal magnetic field that through the Lorentz
force compresses the magnetized plasma and the magnetic flux. The diagnostics
of the magnetic fields are performed using a noninvasive spectroscopic technique
based onthepolarization propertiesoftheZeeman componentsofdifferent atomic
(or ionic) transitions, which enhances the sensitivity of the measurement. In his
Ph.D. research, Dr. Mikitchuk made a highly important contribution to the
understanding of the physics involved in magnetized plasma compression by the
successful determination of the magnetic-field and current-density distributions in
the non-equilibrium, transient plasmas. Specifically, his measurements include:
(i) Development and implementation of localized magnetic-field spectroscopic
diagnostics for pulsed-power systems based on the polarization properties
of the Zeeman effect and using dopant species introduced by laser ablation.
(ii) Direct measurement, for thefirst time, of thecompressed axial magnetic field
evolution and distribution during the implosion and stagnation in a Z-pinch
with preembedded axial magnetic field utilizing noninvasive spectroscopic
methods.
(iii) Simultaneous measurement of the axial and azimuthal magnetic fields
revealing unexpected results of the current distribution and the nature of the
pressure balance of the axial and azimuthal fields.
v
vi Supervisor’sForeword
(iv) Investigationanddemonstrationofthemitigatingeffectsofpreembeddedaxial
magnetic fields on magneto-Rayleigh-Taylor instabilities in Z-pinch implo-
sions, using interferometric and imaging methods.
These measurements are basic and essential for the advancement of the under-
standing of complex plasma systems, both in laboratory and in nature. Since the
magnetic field is a key factor in magneto-hydrodynamics modeling, the results are
highly important for examining simulations, as well as for designing plasma con-
figurations that are particularly relevant to the presently central Magnetized Liner
Inertial Fusion approach.
Rehovot, Israel Prof. Yitzhak Maron
January 2019
Abstract
Inthisresearch,Iinvestigatedfundamentalphenomenaoccurringasmagnetic-field
flux and magnetized plasma are compressed by applied azimuthal magnetic fields.
This subject is relevant to numerous studies in laboratory and space plasmas.
Recently,ithasgainedparticularinterestduetotheadvancesinproducingplasmas
of high temperature and density in experiments based on the approach of magne-
tizedplasmacompression[1].Manyintheplasmaphysicscommunityconsiderthis
approach to be the most promising for achieving controlled nuclear fusion. To
advancethisapproach,itisessentialtostudyexperimentallythegoverningphysical
mechanisms that take place during the compression. Performing the required sys-
tematic experiments is impractical in large-scale facilities designed for fusion
demonstration.
In our experiment, we employ a cylindrical (Z-pinch) configuration, in which a
current (300 kA, rise time 1.6 ls) driven through a cylindrical plasma causes
implosion of the plasma under the self-generated azimuthal magnetic fields (Bh).
However,ourcylindricalplasmaisinitiallyembeddedinanaxialmagneticfieldB .
z
Thefieldisquasi-staticallyappliedpriortothehigh-currentdischarge,withavalue
of 0.4 T.
Here, for the first time in these researches, Zeeman-splitting observations are
used to measure the evolution and spatial distribution of Bz and Bh during the
implosion and stagnation stages. The two fields are measured simultaneously,
which is rather important due to the irreproducibility that characterizes such
experiments of high-current pulses. The difficulties in these measurements are due
to(1)thehighelectrondensitiesintheplasmagivingrisetolargeStarkbroadening
that smears out the Zeeman pattern, (2) the difficulty in distinguishing between B
z
andBh,and(3)theabsenceoflightemissionfromthecenteroftheplasmacolumn.
Indeed, in previous studies, under similar conditions, the B-fields were only indi-
rectly estimated from the plasma radius. These challenges were achieved by
employing a novel spectroscopic technique based on the polarization properties of
Zeeman split emission, combined with a laser-generated doping technique that
provided mm-scale spatial resolution.
vii
viii Abstract
Systematic measurements were performed for different initial conditions of B
z
andgasloads.ThemeasurementsshowedthatestimatesoftheB-fieldsbasedonthe
plasma radius are subjected to large errors and thus unreliable. Indeed, the simul-
taneously measured Bz and Bh, together with the plasma radius and the discharge
current, showed that the application of an initial B has a dramatic effect on the
z
current distribution in the plasma. While without B the entire current is found, as
z
expected, to flow through the imploding plasma, when an initial B is applied, the
z
measured Bh (through r(cid:2)~B¼l0~j) showed that only a small part of the current
flows within the outer radius of the imploding plasma. Specifically, when
Bz0 ¼0:4T,thevalueofBh intheimplodingplasmashellremainsnearlyconstant
(between 1.5 and 2 T) during the implosion, even though the current rises and the
plasma radius drops. This finding indicates that for implosions with B [0 large
z0
fractionofthecurrentflowsintheperipheralplasmaresidingatradiilargerthanthe
imploding plasma radius. A theoretical model, based on the development of a
force-free current configuration in the peripheral plasma, is suggested to explain
this unexpected phenomenon. To rigorously test this model, self-consistent 3D
MHD simulations are required.
In addition to these results, the measurements provide much information useful
for the understanding of the B -embedded plasma implosion. We measure at
z
stagnation a (cid:3)15(cid:2) compression of the initial axial B-field. This compression
factor, together with the observed plasma radius, allows for obtaining the B con-
z
finement efficiency, which is found to be (cid:3)50%. This information is useful for
testing MHD codes. Another phenomenon observed is an axial gradient of B in
z
whichitsmagnitudeincreasesbyafactorof2fromtheanode(lowB )tothemiddle
z
of the plasma column (z(cid:3)5 mm, high B ). This measurement demonstrates the
z
existence of a transition region from the uncompressed B ¼B ðt¼0Þ inside the
z z
electrodes to the compressed B farther away from the electrode surface.
z
The spectroscopic measurements were complemented by 2D images of the
plasma self-emission and by interferometric images. These measurements were
important both for obtaining the B-field evolution and for the study of the depen-
dence of instabilities on the different initial conditions. The measurements clearly
showed the mitigation effect of B on the magneto-Rayleigh-Taylor instabilities.
z
The 2D images have also shown the existence of axially directed, filament-like
regionsthathavesignificantlyhigheremissionthanthesurroundingplasma.These
filaments were found to be plasma regions with higher electron density (by 10–
20%), and slightly lower electron temperature (by a few percent) than of the sur-
rounding plasma.
Abstract ix
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