This PhD thesis studied plasma confinement and heating in the Uragan-3M torsatron device. Experiments varied RF power levels and plasma densities. At RF powers over 100 kW and densities around 1.2×1012 cm-3, signs of improved confinement similar to the H-mode transition were observed, including increased density, temperature, and stored energy. Faster processes like an electric field bifurcation and turbulence suppression triggered the transition, while slower processes like increasing fast ion content and density decay preceded it. Detailed edge plasma turbulence measurements found fluctuations suppressed after the transition. The fast ion loss appears to trigger the transition by initiating the electric field bifurcation. Further study of fast ion generation mechanisms could provide more insight.

1. PhD thesis synopsis
H-mode-like confinement formation in the RF discharge plasma in the
Uragan-3M torsatron
PhD thesis submitted to the Department of Physics and Technology
of V. N. Karazin Kharkiv National University.
Manuscript by Beletskii Aleksey Aleksandrovich. January 2014
The dissertation thesis is devoted to studying plasma production, confinement
and heating in the Uragan-3M device in the Institute of Plasma Physics, National
Science Center Kharkov Institute of Physics and Technology (NSC KIPT), Kharkov,
Ukraine. The Uragan-3M (U-3M) is an  = 3, m = 9 torsatron, major radius R = 1 m,
average plasma radius a ≈ 0.12 m, rotational transform ( ) / 2 0.3aι π ≈ , toroidal
magnetic field is produced by the helical coils only, Вφ ≲ 1 Т. The whole magnetic
system is enclosed into a 5 m diameter and 3.5 m height vacuum chamber, so that
an open natural helical divertor configuration is realized. A hydrogen plasma with
the average density of en ≲ 2×1012
cm-3
is produced and heated by RF fields in the
Alfven range of frequencies (ω ≲ ωсi) with the use of an unshielded frame-like
antenna with a broad spectrum of parallel wavelengths. The fuelling gas (hydrogen)
is leaked into the chamber continuously at the pressure of ~ 10-5
Torr, the typical RF
pulse duration equals 40 ms.
With a sufficient heating power a spontaneous increase of the line-averaged
electron density en , electron cyclotron emission ECE from the central plasma and
diamagnetic stored energy Wdia during the RF discharge have been observed in the
U-3M torsatron. Similar changes take place in all tokamaks and stellarators during
improved plasma confinement formation, in particular, when transition to the H-
mode takes place. An ability to produce and control this kind of transition is of a
great importance for a future thermonuclear reactor, so naturally the aim arose to
investigate the possibility of L-H-mode-like transition (hereinafter – “transition”)
creation in our device. For this purpose, the next points were outlined:
– time evolution analysis of RF discharge parameters depending on plasma
production and heating conditions ( en , Wdia, ECE, charge exchange neutral atoms
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2. PhD thesis synopsis
flux (CXN) from the confinement volume; floating potential flV and ion
saturation current sI at the plasma edge; diverted plasma flows (DPF)
characteristics);
– exploration of the low-frequency electrostatic turbulence and its dynamics in the
edge plasma;
– searching for a relationship between the processes occurring during the transition
in the confinement volume, edge and diverted plasmas.
In this work a so-called radiative layer – the outermost layer of the confinement
volume adjacent inside to the last closed magnetic flux surface (LCFS) and a layer
of open magnetic field lines, a so-called scrape-off-layer (SOL), adjacent outside to
the LCFS, are referred to as the edge plasma.
The main experimental data were obtained using movable arrays of cylindrical
Langmuir probes (LP) in the edge plasma, non-movable arrays of plane Langmuir
probes (divertor probes, DP) and grid analyzers of charged particle energy with
retarding potential (retarding field analyzer, RFA) in the diverted plasma flows. To
generalize the results, measurements provided by other diagnostics (microwave
interferometry and radiometry, magnetic, optical and corpuscular diagnostics) were
involved as well.
In order to find the most favorable conditions when features of a confinement
regime change are sufficiently clear, behaviors of en and ECE were compared at
different levels of the RF power PRF fed to the antenna. It was found, that for the
heating power PRF > 100 kW, when the maximum attainable density in the active
phase of discharge becomes 12 -3
2 10 cmen ≈ × , a high level of ECE is kept over all
the RF pulse, thus evidencing the optimum density for the RF power absorption. In
this conditions, the electron temperature in the central region as estimated from ECE
amounts ~ 500÷700 eV. Regarding ions, two groups with different temperatures
were registered in U-3М: the lower temperature (tens eV) and higher temperature
(300÷600 eV) ones. The more energetic ion content (> 500 eV, fast ions, FI)
increases with PRF. These FI seriously affect plasma characteristics in U-3M. In
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3. PhD thesis synopsis
particular, the FI direct loss results in the DPF up-down asymmetry, discovered
earlier. As shown in this work FI loss play a crucial role in the transition in the U-
3M torsatron.
The most distinct indications of the transition were observed at PRF ≈ 130 kW,
at the value 12
1.2 10en ≈ × cm-3
attained after a slow density decrease, about 20 ms
after discharge beginning. These were a suspension of the en decay, speeding up of
both ECE and plasma energy content Wdia rise and an edge floating potential gradient
significant increase. Energy confinement time τЕ assessment yields 1.5 ÷ 2 times
increase up to 5 ms.
The CXN flux from the confined plasma with different perpendicular energies,
the FI component in the DPF, mean and fluctuating edge plasma parameters were
thoroughly studied in these discharges.
It was found that processes developing in the active phase of RF discharges with
PRF ≈ 130 kW could be divided in two groups by the time scale of their variation.
The density decay from 12 -3
2 10 cmen ≈ × to 12 -3
1.2 10 cmen ≈ × (~ 10 ms), the FI
content increase (units ms) attaining a maximum at 12 -3
1.2 10 cmen ≈ × , the plasma
energy content Wdia and ECE monotonous growth are related to the slower
processes. Taking account of the FI content increasing with power, optimum
conditions for FI generation are supposed to be realized at the combination ω/2π =
8.8 МHz, Вφ = 0.72 Т and 12 -3
1.2 10 cmen ≈ × , with increasing the RF power fraction
deposited in the plasma. On the other hand, the rise of this fraction results in a
stronger confinement degradation and density decrease. Against a background of
slower processes, faster processes (tens – hundreds μs) arise, which determine the
H-like mode transition in itself. The transition is triggered by a sudden single burst
of the FI loss (~500 μs). The burst initiates the edge radial electric field Er bifurcation
to a more negative value with the Еr shear amplified (~50 μs). A rough estimation
yields Er ≈ -20 V/сm before and Er ≈ -100 V/сm after the transition. The stronger
Еr shear suppresses the edge electrostatic turbulence and the turbulence-induced
anomalous transport turbΓ . The Еr bifurcation is supposedly triggered by the ion
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4. PhD thesis synopsis
orbit loss mechanism. Similar to other devices, in U-3M a characteristic form of the
edge potential profile has been already formed before the transition and, most likely,
results from the non-ambipolar ion orbit loss. With the transition, the edge Еr
becomes stronger, with the potential well shifting inside the LCFS. Triggered by a
single burst of FI loss, the H-like mode state persists for a comparatively long period
without recovering the pre-transitional higher level of the edge turbulence. It looks
as if the discharge went from one quasi-steady (L-like) state to another (H-like) state.
Highly important characteristics of the edge plasma turbulence, such as the
fluctuation amplitude, its spectral and statistical properties (wavenumber-frequency
spectra S(k,ω), skewness, kurtosis), turbulence-induced particle transport were
investigated in U-3M using high-resolution measurements of density ( sI ) and
potential ( flV ) fluctuations with Langmuir probe arrays. It was revealed that a
maximal power of density fluctuations corresponds to different spectral subranges
depending on spatial location of measurements, both in DPF and SOL. A supposition
was made that the lower- (higher-) frequency fluctuations were related to a trapped
ion (electron) loss. The lower-frequency subrange corresponds to ~ 5 ÷ 30 kHz, the
higher-frequency one – to ~ 30 ÷ 60 kHz roughly. Investigations of density
fluctuations with the use of probability distribution function analysis were carried
out too. In particular, a relatively large kurtosis (K) of sI observed in DPFs on the
ion B×∇B drift side is apparently concerned with significant amount of FI in these
DPFs. The universal character of the parabolic relation between K and S of sI in the
edge of magnetically confined plasmas was also confirmed for the data obtained in
U-3M with the following fit parameters: K=1.54S2
+ 2.68. The density ( sI ) blobs
and holes birth region was found to occur near the LCFS.
An analysis of the local estimation of the wavenumber-frequency spectra S(k,ω)
for the fV measured with two poloidally separated probes was carried out too. It has
been found that the statistical dispersion relation can be linearly approximated and a
local poloidal wavenumber averaged over spectrum in the vicinity of the LCFS (
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5. PhD thesis synopsis
1/r a ≤ ) lockθ ~ 1 cm-1
, while more distant from the LCFS ( / 1r a > ) lockθ ~ 3 cm-1
.
The direction of the mean poloidal phase velocity of the fluctuations derived from
S(k,ω) attained
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2 10 /ph cm сθυ ≈ ⋅ and appeared to be in the ion diamagnetic direction
for / 1r a > and in the electron diamagnetic direction for 1/r a ≤ .
An Еr shear strong impact on the edge electrostatic turbulence was observed. Just
after transition, phθυ direction changes from the electron to ion diamagnetic one
inside LCFS, probably because of a Doppler effect caused by poloidal ×E B plasma
rotation and its alteration due to Еr shear increase. sI power spectrum drops in all
the spectral range close to the LCFS (a higher Еr gradient), while more distant from
the LCFS the power spectrum does not change significantly (a lower Еr gradient).
S(k,ω) magnitude shows the same dynamics. The frequency-resolved radial
turbulent particle flux ( )turb fΓ estimation yields two spectral subranges depending
on the distance from the LCFS, similar to the sI spectrum. After the transition, turbΓ
is subdued in the layer of strong Еr gradient formation, near the LCFS. A
simultaneous sI and flV coherency and phase spectra significant change confirms
the conventional turbulence suppression mechanism. There is a distinct time
correlation between the level of edge turbulent transport (a surface process) and
plasma confinement (a volume characteristic defined in this work by the rate of en
decay). A short-time ( )turb tΓ increase just before the transition apparently relates to
the burst of FI loss that triggers the transition.
A phenomenon has been discovered that fluctuations statistics change from Levy
to Gaussian during the transition is of a particular interest.
Since FI loss is found to be the trigger of the transition, discovering of FI
generation mechanism is of a great importance and further investigations are needed.
A one possible explanation is that a turbulent heating of ions and electrons can take
place owing to parametric excitation of small-scale ion Bernstein waves in the U-
3M torsatron.
Thus, evidences of the transition to the H-mode were successfully revealed.
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