Published by Fusion Energy Division, Oak Ridge National Laboratory
Building 5700 P.O. Box 2008 Oak Ridge, TN 37831 -6169, USA
Editor: James A. Rome Issue 114
E-Mail: jar@ornl.gov Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
April 2008
Wendelstein 7-X newsletter
available
The first issue of a newsletter on the Wendelstein 7-X
project has been launched by the Max-Planck-Institut fur
Plasmaphysik, Greifswald, Germany. The newsletter is
scheduled to be available on the Web about four times a
year:
German: http://www. ipp. mpg.de/ippcms/de/for/pub 1 ikationen/
w7xletters/
English: http://www.ipp.mpg.de/ippcms/eng/for/publikationen/
w7xletters/
Wendelstein 7-X
NEWSLETTER
No. 1 / April 2 0 0 8
First major m i l e s t o n e a c h i e v e d
The first two half modules of Wendelstein 7-X have
left their assembly stands and have been moved to
the next stand for joining them together to a whole
module. The next assembly steps will include the
installation of the superconducting bus and the cryopiping,
connecting the coils to power and liquid helium
supplies. In parallel the assembly of the second pair
of half-modules will be started.
Initially hampered by quality problems, in particular
with the electrical insulation of the super-conducting
coils, and requiring considerable additional
engineering efforts to improve the coil support
structure, the assembly proper of the device finally
started mid 2007 and since then proceeded without
delays.
Fig-1- The Wendelstein 7-X Newsletter as available
online.
Persons interested in subscribing to the newsletter may
contact Dr. Andreas Dinklage at
w7 xne w s letter @ ipp. mpg. de.
The content of the first issue is also given in the following
article.
In this issue...
Wendelstein 7-X newsletter available
The first issue of a newsletter on the Wendelstein 7-X
project has been launched by the Max-Planck-Institut
fur Plasmaphysik, Greifswald, Germany. The newsletter
is scheduled to be available on the web about four
times a year 1
Wendelstein 7-AS review published
The review "Major Results from the Stellarator Wendelstein
7-AS" has now been published in Plasma
Physics and Controlled Fusion. A summary is presented
here 3
Evidence of long-distance correlation of fluctuations
during edge transitions to improved confinement
regimes in the TJ-II stellarator
The discovery of long-range correlations in potential
fluctuations, which are amplified by the development
of radial electric fields during transitions to improved
confinement regimes in the TJ-II stellarator, is
reported. Results show a long-distance correlation
between floating potential signals that increases when
probes are approximately at the same radial location,
whereas there is no correlation between ion saturation
current signals. Cross-correlation shows a maximum
value when plasma density is close to the threshold
for the development of spontaneous edge sheared
flows with cross-phase close to zero. Furthermore,
correlation between potential signals increases in
plasma regimes with edge biasing induced enhanced
confinement. These experimental findings suggest the
importance of long-range correlations as a new fingerprint
of the plasma behavior during the development
of edge shear flows 5
All opinions expressed herein are those of the authors and should not be reproduced, quoted in publications, or
used as a reference without the author's consent.
Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy.
Wendelstein 7-X achieved major assembly
milestone
The first two half modules of Wendelstein 7-X have left
their assembly stands and have been moved to the next
stand for joining them together to create a whole module.
The next assembly steps will include the installation of the
superconducting bus and the cryo-piping that connect the
coils to the power and liquid helium supplies. In parallel
the assembly of the second pair of half-modules will be
started.
Initially construction was hampered by quality problems.
In particular, there were issues with the electrical insulation
of the superconducting coils, which required considerable
additional engineering efforts to improve the coil
support structure. The assembly proper of the device
finally started in mid-2007 and since then has proceeded
without delays.
Fig. 2. A completed half-module on its way to the next
assembly stand (photo: Beate Kemnitz, IPP).
Fig. 3. The two half-modules in assembly stand II (photo:
Antje Richter, IPP).
By now the assembly progress is no longer determined by
the delivery of the coils—all 70 superconducting coils
Fig. 4. The next nonplanar coil is waiting for assembly
(photo: Antje Richter, IPP).
W7-X Newsletter
Coordination: Prof. a. D. Dr. Robert Wolf
Contact: Dr. Andreas Dinklage
E-mail: andreas.dinklage@ipp.mpg.de
have been manufactured. Altogether, about half of the
coils have successfully passed cryo and high-voltage tests.
After a revision of the assembly schedule and a reorientation
of the start-up requirements of Wendelstein 7-X, the
completion of the assembly is now foreseen for 2014. The
main measures to achieve this completion date are intensified
shift work, the omission of 45 ports, and a start-up
configuration without actively cooled in-vessel components.
Although in the long run the omission of ports may
reduce the experimental flexibility, none of the scientific
objectives of Wendelstein 7-X will be affected.
During the first two years of plasma operation, without
active cooling, high power operation of up to 8-10 MW
will be limited to 5-10 s. The available heating systems
will be either 8 MW of electron cyclotron resonance heating,
already prepared for steady-state operation, or 7 MW
of neutral beam injection, resulting in a total of about 11
MW limited by the power supplies. At a 1 MW power
level, discharge durations of about 50 s will be possible.
First experiments will focus on preparing for the later
steady-state operation by verifying the divertor heat load
distribution, on demonstrating improved neoclassical confinement
in the low mean free path regime, and on first
attempts to develop an integrated high-density, improved
confinement regime.
This initial phase will be followed by a 18-month shutdown
for upgrading Wendelstein 7-X to its full steadystate
capability. This includes in particular the installation
of an actively cooled divertor for heat fluxes of up to 10
MW/m2.
Stellarator News -2- April 2008
Wendelstein 7-AS review
published
The review "Major Results from the Stellarator Wendelstein
7 -AS" has now been published online by Plasma
Physics and Controlled Fusion at http://www.iop.org/EJ/
journal/-page=featured/0741-3335. The printed version
will appear in the May issue.
Wendelstein 7-AS (W7-AS) was the first stellarator device
that used modular field coils instead of large helical windings.
The successful operation proved that three-dimensional
(3D) magnetic field configurations with flux
surfaces of high quality can be produced by modular coils.
Thus this concept provides the technical basis for building
optimized stellarators, which use modular coils. W7-AS
was the first step to test some of the basic elements of stellarator
optimization, and it showed that essential optimization
criteria can be met simultaneously, namely:
An improved equilibrium with reduced Shafranov shift
and improved stability properties resulted in P values up
to 3.4% (at 0.9 T). The magnitude of the observed Shafranov
shift as well as its dependence on the rotational transform
are consistent with predictions. The P limit was
determined by power balance and impurity radiation without
noticeable degradation of stability or a violent collapse.
Of important operational value are the slow time
scales of the plasma decay and the absence of currentdriven
instabilities close to the operational boundaries at
the density and P limits.
A partial reduction of neoclassical transport in the plateau
regime was achieved by an average elongation of the flux
surfaces and beyond this by the neoclassical impact of the
radial electric field. For high core temperatures, neoclassical
transport with its strongly rising temperature dependence
prevails. For these conditions, the experimental
results were in good agreement with calculations confirming
the reliability of neoclassical predictions and the feasibility
of the concept of drift optimization. A full
neoclassical optimization was beyond the scope of this
project. Because of this the bootstrap current was still
"tokamak-like" and no design effort has been made to
reduce it by further optimization, but the measurements
agree well with the expectations, giving confidence in the
desired minimization of the bootstrap current in W7-X.
The energy confinement was about a factor of 2 above
ISS95 without degradation near operational boundaries. A
number of improved confinement regimes, such as core
electron-root confinement with central Te < 7keV and
regimes with strongly sheared radial electric field at the
plasma edge resulting in Tx < 1.7keV , were obtained.
W7-AS is the first non-tokamak device to have achieved
the H-mode and moreover developed a high density Hmode
(HDH) regime with strongly reduced impurity confinement
that allowed quasi steady-state operation
(x « 65 x£ ) at 2.5 T and densities hQ = 4 x 102°nf3
(exceeding the Greenwald limit of an equivalent tokamak
by a factor of 2.5) under demanding conditions (p > 3%,
ZplxE = 2).
A variety of non-ohmic heating and current drive scenarios
by ICRH, NBI, and, in particular, ECRH were tested
and compared successfully with theoretical predictions. In
order to explore the stellarator-specific high-density path
to a reactor regime, new heating schemes for overdense
plasmas were developed, such as RF mode conversion
heating—Ordinary mode, Extraordinary mode, Bernsteinwave
(OXB) heating—or second harmonic O-mode (02)
heating.
For the first time in a helical system, a modular island
divertor was tested, with applicability to W7-X and conceptually
to a stellarator reactor. The divertor was operated
with stable partial detachment, in agreement with
numerical simulations.
W7-AS laid the physics background for operation of an
optimized low-shear steady-state stellarator, but its complementary
characteristics also have helped to better
understand the tokamak. This applies to the differences in
field structure, e.g., low global shear or strong toroidal viscosity;
to features of the H-mode, e.g., the existence of iota
windows or the role of the neoclassical electric field at the
plasma edge; to the spectrum of Alfven waves under
strong shaping; to the bootstrap current, which can be
much more easily measured without inductive background;
to the boundary-layer and divertor physics with
3D effects e.g., with local limiters or ergodic boundary
layers at the tokamak edge; and to the physics of operational
limits—those subject to MHD or power balance.
This list could be extended.
W7-AS ceased operation mid-2002 (Fig. 1). The continuation
of the experimental stellarator work of IPP is expected
by 2014. This gap reflects the step from a laboratory-based
experiment, W7-AS, to the superconducting steady-state
system W7-X, with fabrication and assembly complexity
and the challenge of high-performance operation requiring
a completely actively cooled first wall, divertor, and invessel
components. Fusion power plants must operate in
steady state; this demand is imposed by material aspects,
by their large power to the grid, and possibly by competitive
considerations with regard to the next generation of
fission reactors (Generation IV). The choices in fusion are
either 3D systems with increased mechanical complexity
but benign operational characteristics or 2D systems with
higher operational complexity. The performance of W7-
AS has shown that optimized stellarators are an option
with high potential.
Stellarator News -3- April 2008
Fig. 1. The W7-AS Team in the experiment hall at the final shutdown in July 2002.
Matthias Hirsch for the Wendelstein Team
Division W7-X Physics
Max-Planck-lnstitut fiir Plasmaphysik
D-17491 Greifswald, Germany
E-mail: Matthias.Hirsch@ipp.mpg.de
Stellarator News -4- April 2008
Evidence of long-distance correlation
of fluctuations during
edge transitions to improved
confinement regimes in the
TJ-II stellarator
First- and second-order phase transition models with ExB
sheared flow as a key ingredient have been invoked to
explain the transition to improved confinement regimes
[1-4]. Zonal flows have also been suggested as an important
ingredient to explain the Low to High (L-H) transition
in magnetic confinement devices [5, 6 and references
therein]. In the framework of the physics of second-order
phase transition, the correlation length of fluctuations and
the relaxation time (in the order parameter) are expected to
diverge at the transition point. Thus, the development of
edge plasma diagnostics to characterize the emergence of
sheared flows and to quantify the degree of long-range
correlation can provide relevant information on mechanisms
involved in the transition to improved confinement
regimes.
In the TJ-II stellarator, sheared flows can be easily driven
and damped at the plasma edge by changing the plasma
density [7, 8] or during biasing experiments [9]. The
experimental results on the emergence of the shear flow
layer in TJ-II have some of the characteristics of a transition
and are consistent with the expectations of secondorder
transition models of turbulence-driven sheared flows
[3]. In particular, measurements of the relaxation time of
externally induced electric fields show that an increase
above the threshold gradient triggers the development of
sheared flows, in agreement with the model [10]. The
recent development of two sets of Langmuir probes
located at two toroidal positions in the TJ-II stellarator
makes this device an excellent laboratory for investigating
long-distance correlations during the transition to
improved confinement regimes.
Experiments were carried out in the TJ-II stellarator in
electron cyclotron resonance heated plasmas ( ^ecrh -
400 kW, BT= I T , (R> = 1.5 m, (a) < 0.22 m, I(a)/2n «
1.5-1.9). The plasma density was modified in the range
(0.35-1) x 1019 m~3. Different edge plasma parameters
were simultaneously characterized at two toroidal positions
approximately 160° apart using two similar multi-
Langmuir probes, installed on fast reciprocating drives
(approximately 1 m/s) [11]. The arrangement of both
probes in TJ-II is illustrated in Fig. 1.
plasma.
One of the probes (Probe 1) is located in a top window
entering vertically through one of the "corners" of its
beam-shaped plasma and at (J) = 35° (where (|) is the toroidal
angle in the TJ-II reference system). Probe 2 is
installed in a bottom window at <\> = 195° and enters the
plasma through a region with a higher density of flux surfaces
(i.e., lower flux expansion) than Probe 1. Note that
the field line passing through one of the probes is approximately
150° poloidally displaced when reaching the toroidal
position of the other probe, which is more than 5 m
away. A graphite electrode (12 mm high, 25 mm diam)
was developed for biasing experiments on TJ-II and has
proved to be a valuable tool for controlling the edge
plasma electric field and consequently for placing the
plasma in an enhanced confinement regime. Edge radial
profiles of different plasma parameters have been measured
simultaneously at the two separate toroidal locations.
The development of sheared flows at the plasma edge of
the TJ-II requires a critical value of plasma density or density
gradient that depends on global plasma parameters
[12]. Fast imaging of the plasma edge in shots with different
values of density also revealed an effect of the shear
layer on turbulent structures, in good agreement with
probe results [13].
The perpendicular electric field fluctuations (i.e., the turbulent
radial velocity vr = EQ/B ) and the perpendicular
phase velocity measured at the plasma edge as a function
of plasma density are shown in Fig. 2. Measurements have
been obtained simultaneously with both probe systems,
located at approximately the same radial position (p = r/a
« 0.9), while changing density from shot to shot. The fluctuation
levels and the turbulent transport increase as density
increases to the critical value for which sheared flows
are developed. For densities above the threshold, and once
sheared flows are fully developed, the fluctuation level
and the turbulent transport slightly decreases and the edge
—_*>
R
Probe
Stellarator News -5- April 2008
gradients become steeper. Edge sheared flows are developed
at the same threshold density in the two toroidal
positions.
4500
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3500
2500
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19 Line-averaged density (x 1 0 m
Fig. 2. Averaged electric field fluctuations and perpendicular
velocity measured at two toroidal locations and at
approximately the same radial position (pla ~ 0.9) as a
function of plasma density.
The development of edge sheared flows has also been
induced in TJ-II using an electrode that externally imposes
a radial electric field at the plasma edge. The response of
the plasma to biasing is different at densities below and
above the threshold value needed to trigger the spontaneous
development of ExB sheared flows [9], but it is similar
at the two toroidal locations.
Floating potential signals measured at both toroidal locations
show a striking similarity mainly for low-frequency
components, contrary to that observed in the ion saturation
current signals. This similarity is observed at different
time scales, but it is more clear during fluctuation events
with time scales in the range of 0.1-1 ms related to the
shear flow development [10]. To quantify the similarity
between probe signals, the toroidal cross-correlation has
been computed for a wide range of TJ-II plasma conditions,
including a line-averaged density scan as well as
with and without electrode bias in plasmas without MHD
activity as shown by the pick-up coils installed in TJ-II.
The toroidal cross-correlation of the floating potential and
the ion saturation current signals measured at different
radial positions of Probe 1 while Probe 2 is fixed at p/a ~
0.95 is shown in Fig. 3 with and without biasing, for
ECRH plasmas and with similar line-averaged density (ne
« 0.65 x 1019 m~3), close to the critical value. The ion saturation
current toroidal correlation is very low in agreement
with previous measurements of the parallel
correlation in the scrape-off layer (SOL) region that have
shown an increase of correlation only when probes were
located at the same field line [14, 15]. However, the correlation
between floating potential signals is significant, particularly
during biasing when it increases while the ion
saturation current correlation is in the noise level range.
The maximum floating potential correlation is observed
when the probes are approximately at the same radial location.
The toroidal correlation shows a maximum in the
region just inside the LCFS, both with and without bias,
being negligible near the SOL.
0.6
0.4
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p=.84/.95, #16012
p=.82/.95, #16014
-60 -40 -20 0 20 40 60
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tGUI'
Fig. 3. Cross-correlation function for floating potential and
ion saturation current signals measured at different radial
positions of probe 1 (while probe 2 is fixed) with and without
biasing in ECRH plasmas with similar line-averaged
density (ne ~ 0.65 x 1019 m"3) in both regimes.
Figure 4 illustrates the dependence of the toroidal floating
potential correlation on the line-averaged density (for the
same shots presented in Fig. 2). It is observed that the
cross-correlation depends on the density, being larger as
density increases to nt« 0.6 x 1019 m~3, which corresponds
to the threshold density for shear flow development
for the selected plasma configuration (see Fig. 2).
Although experimentally the external control used is the
plasma line-averaged density, the local plasma parameters
(i.e., ion saturation current gradient) should be used as a
Stellarator News -6- April 2008
more suitable control parameter [3]. The increase of correlation
with density results mainly from the rise in the correlation
at low frequencies (below 20 kHz).
0.8
_ 0.6
4— >
0.4
x
E 0.2
00 .3 0.4 0.5 0.6 0.7 0.8 0.9
Line averaged density (101 9 m~3)
Fig. 4. Maximum value of the cross-correlation function
between floating potential signals measured at approximately
the same radial positions of both probes (pla ~ 0.9)
as a function of plasma density. Shadowed area indicates
the critical density region.
Figure 5 shows the time evolution of plasma density and
the cross-correlation between ion saturation current and
floating potential signals (measured by Probes 1 and 2). It
shows clearly the increase in the floating potential crosscorrelation
during the biasing phase, with a time delay for
the maximum cross-correlation in the range of 5-10 (is in
the different scenarios, as the radial electric field (and so
the sheared flows) develop first at Probe 2, rather than at
Probe 1. On the contrary, the degree of long-range correlation
is negligible in density fluctuations (as shown in
Fig. 3). Note that in the biasing experiments reported in
Fig. 5, the plasma density is below the critical value in the
phase without biasing (t < 100 ms), exceeding the critical
value during the biasing phase (100 < t < 150 ms). Once
the biasing is turned off, the density decreases on the time
scale of the particle confinement time (in the range of
10 ms), whereas both the electric field and the degree of
long-range correlation decrease in a much faster time
scale. These results show that the high degree of longrange
correlation observed in floating potential signals is
coupled to the value of radial electric field and not to the
plasma density.
In conclusion, TJ-II results show the importance of longdistance
correlation as a first step in the transition to
improved confinement regimes and the key role of electric
fields to amplify them. The present findings point out the
important role of edge diagnostic development to characterize
simultaneously at different plasma locations the
structure of sheared flows and fluctuations to unravel of
physics of sheared flows. Comparative studies stellarator -
tokamak (during L-H transition) should be stimulated to
provide a critical test for the L-H transition physics mechanisms.
Time (ms)
Fig. 5. Delay of cross-correlation function between (b) ion
saturation current and (c) floating potential signals measured
toroidally apart and at the plasma edge as a function
of time for one shot during biasing experiments. The overplotted
solid line in (c) represents the evolution of the time
delay where the correlation is maxima. Line averaged density
for the same shot is also shown (a).
M. A. Pedrosa, C. Hidalgo, D. Carralero and C. Silva*
EURATOM-CIEMAT (Spain) and * EURATOM-IST (Portugal)
E-mail: angeles.pedrosa@ciemat.es
References
[1] P. W. Terry, Rev. Mod. Phys. 72 (2000) 109.
[2] P. Diamond et al., Phy. Rev. Lett. 72 (1994) 2565.
[3] B. A. Carreras et al., Phys. Plasmas 13 (2006) 122509.
[4] P. Mantica et al., Phys. Rev. Lett. 96 (2006) 095002.
[5] P. N. Guzdar et al., Phys. Rev. Lett. 87 (2001) 015001.
[6] P. H. Diamond et al., Plasma Phys. Control. Fusion 47
(2005) R35.
[7] C. Hidalgo, M. A. Pedrosa, L. Garcia, and A. Ware,
Phys. Rev. E 70 (2004) 067402.
[8] M. A. Pedrosa et al., Plasma Phys. Control. Fusion 47
(2005) 777.
[9] C. Silva et al., Czech. J. Phys. 55 (2005) 1589.
Stellarator News -7- April 2008
[10] M. A. Pedrosa et al., Plasma Phys. Control. Fusion 49
(2007) B303.
[11] M. A. Pedrosa, et al., Rev. Sci. Instrum. 70 (1999) 415.
[12] M. A. Pedrosa, C. Hidalgo, et al., Czech. J. Phys. 55
(2005) 1579.
[13] J. A. Alonso et al., Plasma Phys. Control. Fusion 48
(2006) B465.
[14] Ch. P. Ritz et al., Rev. Sci. Instrum. 59 (1988) 1739.
[15] H. Thomsen et al., Phys. Plasmas 9 (2002) 1233.
Stellarator News -8- April 2008