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Published by Fusion and Materials for Nuclear Systems Division
Oak Ridge National Laboratory
Building CR 5600 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 140 April 2013
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
Plasmas of an arbitrary degree
of neutrality confined in a
stellarator
Non-neutral plasmas have been traditionally confined in
Penning-Malmberg and purely toroidal traps. However,
stellarators can also confine non-neutral plasmas, and the
confinement of pure electron plasmas has been extensively
studied using the Columbia Non-neutral Torus (CNT) stellarator
since 2004 [1]. Indeed, stellarators present some
advantages and flexibility for the study of non-neutral
plasmas. A stellarator can confine plasma even in the
absence of significant space charge (in contrast to purely
toroidal traps), can confine both signs of charge simultaneously
(unlike Penning traps), and does not require large
internal currents for confinement (in contrast to tokamaks).
Thus, plasmas of an arbitrary degree of neutralization
(ranging from pure electron to quasi-neutral) can be
confined in a stellarator. A recent paper [2] shows that it is
experimentally possible to create, sustain, and study plasmas
at any degree of neutralization in a stellarator. This
represents a first step in the characterization of plasmas of
arbitrary degree of neutrality.
Steady-state plasmas are created in CNT by thermionic
emission. In non-neutral plasmas, confinement is limited
by a rather low density limit [3, 4]. In CNT, a typical pure
electron plasma created using a filament biased at e =
200 V held on the magnetic axis is characterized by ne
3 1012 m3, and Te ranging between 2 eV and 7 eV. This
implies that many Debye lengths of pure-electron plasma
are confined in CNT.
The emitter filament creating the plasma and the probes
for diagnostics are mounted on alumina rods held inside
the plasma. These ceramic rods charge negatively with
respect to the plasma and act as steady-state sinks for ions
[5]. Thus, the degree of neutralization of the plasma in
CNT can be varied continuously between pure electron
and quasi-neutral by adjusting the neutral pressure in the
chamber, which determines the steady-state balance
between volumetric ionization of neutrals and recombination
on the rods.
The physics of the detected fluctuations at the two
extremes of the neutralization scale is completely different.
Quasi-neutral plasmas in CNT develop spontaneous
oscillations driven by density gradients (drift waves). The
observed global mode presents an approximately offset
linear relationship with E/B, and a small local phase shift
between density and potential oscillations. Measurements
with a set of capacitive probes and a high-speed camera
show that these oscillations are almost perfectly aligned
In this issue . . .
Plasmas of an arbitrary degree of neutrality
confined in a stellarator
Stellarators are capable of confining neutral and nonneutral
plasmas (of either sign). Steady-state plasmas
at the two extremes of neutralization were studied in
the Columbia Non-neutral Torus (CNT). Quasi-neutral
plasmas in CNT develop spontaneous oscillations
driven by density gradients. The physics of the unstable
fluctuations in electron-rich plasmas with a small
but non-negligible amount of ions is dominated by
electrostatics. .......................................................... 1
Wendelstein 7-X news
The long-pulse heat loads on the in-vessel components
are greater than those experienced by the leading
edges of the space shuttle during re-entry.
Because of this, the heat loads and their patterns will
be measured during the initial short-pulse operation,
and the results will be used to design and manufacture
the final configuration of heat protection. The current
state of W7-X construction is also described. .. 3
Next International Stellarator-Heliotron Workshop
The next International Stellarator-Heliotron Workshop
will be combined with the RFP workshop and held in
Padua, Italy, in September 2013. See the Web page
for more information:
http://www.igi.cnr.it/ish_rfp_ws2013/.................... 4
Stellarator News -2- April 2013
with the magnetic field lines and present a resonant m=3
poloidal structure.
At the other extreme of the neutralization scale, the physics
of the unstable fluctuations in electron-rich plasmas
with a small but non-negligible amount of ions is dominated
by electrostatics. The observed mode presents a nonresonant
m=1, n=0 spatial structure under exactly the same
magnetic configuration (as the quasi-neutral case), which
has an almost flat profile, 1/3. First observations of
these oscillations were reported by Marksteiner in Ref. [6].
The behavior of this mode is identical to the ion-driven
instability detected in Penning traps [7] and purely toroidal
traps [8]. However, such a spatial structure in a stellarator
implies that the instability breaks the parallel force balance
(even in the presence of low order rational magnetic surfaces),
and the poloidal magnetic flux is not conserved.
Figure 1 compares measurements of the poloidal structure
of the mode observed in CNT’s quasi-neutral plasmas (b)
with the mode detected in electron-rich non-neutral plasmas
(c). Partially neutralized plasmas lying between these
two extremes of the neutralization scale present broadband
behavior.
X. Sarasola and T. Sunn Pedersen
Max Planck Institute for Plasma Physics
EURATOM Association,
Wendelsteinstr. 1, 17491 Greifswald, Germany
Deptment of Applied Physics and Applied Mathematics, Columbia
University, New York, NY 10027, USA
References
[1] J. P. Kremer et al., Phys. Rev. Lett. 97, 095003 (2006).
[2] X. Sarasola, and T. S. Pedersen, Plasma Phys. and Controlled
Fusion 54, 124008 (2012).
[3] L. Brillouin, Phys. Rev. 67, 260–266 (1945).
[4] A. H. Boozer, Phys. Plasmas 12, 104502 (2005).
[5] J. W. Berkery et al., Phys. Plasmas 14, 084505 (2007).
[6] Q. R. Marksteiner et al., Phys. Rev. Lett. 100, 065002
(2008).
[7] R. C. Davidson, and H. S. Uhm, Phys. Fluids 21, 60–71
(1978).
[8] M. R. Stoneking et al., AIP Conf. Proc. 692, 310–319
(2003).
Fig. 1. (a) Layout of the array of capacitive probes around
the poloidal cross section of the plasma. (b) and (c) show
respectively the measured phase shift of the oscillations
detected in quasi-neutral and electron-rich plasmas using
these probes. TP #3 is used as the phase reference.
Stellarator News -3- April 2013
Wendelstein 7-X news
Continuous heat flows are a challenge for the in-vessel
components of the Wendelstein 7-X (W7-X) stellarator,
which is under construction in Greifswald, Germany. For
experiments with 30 min duration, as planned for W7-X, a
discharge corresponds to 200–2000 conventional pulses.
Therefore the relevant W7-X components must be continuously
water-cooled. Eighty ports (one third of the ports
on W7-X) are used to feed water pipes into the plasma
vessel. Inside the plasma vessel, 4 km of piping will be
installed. Accurate predictions for the expected thermal
load are critical for the design of these components, in particular
for the divertor. The 10 divertor modules are
designed for a continuous heat load of 10 MW/m2 occurring
in footprints of the plasma with a poloidal width of
about 10 cm (half width around 5cm) and 150 cm in toroidal
length. This load is about 20 times higher than usual in
heat exchangers used for conventional power plant technology.
It is also higher than the load placed on the edges
of the wings of the space shuttle when it re-enters the
atmosphere—6 MW/m2 for “only” several hundred seconds.
Figure 1 shows the geometry of these W7-X components.
Figures 2 and 3 are photos of some of the actual
components.
As a result of these cooling issues, Wendelstein 7-X will
go through two different operating phases. For the first
phase with short pulses of 5 to 10 s, a temporary test divertor
unit (TDU) with inertially cooled copper plates coated
with graphite tiles will be installed. This TDU allows precise
measurements of the local thermal loads for all important
operating scenarios. These measurements form the
basis for optimization of the water-cooled target modules
of the high-performance heat exchanger. After about 2
years of operation, the test divertor will be replaced by the
actual high-heat-flux (HHF) divertor, this shutdown being
scheduled to last another 2 years. The HHF divertor consists
of 890 elements with tiles made of 8 mm thick carbon-
fibre-reinforced carbon which are connected to watercooled
metal blocks by means of a patented process. Serial
production of these elements is going on and their installation
is scheduled for 2017.
Fig. 1. In-vessel components for one of five modules: top
and bottom divertor modules, baffle modules, heat shields,
and stainless steel wall panels, the latter for the lower
loaded areas of the wall. Protection structures for the ports
will be installed in the plasma vessel also but are not
shown for clarity. A complex system of cooling water supply
lines for these elements uses 80 of the ports. Additionally,
control coils and cryopumps and diagnostics are integrated
in the divertor as well as in and behind the first wall elements;
altogether, the 2500 in-vessel components comprise
about 710,000 parts. Graphic: IPP, Jean Boscary
Pipe work
Top divertor unit
Lower divertor plate
Vacuum vessel panels
Correction coil
Heat shields
Fig. 2. Heat shields at higher loaded parts of the wall (local
design heat load 500 kW/m2, average load 250 kW/m2)
during installation in the plasma vessel of W7-X. The
water-cooled CuCrZr heat sinks of the shown heat shield
segment are not yet covered by their graphite tiles.
Photo: IPP, Torsten Bräuer
Fig. 3. Wall protection prepared for a port to be used for
neutral particle heating. Photo: IPP, Robert Haas
Stellarator News -4- April 2013
Status of Wendelstein 7-X:
In-vessel components: Most of the in-vessel components
have already been delivered to Greifswald, and in-vessel
assembly including also diagnostic components and cable
routing has started. To attach the different components,
1200 bolts and brackets are being welded to the vessel
wall in each module using a positioning robot. First experience
with assembly of the in-vessel components themselves
shows that the detailed adaptation to the vessel
shape is rather time consuming.
Vacuum vessel: The assembly of the plasma vessel modules
has been completed, and the connection of the modules
with each other is proceeding as scheduled. The ports
of the modules are completely assembled and welded to
the vessels. For three of the five module separation planes,
the ports in the transition zone between two neighboring
modules are welded as well. The welding of the ports at
the fourth and fifth module separation planes will be finished
during the first and second quarter, respectively.
Trim coils: The four trim coils of type A manufactured in
cooperation with Princeton Plasma Physics Laboratories
and Everson Tesla have been delivered to IPP on schedule.
Manufacturing of the type B coil is still proceeding.
Current leads: The serial production of the current leads
at the Karlsruhe Institute for Technology has been completed
and the first pair has been installed on Wendelstein
7-X, as shown in Fig. 4.
from Wendelstein 7-X Newsletter
contact: Prof. Dr. Robert Wolf (robert.wolf@ipp.mpg.de)
Next Stellarator-Heliotron
Workshop
The next International Stellarator-Heliotron Workshop,
will be combined with the RFP workshop and held in
Padua, Italy, in September 2013. See the Web page for
more information:
http://www.igi.cnr.it/ish_rfp_ws2013/
The range of topics covered this year is even broader than
previously, and thus will accommodate talks on 3D configurations
generally (tokamaks, stellarators, RFPs, and
anything else toroidal with bumpy fields).
Although the deadline for invited papers has passed,
abstracts for contributed papers may be submitted through
the Web site between 30 April and 15 May.
Dr. Jeffrey Harris
Fusion Energy Division
Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831-6169
Telephone: +1 865 241 6546
Fax: +1 530 420 4650
Mobile: +1 865 274 8631
Skype: jhh112
E-mail: harrisjh@ornl.gov
Fig. 4. Current leads for the superconducting coils.
Photo: IPP, Beate Kemnitz