Determination of the plasma potential in the edge region of magnetised fusion plasmas by means of el

1
Association EURATOM-Austrian Academy of Sciences
P r o j e c t P 5 Innsbruck Experimental Plasma
Physics Group (I E P P G)Institute for Ion
Physics and Applied Physics University of
Innsbruck, Austria
Edge plasma turbulence and its diagnostics
2
List of collaborations during the period of
report
3
Outline

• General objectives of the subproject
• Most important new results

• IST (in collaboration with RISØ)
• MHEST
• IPP-Garching (in collaboration with ENEA-RFX,
  RISØ and ÖAW-P8)
• IPP.CR (in collaboration with IPP-Garching)
• EFDA-JET (in collaboration with RISØ)

• Conclusion
• Outlook

4
General objectives of the subproject

• Studying phenomena related to radial turbulent
  particle flux caused by electrostatic and/or
  magnetic fluctuations in the L-mode and during
  and in between ELMs.
• To gain new insight into the fine structure of
  ELM current filaments.
• Studying the underlying physics of these
  phenomena in various fusion devices.
• To obtain better insight into transport events
  and to contribute to an improved general model of
  all aspects of edge plasma turbulence in toroidal
  magnetic fusion experiments.
• To derive general conclusions from these results,
  which can be rescaled also to larger machines
  such as ITER.

5
Diagnostics tools

• Cold probes (with the restriction that the plasma
  potential cannot be determined directly)
• Emissive probes
• Ball-pen probes
• Magnetic probes (in connection with cold probes)

6
New results in ISTTOK, I
In ISTTOK, Lisbon, an arrangement of four
emissive and one cold probes was used
The probe head is inserted perpendicularly, thus
in poloidal direction, to measure the poloidal
electric field and its fluctuations. The
"emissive probes" could also be used as cold
probes by not heating them. The additional cold
probe allows to measure also the ion density.
7
New results in ISTTOK, II
Cross correlations ? between two poloidally
staggered probes, to compare heated and
non-heated probes in the same discharge one
pair heated to electron emission, one pair not
heated.
Position of the poloidal graphite limiter
Cross correlation between the potentials of the
cold Langmuir probes 2 and 5
Cross correlation between the floating potentials
of the emissive probes 1 and 4
Horizontal axis indicates the time lag between
the two signals. Inside the limiter (r lt 85 mm)
at around 65 mm the signals are highly correlated
(? ? 0,9). With increasing radius the correlation
decreases having a minimum at 74 mm (? ? 0,7).
Usually this is associated with a de-correlating
shear layer at the limiter position. Therefore
the "true" position of the limiter lies
approximately at 74 mm instead of 85 mm. Cold
probes show broader correlation functions but the
same qualitative behaviour as emissive probes.
Emissive probes show a better signal to noise
ratio than cold probes.
8
New results in ISTTOK, III
Radial profiles of the floating potentials of
cold and emissive probes and zonal flows
Schematic of a zonal flow in a tokamak
Floating potentials of cold and emissive probes
The radial profiles of the floating potential of
probe 4 in heated (red) and unheated mode (blue)
show slopes of different signs. This indicates an
outward and inward radial electric field
associated with a zonal flow. Again we see that
the "true" limiter position and therefore a shear
layer is situated around 74 mm. This behaviour
can be explained in a neo-classical picture by
finite Larmor radius (FLR) losses or by Reynolds
stress which drives flows without external
momentum. Again emissive probes show a higher
signal-to-noise-ratio than cold probes.
9
Investigations on emissive probes in the Linear
Magnetized Plasma Machine at MHEST, I
Influence of an emissive probe on a magnetized
plasma, measured by a Langmuir probe on the same
magnetic field line
The emissive probe causes strong pertur-bations
if it is biased negatively, i.e., when an
emission current flows into the plasma. We even
see indications for additional ionisation. However
, when it is floating, there is no perturbation,
be the emissive probe heated or not.
10
Investigations on emissive probes in the Linear
Plasma Machine at MHEST, II
This result is also confirmed by measurements of
the hydrogen spectra of the plasma
I.e. also in the very sensi-tive vibrational
Fulcher bands of molecular hydro-gen show now
change for a floating emissive probe.
This result we can take as an additional
justi-fication for the use of emissive probes in
magnetized fusion plas-mas for direct
unper-turbed measurements of the plasma
potential.
Observed vibrational Fulcher band spectra in
hydrogen plasma.
11
New results in ASDEX Upgrade, I
Probe head for simultaneous measurements of
electrostatic and magnetic fluctuations in the
scrape-off layer of AUG
Six Langmuir probes and one magnetic triple
sensor 20 mm behind the front side. Only one
triple magnetic sensor was used to measure
varia-tions of br, b? and b?
12
New results in ASDEX Upgrade, II
Front view of the probe head(as seen from the
plasma)
Probe 1 floating, radially protruding by 3 mm,
Vfl,1 Probe 2 floating, Vfl,2 Probe 3 floating,
Vfl,3 Probe 4 negatively biased, Iis Probe
5 swept, Te Probe 6 floating, Vfl,6
10 mm

This electric probe arrangement allows the
determina-tion of the poloidal and radial
electric field (d36 10 mm, d12 3 mm)
Probe head mounted on the midplane ma-nipulator
of AUG
13
New results in ASDEX Upgrade, III
This probe head is mounted on the midplane
manipulator and inserted up to three times during
an average AUG shot of 7 s length
Experimental arrangement
Distances between probe head and separatrix
Penetration of the probe into the scrape-off
layer (SOL)
Last closed flux surface during a typical H-mode
shot probe head is about 5 cm in front of the
LCFS. Same for a L-mode shot 8 cm
Radial position of the probe head with respect to
the rest position
14
Flux measurements during type-I ELMs, I
Sequence of type-I ELMs
Temporal evolution during the entire probe
insertion (a) of the instantaneous radial
turbu-lent flux (red) and its integral (blue)
showing typical type-I ELMs, (b) ion saturation
current to probe 4 during the same interval.

The slope of the blue line shows the strong
difference between the trans-port during ELM
intervals (strong particle loss) and inter-ELM
intervals (good confinement).
The two green arrows indicate the detail shown
below.
15
Flux measurements during type-I ELMs, II
Fine structure of one ELM
Detail of temporal evaluation during the interval
indicated by two green arrows above, i.e.,
transport during one ELM.
We see the fine structure of one type-I ELM Each
of the small step corre-sponds to a current
filament ii,iii. We observe also negative (
inward) transport, in particular after strong
positive transport events.

ii M. Endler, I. Garcìa-Cortès, C. Hidalgo,
G.F. Matthews, ASDEX Team, JET Team, Plasma Phys.
Control. Fusion 47, 219 (2005). iii A. Kirk,
N. Ben Ayed, G. Counsell, B. Dudson, T. Eich, A,
Herrmann, B. Koch, R. Martin, A. Meakins, S.
Saarelma, R. Scannell, S. Tallents, M. Walsh, H.R
Wilson, MAST Team, Plasma Phys. Contr. Fusion 48,
B433 (2006).
16
Flux measurements during type-I ELMs, III
H- to L-mode transition
Temporal evolu-tion of the turbu-lent flux during
a transition from the H-mode to the L-mode

The green dotted line indicate the approximate
border between the two modes
17
Magnetic measurements during type-I ELMs, I
Magnetic signals of shot 23161
Temporal evolution during almost the same time
window as above (a) Current in the SOL (black)
and D?-line intensity at the inner divertor
(orange). (b) Ion saturation current (same as
above). (c) Magnetic signals br and b? from the
radial and poloidal sensor coils, respectively.
The red and blue arrow in (a) show the time
intervals for the hodograms in the respective
colors in the next trans-parency.

18
Magnetic measurements during type-I ELMs, II
Magnetic signals of shot 23161
Hodograms of the poloidal and radial magnetic
signals, b? and br for the time intervals shown
by the arrows in the respective colors in the
figure above Red arrow during an ELM interval
(in the timeframe of Iis). Blue arrow in during
an inter-ELM interval.

The closed-loop hodograms deliver a direct
picture of cur-rent filaments during an ELM
19
Some of the newest results from May 2009, II
Shot 24470
L-H transition

20
Ball-pen probe for ASDEX Upgrade, I
Arrangement of ball-pen probes in ASDEX Upgrade
Ball-pen probes have retracted collectors which
enable them in a strong magnetic field to
col-lect electron and ion saturation currents of
about equal magni-tudes. Therefore their floating
potential is equal to the plasma potential.
21
Ball-pen probe for ASDEX Upgrade, II
Comparison between Te measurements by Thomson
scattering and difference between ?pl and Vfl
From ideal probe theory we have
By turning this around we get
22
Evaluation of magnetic measurements in JET, I
Magnetic coils in the inner wall of JET see
signals of a type-I ELM post-cursor, the palmtree
mode
23
Evaluation of magnetic measurements in JET, II
The palmtree mode, following a type-I ELM, only
seen in JET so far, close to a rational q 3
magnetic surface near the pedestal
Core MHD activity ELM Palmtree mode
A sign of a "hole" in the plasma propa-gating
inward after an ELM has caused turbulent
transport of plasma outward in form of a "blob"?
Shot 77188 in JET (18.02.2009, 2107) Plot coil
PP404
24
Evaluation of magnetic measurements in JET, III
Cross correlation between two toroidally
separated coils
The vertical blue lines indicate the regions for
which the cross correlation functions were
computed.
Core MHD activity ELM Palmtree mode
Shot 77188 in JET (18.02.2009,
2107) Plot Poloidal limiter coil PP404 Poloidal
limiter coil PP804 Toroidally separated by 180
25
Evaluation of magnetic measurements in JET, IV
Poloidal motion of palmtree modes
Signals from the two poloidal coil sets,
toroidally separated from each other by 180º
Core MHD activity ELM Palmtree mode
Shot 77188 in JET (18.02.2009,
2107) Plot Poloidal limiter coil array (PP401,
PP404, PP407) Poloidal limiter coil array (PP801,
PP804, PP807)
26
Evaluation of magnetic measurements in JET, V
Poloidal motion of palmtree modes
Contour plot of B for all coils on one limiter.
We see the poloidal motion of the mode from up to
down. PP801 shows the strongest signal, PP807 the
weakest.
Shot 77188 in JET (18.02.2009,
2107) Plot Poloidal limiter coil array (PP801 -
PP807)
27
Conclusion

• We have determined several essential
  characteristic para-meters concerning turbulence
  and radial particle trans-port in the edge region
  of various toroidal fusion experiments.
• We have performed these measurements with
  specialised probes for direct fluctuation
  measurements and the direct determination of more
  complex parameters.
• We have found new interesting results concerning
  the edge plasma as such and the turbulence
  therein, which also sup-port existing models of
  ELMs.

28
General outlook, I
The most important milestones for the next years

• Further investigations of fluctuations and
  related pheno-mena in the edge region of
  magnetized fusion plasmas, in particular during
  and in between ELMs.

in ASDEX Upgrade, JET, ISTTOK, COMPASS-D

• By special combinations of probes, measurements
  also of magnetic fluctuations, simultaneously and
  almost on the same position as the electric
  fluctuations, especially during ELMs

in particular in ASDEX Upgrade, JET and
COMPASS-D.
29
General outlook, II

• Comparison of the results with those of other
  diagnostics, with various ELM-models and with
  turbulence simula-tions.
• Thereby to gain new insight into radial transport
  pheno-mena and,

• in particular, into the transition from the L- to
  the H-mode and the temporal development and
  spatial propagation of ELMs.

30
Planned activities in more detail

• Localized, simultaneous measurements of the
  magnetic field, density, and potential
  fluctuations associated with ELM filaments.
• Magnetic field perturbations allow to obtain good
  estimates of the current structure inside the
  filaments.
• The structure of ELM filaments from natural
  type-I ELMs and from triggered ELM events should
  be compared.
• The current structure will potentially allow
  conclusions on the ELM instability mechanism
  (current vs. pressure driven).
• Same measurements for type III ELMs and blob
  filaments.
• Localised measurements provide restraints to
  modelling magnetic signals on wall probes.

31
Project relevant publications in the reporting
period

• R. Schrittwieser, C. Ionita, P. Balan, C. Silva,
  H. Figueiredo, C.A.F. Varandas, J. Juul
  Rasmussen, V Naulin, "Turbulence and transport
  measurements with cold and emissive probes in
  ISTTOK", Plasma Phys. Contr. Fusion 50 (2008)
  055004 (8pp) (This paper was identified by the
  referee as an article of particular interest.
  Moreover it is listed as "Featured Article" on
  the webpage of the IOP. Moreover, the paper was
  among the 25 most popular articles downloaded
  from the Plasma Physics and Controlled Fusion
  website in 2008. The article was downloaded 204
  times.)
• R. Schrittwieser, C. Ionita, P. Balan, R.
  Gstrein, O. Grulke, T. Windisch, C. Brandt, T.
  Klinger, R. Madani, G. Amarandei, "Laser-heated
  emissive plasma probe", Rev. Sci. Instrum. 79
  (2008), 083508 (9pp).
• G. Amarandei, D.G. Dimitriu, A.K. Sarma, P.C.
  Balan, T. Klinger, O. Grulke, C. Ionita, R.
  Schrittwieser, "Studies on Suitable Materials for
  a Laser-Heated Electron-Emissive Plasma Probe",
  Rom. J. Phys. 53 (2008), 311-316.
• J. Adámek, J. Stöckel, H. Horacek, V. Rohde, H.W.
  Müller, A. Herrmann, C. Ionita, F. Mehlmann, J.
  Brotankova, R. Schrittwieser, ASDEX Upgrade Team,
  "Direct measurements of the plasma potential in
  ELMy H-mode plasma with ball-pen probes on ASDEX
  Upgrade tokamak", J. Nucl. Mat. 390-391 (2009),
  1114-1117.
• C. Ionita, N. Vianello, H.W. Müller, F. Mehlmann,
  M. Zuin, V. Naulin, J.J. Rasmussen, V. Rohde, R.
  Cavazzana, C. Lupu, M. Maraschek, R.W.
  Schrittwieser, P.C. Balan, ASDEX Upgrade Team,
  "Simultaneous measurements of electrostatic and
  magnetic fluctuations in ASDEX Upgrade edge
  plasma", J. Plasma and Fusion Res. Ser., in
  print.
• M. Cercek, T. Gyergyek, B. Fonda, C. Ionita, R.
  Schrittwieser, "Electric and spectroscopic
  characterization of magnetized hydrogen and
  helium hot cathode discharge plasma", J. Plasma
  and Fusion Res. Ser., in print.
• R.W. Schrittwieser, R. Stärz, C. Ionita, R.
  Gstrein, T. Windisch, O. Grulke, T. Klinger, "A
  radially movable laser-heated emissive probe", J.
  Plasma and Fusion Res. Ser., in print.
• M.P. Gryaznevich, , P. Balan, (in total 69
  authors), "Results of joint experiments and other
  IAEA activities on research using small
  tokamaks", Nuclear Fusion, in print.
• H. Zohm, , R. Schrittwieser, (in total 181
  authors), "Overview of ASDEX Upgrade Results",
  Nuclear Fusion, in print.

32
Project-relevant conference contributions and
invited talks in the reporting period
18th International Conference on Plasma Surface
Interactions (Toledo, Spain, 26-30 May 2008)1
Poster contribution 35th EPS Conference on
Plasma Physics (Hersonissos, Crete, Greece, 9-13
June 2008)3 Poster contributions 23rd Symposium
on Plasma Physics and Technology (Prague, Czech
Republic, 1619 June 2008) 1 Invited
lecture 3rd International Workshop Summer
School on Plasma Physics (Kiten, Bulgaria, 30
June-5 July 2008) 1 Invited lecture, 1 oral
contribution International Interdisciplinary-Sympo
sium on Gaseous and Liquid Plasmas (ISGLP2008)
(Akiu/Sendai, Japan, 5-6 September 2008)1 Oral
contribution 13th EU-US Transport Task Force
Workshop (Kopenhagen, Denmark, 1-4 September
2008) 1 Poster contribution 14th International
Congress on Plasma Physics (ICPP2008) (Fukuoka,
Japan, 8-12 September 2008)1 Oral contribution,
2 Poster contributions 22nd IAEA Fusion Energy
Conference (Geneva, Switzerland, 13-18 October
2008)1 Invited lecture 8th Workshop on
Frontiers in Low Temperature Plasma Diagnostics
(Blansko, Czech Republic, 19-23 April 2009)1
Invited lecture, 1 Poster contribution 36th
European Physical Society Conference on Plasma
Physics (Sofia, Bulgaria, 29 June-3 July 2009)
1 Invited lecture, 5 Poster contributions EFTSOMP
2009 - Workshop on Electric Fields, Turbulence
and Self-Organisation in Magnetized Plasmas
(Sofia, Bulgaria, 67 July 2009) 1 Oral
contribution
33
What is the fluctuation-induced particle flux?
Fluctuations at the edge of a plasma column or
torus are inevitable and unfortunately they lead
to an enhanced particle transport in the outward
radial direction, i.e., to a loss of plasma. Also
ELMs do this but to an even higher extent. The
radial plasma flux ?r with the velocity vr,
induced by edge fluctuations, is mainly due to an
E?B drift in the radial direction. Since B is
mainly in toroidal direction and more or less
constant, a radial E?B drift can only be caused
by an electric field in the poloidal direction,
E?, respectively by its fluctuations . Thus
the formula for the fluctuation-induced radial
flux ?r reads
34
How can the fluctuation-induced flux be measured?
So for measuring ?r we need to know the
fluctuations of the plasma density and of the
poloidal component of the electric field. The
former can be mea-sured by a cold probe by
assuming that , which can be
determined from the ion saturation current Ii,sat
to the probe by using the formula
, with Ap
being the probe area. The fluctuations of E? can
be measured by two electron-emissive probes,
aligned in poloidal direction at a distance d?
from each other. These measure ?pl,1 and ?pl,2,
respectively. Thus the final formula, to be
evaluated for the calculation of the
fluctuation-induced radial particle flux ?r,
reads
35
What is Reynolds stress? (I)
The Reynolds stress is a measure of the
anisotropy of the turbulent velocity
fluctuations, which produce a stress on the mean
flow. This may cause an acceleration of the flow
in the plasma, which e.g. could be a poloidal
flow. There are reasons to assume that sheared
poloidal flows can be generated in the presence
of a radially varying Reynolds stress in fusion
plasmas. This is usually considered beneficial
since it tends to quench the radial flow since a
sheared poloidal flow can tear apart larger
eddies at the edge that would cause a strong
radial particle transport. The mean poloidal flow
in a cylindrical or toroidal geometry is given
by
36
What is Reynolds stress? (II)
In the case of purely electrostatic fluctuations,
i.e., neglecting magnetic field fluctuations, the
Reynolds stress Re is simply given by
Also here we have assumed that the two velocity
components are given by the respective E?B-drift
velocities, and the electric field components are
determined by the difference of the plasma
potentials of two electron-emissive probes
aligned in the appropriate directions. In
principle, also magnetic field fluctuations
contribute to the Reynolds stress, but we did not
yet to take them into account, and had also no
chance to measure them. But in future experiments
like ASDEX Upgrade and larger we have to take
them in account and will also be able to measure
them by special probes.