1. Edge plasma diagnostics in tokamaks (advanced electrical probes)
Jan Stöckel + CASTOR team
Institute of Plasma Physics, Association EURATOM/IPP.CR
Prague, Czech Republic
In close collaboration with
G. Van Oost, Ghent University
J. Gunn, M. Kocan, P. Devynck, CEA Cadarache, France
E. Martines, Consorzio RFX, Padova, Italy
R. Schrittwieser, C. Ionita, P. Balan, Innsbruck University, Austria
Decisive role of the edge plasma in global plasma confinement in tokamaks (what to be measured?)
Fluctuation and flow measurements by electric probes (how to measure?)
IAEA TCM RUSFD, Lisabon, October 24-26, 2007

2. CASTOR -Czech Academy of Sciences TORus
Built in Kurchatov Inst. Moscow 1958
Operational in IPP Prague since 1977
Reconstructed (new vessel)
Associated to EURATOM
Last shot Oct 2007
Will be replaced soon by COMPASS tokamak (UKAEA):
see posters R. Panek and M Hron
Major radius m
Minor radius m
Toroidal magnetic field <1.3 T
Plasma current <15 kA
Pulse length 50 ms
Plasma temperature keV
Plasma density ~ 1*1019 m-3
Confinement time < 1 ms
Edge density ~2*1018 m-3
Edge temperature eV
Research topics
Edge plasma physics
Plasma turbulence
Diagnostics development
LHCD
Physics at the plasma edge is almost independent on machine size

3. Edge of a tokamak (schematically) Last Closed Flux Surface
Scrape off layer (open magnetic field lines – connected to a material surface (divertor or limiter)
Plasma Core
x Btor
Last Closed Flux Surface
(separatrix)
Edge plasma - a „layer“
insulates the hot core from the cold wall
its importance for plasma confinement recognized in 80s – transport barriers
Heat and particles are transported from the core to wall through the edge plasma
Transport coefficients  and D must be as low as possible
However, the opposite is true -  and D are x larger than predictions because of plasma turbulence
Decisive role of plasma turbulence was recognized ~30 years ago, but its nature is not yet fully understood

4. What is the edge turbulence (result of fluid modelling of plasma edge in CASTOR)
WALL
Central part of
plasma column
is not modelled
LFS
HFS
20 cm
Flute-like structure(s) of density or potential), which follow the magnetic field lines and propagates poloidally
BRIGHT COLORS
Density is higher than the average
DARK COLORS
Density is lower than the average

5. What diagnostics are required?
It is evident that understanding of underlying physics in the edge plasma requires simultaneous measurement:
Plasma parameters like density, temperature, potential with a good spatial and temporal resolution (turbulence)
Plasma flows in poloidal (and toroidal) direction
A practical solution for small scale experiments => electric (Langmuir) probes
Alternatively in large experiments (more sophisticated diagnostics):
Fast cameras are used to visualize the density fluctuations ( ~100 k USD)
Beam emission spectroscopy
Microwave scattering ……

6. Ion saturation current
Single Langmuir probe
Ion saturation current
Floating potential
Iprobe =Iionsat {1 - exp [- e(Vfloat-Vprobe)/kTe]}

7. Design of Langmuir Probe Array
(radial profiles on a small-scale tokamak)
Rake probe on CASTOR
floating potential
16 small tips (diam.=0,7 mm, l=2 mm)
Distance of tips 2.5 mm
Total extension 35 mm
LCFS
LCFS
Limiter
Wall
Isat ~ density
All tips are biased simultaneously
Probe currents are measured
I-V characteristics are fitted
Radial profiles averaged over six
identical discharges
El. temperature
Radius [mm]

8. Fast-scanning probe on Tore Supra
Design of Langmuir Probes on Large Tokamaks
has to be quite different!!
Fast-scanning probe on Tore Supra
Probes
Probe construction must be sufficiently robust to survive extreme heat loads
Probe head must reciprocate during discharge.
Typically
200 ms - inward motion
200 ms - outward motion
fast radial motion
Graphite shield

9. Reciprocating probe head on TORE-SUPRA
(5 insertions of the probe head will be seen on movie)
Might not be enough!

10. Single Langmuir probe for fluctuation measurement
Time required to measure a single
I-V characteristic is typically >1 ms
Example of a simple circuit (just Isat or Vfloat) used on CASTOR for > 100 probes simultaneously
Typical power spectrum
Mean value of the signal
Level of fluctuations
Frequency spectrum

11. Double Langmuir probe for fluctuation measurement
Cross-correlation function is
calculated and
Mean propagation velocity of fluctuations
v = d/ t
can be determined from the delay t

12. But what about temperature fluctuations
But what about temperature fluctuations? A solution - Segmented Tunnel Probe on CASTOR
Advanced Langmuir probe measuring simultaneously ne, Te, T//,i Just the ion saturation currents from individual electrodes are measured Temporal resolution is determined only by data acquisition system. Low-cost and robust - only three DC signals of ion saturation current are measured No expensive electronics. Immediate access to fluctuating quantities
Diameter of the tunnel ~ several Larmor radii (CASTOR d=5 mm)
ne ~ I1 + I2 +Ib
Te ~ (I1 + I2) / Ib
T//,i ~ I1 / I2

13. Two examples of time resolved Measurements with STP
However, massive PIC simulations are needed in order to absolutely calibrate the probe for Te and T//,i measurements, for particular probe design, tokamak configuration and the range of plasma parameters to interpret measured signals
for J//,i = 1 ÷ 3 kAm-2
for J//,i = 3 ÷ 6 kAm-2
Electron
temperature
Ion
temperature
Jsat
(~ density)
Floating
potential

14. Space/time resolved measurements of plasma turbulence
Poloidal array of 124 probes
(for a better insight to the physics of the edge turbulence)
Array surrounds the whole poloidal circumference of the tokamak
Poloidal resolution  = 2.9 deg (3 mm)
Metallic support represents the poloidal limiter
64 fast data acquisition channels available (1 s sampling) B

15. Poloidal structure of edge turbulence

16. Cross-Correlation in the Poloidal Direction
Poloidal periodicity is more evident from cross-correlation analysis
Poloidal mode number can be easily determined
The reference probe
is located at the top
of the torus
Poloidal direction
Time lag [ms]

17. 2D Array of Langmuir Probes
2D matrix of 64 tips
Poloidal resolution ~ 6 mm
Radial resolution ~ 4.5 mm
As shown before by numerical modelling, the character of the edge turbulence is rather complex.
To know as much as possible about turbulent structures (characteristic dimensions, life time, wavelength, …)
2D arrays of the Langmuir probes should be used!

18. 2D Structure of Edge Turbulence on the CASTOR Tokamak
2D matrix of 64 Langmuir probes
limiter
separatrix
radial position [mm]
poloidally: 42 mm
A snapshot of potential structures

19. 2D Structure of Edge Turbulence as measured by the matrix of Langmuir probes
42 mm in the poloidal direction
POTENTIAL
HILL
POTENTIAL
VALLEY
22 mm in radial direction
Movies: 1000 frames by 1 s  Total duration = 1 ms

20. Measurement of ion flow velocity by Planar (Mach) Probe
Mach number in the direction parallel
to the magnetic field lines is calculated
from ion saturation currents measured
by the upstream and downstream collectors using a simple formula
vII
Isat(upstream)
Isat(downstream)

21. Alternative approach - Gundestrup Probe
Polar diagram of Ion saturation current
The Gundestrup
cauldron B
Top View
Bt, Ip
Parallel & perpendicular Mach numbes
are derived with a high temporal resolution
from the shape of the polar diagram
Several (eight) segments with a different orientation with respect to magnetic field lines
Used on ISTTOK as well

22. Tools for edge plasma diagnostic and fluctuation measurements on CASTOR (Summary)
Classical Langmuir probes – IV characteristics, local Te, ne, Ufl at the plasma edge, routine measurements
Radial & Poloidal arrays of Langmuir probes for spatially-temporally resolved measurements of plasma fluctuations
Oriented probes - Gundestrup and Mach probe for flow measurements during biasing experiments
Advanced probes – Segmented tunnel probe - a quite novel concept for fast Te and Ti measurements
Ball pen probe & Emissive probe – Direct measurement of plasma potential (not discussed here)

23. Conclusions
Edge plasma is an important region in tokamaks – confinement, transport barriers
Edge plasma diagnostics with a good spatial and temporal resolution are required to understand the underlying physics
Electric probes (arrays) are extremely useful tools for that purpose
Small tokamaks (flexibility, routine operation) are suitable for:
Testing of novel diagnostics
Investigation of relevant edge physics (in particular the plasma (turbulence)
Training
J Stöckel,et al,
Advanced probes for edge plasma diagnostics on the CASTOR tokamak,
Journal of Physics, Conference Series, 63 (2007),