Edge plasma diagnostics in tokamaks(advanced electrical probes)

Jan Stöckel + CASTOR team Institute of Plasma Physics, Association EURATOM/IPP.CR

Prague, Czech Republic

IAEA TCM RUSFD, Lisabon, October 24-26, 2007

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?)

Built in Kurchatov Inst. Moscow 1958Operational in IPP Prague since 1977Reconstructed (new vessel) 1985Associated to EURATOM 1999Last shot 26 Oct 2007

CASTOR -Czech Academy of Sciences TORus

Major radius 0.4 m

Minor radius 0.1 m

Toroidal magnetic field <1.3 T

Plasma current <15 kA

Pulse length 50 ms

Plasma temperature 0.20 keV

Plasma density ~ 1*1019 m-3

Confinement time < 1 ms

Edge density ~2*1018 m-3

Edge temperature 10-40 eVPhysics at the plasma edge is almost independent on machine size

Research topics• Edge plasma physics• Plasma turbulence• Diagnostics development• LHCD

Will be replaced soon by COMPASS tokamak (UKAEA):

see posters R. Panek and M Hron

Edge of a tokamak (schematically)

Scrape off layer (open magnetic field lines – connected to a material surface (divertor or limiter)

Edge plasma - a „layer“ insulates the hot core from the cold wallits importance for plasma confinement recognized in 80s – transport barriers

Last Closed Flux Surface(separatrix)

Plasma Corex Btor

• 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 100-1000x 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

What is the edge turbulence (result of fluid modelling of plasma edge in CASTOR)

BRIGHT COLORSDensity is higher than the averageDARK COLORSDensity is lower than the average

WALL

Central part ofplasma columnis not modelled

20 cm

HFS LFS

Flute-like structure(s) of density or potential), which follow the magnetic field lines and propagates poloidally

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• ……

What diagnostics are required?

Iprobe =Iionsat {1 - exp [- e(Vfloat-Vprobe)/kTe]}

Single Langmuir probe Floating potentialIon saturation current

• 16 small tips (diam.=0,7 mm, l=2 mm) • Distance of tips 2.5 mm• Total extension 35 mm

Design of Langmuir Probe Array (radial profiles on a small-scale tokamak)

LCFS

Wall

Limiter

LCFS

•All tips are biased simultaneously•Probe currents are measured•I-V characteristics are fitted

Radial profiles averaged over sixidentical discharges

Radius [mm]

Rake probe on CASTOR floating potential

Isat ~ density

El. temperature

Fast-scanning probe on Tore Supra

Design of Langmuir Probes on Large Tokamaks has to be quite different!!

Probe construction must be sufficiently robust to survive extreme heat loads

Probe head must reciprocate during discharge. Typically200 ms - inward motion200 ms - outward motion

fast

radi

al m

otio

n

Probes

Graphite shield

Reciprocating probe head on TORE-SUPRA(5 insertions of the probe head will be seen on movie)

Mig

ht n

ot b

e eno

ug

h!

Single Langmuir probe for fluctuation measurement

• Mean value of the signal• Level of fluctuations • Frequency spectrum

)()( txxtx

x

Typical power spectrum

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

Double Langmuir probe for fluctuation measurement

Cross-correlation function is calculated and

Mean propagation velocity of fluctuations

v = d/ can be determined from the delay

d

B

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 measuredTemporal resolution is determined only by data acquisition system. Low-cost and robust - only three DC signals of ion saturation current are measuredNo expensive electronics. Immediate access to fluctuating quantities

ne ~ I1 + I2 +Ib

Te ~ (I1 + I2) / Ib

T//,i ~ I1 / I2

Diameter of the tunnel ~ several Larmor radii (CASTOR d=5 mm)

Two examples of time resolved Measurements with STP

for J//,i = 1 ÷ 3 kAm-2

for J//,i = 3 ÷ 6 kAm-2

c1i//,

2e

42i 51.37452.7210539.210488.1 RJTT

c1i//,

2e

2i 488.9110107.2347.010081.3 RJTT

Electron temperature

Ion temperature

Floatingpotential

Jsat(~ density)

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

• 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)

Space/time resolved measurements of plasma turbulencePoloidal array of 124 probes

(for a better insight to the physics of the edge turbulence)

B

Poloidal structure of edge turbulence

Cross-Correlation in the Poloidal Direction

Polo

idal d

irecti

on

Time lag [ms]

Poloidal periodicity is more evident from cross-correlation analysis

Poloidal mode number can be easily determined

The reference probeis located at the top of the torus

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!

2D Array of Langmuir Probes

2D matrix of 64 tips

Poloidal resolution 6 mmRadial resolution 4.5 mm

2D Structure of Edge Turbulence on the CASTOR Tokamak

limiter

separatrix

A snapshot of potential structures

poloidally: 42 mmra

dia

l p

osi

tion [

mm

]

2D matrix of 64 Langmuir probes

2D Structure of Edge Turbulenceas measured by the matrix of Langmuir probes

Movies: 1000 frames by 1 s Total duration = 1 ms

42 mm in the poloidal direction

22 m

m in

rad

ial

dir

ecti

on

POTENTIALVALLEY

POTENTIALHILL

Measurement of ion flow velocityby Planar (Mach) Probe

B

Isat(upstream) Isat(downstream)

downs

upsII /II2.3lnM

s

II

c

v

Mach number in the direction parallelto the magnetic field lines is calculatedfrom ion saturation currents measuredby the upstream and downstream collectors using a simple formula

vI

I

Alternative approach - Gundestrup Probe

B,I

1

2

34 5

6

78

Bt, Ip

Top View

B

Polar diagram of Ion saturation current

Several (eight) segments with a different orientation with respect to magnetic field lines

The Gundestrup cauldron

Parallel & perpendicular Mach numbesare derived with a high temporal resolutionfrom the shape of the polar diagram

Used on ISTTOK as well

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)

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), 012001http://www.iop.org/EJ/journal/conf