probes for plasma potential measurements - rsphys -...

06/08/2007 IPELS 2007, Cairns, Australia, August 2007 1

Probes for direct determination of the

plasma potentialC. Ioniţă,1

P.C. Balan,1 C. Silva,2 H.F.C. Figueiredo,2J. Adámek,3 M. Tichý,4 E. Martines,5

A. Marek,4 C. Brandt,6 O. Grulke,6 R. Gstrein,1T. Windisch,6 T. Klinger,6 R. Schrittwieser,1J. Stöckel,3 G. Van Oost,7 C.A.F. Varandas2

1. Institute for Ion Physics and Applied Physics, Leopold-Franzens University of Innsbruck, Innsbruck, Austria

2. Association EURATOM/IST, Centro de Fusão Nuclear, Instituto Superior Técnico, Lisboa, Portugal

3. Association EURATOM/IPP.CR, Institute of Plasma Physics, Prague, Czech Republic

4. Charles University, Prague5. Associazione EURATOM/ENEA, Consorzio RFX, Padova, Italy6. Max Planck Institute for Plasmaphysics, Greifswald Branch,

Greifswald, Germany7. Department of Applied Physics, University of Gent, Gent,

Belgium


Why is it important to measure Φpl and how can we do it?Can we make Vfl equal to Φpl?Plasma potential measurements with cold probes. Conventional emissive probes.Laser-heated emissive probes.The ball-pen probe. Plug probe and baffled probe.A few results.Conclusion

Outline


The plasma potential controls the general particle transport in a plasma. Electric field fluctuations control the radial loss of plasma parti-cles across the magnetic field and are therefore essential for the confinement of the plasma.Theories and numerical simulations of the edge region deliver the electric field or the plasma potential as most important parameter. Therefore it would be essential to measure Φpl and its fluctua-tions as fast as possible and with a sufficient spatial resolution on as many positions as possible simultaneously.

Why is it important to measure the plasma potential Φpl(x,t), and thereby the electric field

E(x,t)?


How can the plasma potential be measured?

In contrast to the measurement of other plasma parameters the determination of the plasma potential Φpl is more difficult. Φpl can be measured by electron or ion beams, however, these methods are not only intricate but also expensive and do not have a good temporal or spatial resolution. The simplest, least expensive and most straightforward method with good temporal and spatial resolution is the plasma probe. However, although in principle we can derive Φpl from the current-voltage characteristic of a cold probe through the maxi-mum of the first derivative, this process either takes time and/or is also not free of errors in a non-Maxwellian plasma.


I-V characteristics of various probes in a magnetized plasma which in principle allow a

direct determination of Φpl

Ip

VpΦpl

Conventional cold probe characteristic

Emissive probe characteristic

Ball pen probe, plugged probe or baffled probe characteristic

For simplicity, Φpl is assumed to

be zero. Vfl

Why is the floating potential of a cold probe not simply the plasma potential?This is due to the strong imbalance of the masses of the negative and the positive charge carriers in a conventional plasma. Therefore electrons have a much higher mobility, which leads to a negative charging of all floating electrodes in touch with a plasma.

|dIp/dVp|


What can we do to make the floating potential of a probe identical with the plasma potential?

• Either we compensate the plasma electron saturation current by an equally strong current on the negative side of the characteristic; this is the principle of the emissive probe.

• Or we reduce the plasma electron saturation current, until its magnitude becomes equal to that one of the ion saturation current; this is the principle of the ball-pen, plug and baffled probe.

In principle there are two possibilities:


The simplest way − from ideal probe theory:

So we have to know the electron temperature.

But:The electron temperature can fluctuate during the measurement and there can be temperature gradients in the region of investigation (which is always the case in the edge region of a hot magnetized plasma).

In addition:In addition:A cold probe will deliver erroneous results for the plasma potential whenever there is a stronger deviation of the electron velocity distribution function from a Maxwellian, for instance when there is an electron drift or an electron beam or runaway electrons.

Plasma potential measurements with cold probes

eflis

eseflpl TVI

ITV Δ+=⎟⎟⎠

⎞⎜⎜⎝

⎛+=Φ ln


Basic equation for floating potential of a cold probe in a Maxwellian plasma

Total current of the probe:( ) ( ) ( )pepipp VIVIVI −=

⎟⎟⎠

⎞⎜⎜⎝

⎛ Φ−=

e

plpese T

VII exp

Electron and ion currents for : plpV Φ≤

(1)

(2)isi II =

Floating potential flpp VVI =⇒=0

eflis

eseflpl TVI

ITV Δ+=⎟⎟⎠

⎞⎜⎜⎝

⎛+=Φ ln (3)

from which we get the equation mentioned above:


In this case equations (3) becomes:

(4)

Normalized difference Δem between plasma potential and floating potential with and without electron emission:

isesemem

emis

es

e

emflplem

IIIII

IT

V

−==Δ→

⎟⎟⎠

⎞⎜⎜⎝

⎛+

=−Φ

=Δ

for0

ln, (5)

Floating potential of an emissive probe

⎟⎟⎠

⎞⎜⎜⎝

⎛≡Δ

is

es

IIln

For a magnetizedhydrogen plasma Δ = 2.5

eememflemis

eseemflpl TVII

ITV Δ+=⎟⎟⎠

⎞⎜⎜⎝

⎛+

+=Φ ,, ln

emflpl V ,=Φ→


Difference between unheated and heated

floating probe potential

(a) versus emission current, normalized to electron saturation current

(b) versus temperature of the probe wire, assuming tungsten as wire material

ne = 1017 m-3, Te = 10 eV, hydrogen plasma 2000 2200 2400 2600 2800 30000,0

0,5

1,0

1,5

2,0

2,5

(b)

Tw(K)

0,0 0,2 0,4 0,6 0,8 1,00,0

0,5

1,0

1,5

2,0

2,5

(a)

Δ em

-Δ

Δ em

-Δ

For jem/jes = 0,92Δem - Δ = 2,5, i.e. Vfl = Φpl

jem/jes

For Tw = 2900 KΔem - Δ = 2,5, i.e. Vfl = Φpl


Emissive probe construction of the IEPPG(Innsbruck Experimental Plasma Physics Group)

Inside the bores the tungsten wire is spliced with thin copper threads to increase the conductivity. This guarantees that only the probe loop is glowing when a current is passed through.

The probe holder is a ceramic tube (Al2O3) of oval cross section (1,4/2,3 mm outer diameters) and a length of 8 cm. The tube has two bores of 0,7 mm diameter each.


A typical small tokamak for fusion experimentsas e.g. ISTTOK (Instituto Superior Técnico Tokamak), Lisbon

or CASTOR (Czech Academy of Science Torus, Prague)

Major radius R ≅ 0,40 m, minor radius r ≅ 80 mm, magnetic field B = 0,5 – 1 T, toroidal current It ≅ 10 kA, hydrogen plasma

Typical core plasma electron density and electron temperature:

ne ≅ 1019 m-3

Te ≅ 180 eV

Typical edge plasma electron density and electron temperature:

ne ≅ 1018 m-3

Te ≅ 10 eV


Typical probe characteristics from the CASTOR tokamak, Prague, Czech Republic

Current-voltage characteristic with increasing wire heating

Floating potential with increasing wire heating


The VINETA Experiment, IPP Greifswald

Magnetic field B [mT] 100Plasma density [1019 m-3] ≤ 2Electron temperature Te [eV] 3Ion temperature Ti [eV] 0.2


Laser-heated probe in the VINETA machine I

A cylindrical piece of LaB6 or graphite irradiated from above by infrared light of 808 nm from a diode laser (JenLas HDL50F from JenOptik, Jena, Germany) with up 50 W.

• Graphite or LaB6 are heatable to much higher temperatures, therefore higher elec-tron emission and longer lifetime.

• No current flows through the probe, therefore no danger of deformation in a magnetic field.

• Lower capacity of the probe system, there-fore better time response.

• Surface of the probe is an equipotentialarea.

A laser-heated probe has several advan-tages as compared to a conventionalemissive wire probe:


Laser-heated probe in the VINETA machine II

Glass fibrecable

Lens head

Plasma column

VINETA vacuumvessel

Graphite probe pins Ceramic

tubes

Electric connection


Recent results of the laser-heated probe in the VINETA plasma

Current-voltage characteristic with increasing laser heating

Floating potential with increasing laser heating

-50 -40 -30 -20 -10 0 10 20 30 40 50-0,3

-0,2

-0,1

0,0

0,1

0,2

I p (A

)

Vp (V)

Laser Power 0 21,5 W 23,5 W 26,5 W 31 W 34 W 35->45 W

r = 0 mm = center of the plasma column

Φpl,em = 11,4 V

Φpl,cold = 13,7 V, determined

from the 1st derivative of the cold probe characteristic

p = 0,23 Pa, Ar, helicon discharge

0 10 20 30 40

-10

-5

0

5

10

15

V fl,e

m (V

)

Laser power (W)

Vfl,em

Φpl,em

Φpl,cold


The ball-pen probe

h

boron nitride tube

plasma sprayed corundum

stainless steel collector

Additional Langmuir probe

Additional Langmuir probe

( )( )

( )( ) plfliis

ees

iis

eeseflpl VhAj

hAjhAjhAjTV Φ=⇒=⎟⎟⎠

⎞⎜⎜⎝

⎛+=Φ 1;ln


Some measurements with the ball-pen probe in CASTOR

I-V characteristics for various collector positions; h is negative when the collector is inside the shielding tube;

Floating potential (blue squares) and ln(Ies/Iis) (black dots) with respect to h. The radial position of the probe head is at r = 75 mm inside the edge region of CASTOR.


XOOPIC simulations of the IV-characteristic of the ball-pen probe

With the depth h of the collector as parameter


Other probes based on this principle

1) The baffled probe:*)

Vfl Фpl

*) V. I. Demidov,a) S. M. Finnegan, M. E. Koepke, E. W. Reynolds, Rev. Sci. Instrum. 74 (2003), 4558-4560.


Other probes based on this principle

2) The plug probe:*)

*) S. V. Ratynskaia, V. I. Demidov, and K. Rypdal, Phys. Plasmas 9 (2002), 4135-4143.


By the way: Emissive probes were even used on space crafts!

From: R.H. Vernon, H.L. Daley, “Emissive probes for plasma potential measurements on the Sert II spacecraft”, J. Spacecraft and Rockets 7 (1970), 482-484.


• We have discussed various plasma probes, by which a direct determination of the plasma potential is possible.

• These probes are constructed in such a way that their floating potential yields an acceptable measure of the plasma potential.

• Therefore also the temporal and spatial resolution for the deter-mination of Φpl with these probes is relatively good; the latter in particular of the laser-heated emissive probe.

• A certain perturbation of the plasma by such a probe can unfortunately not be avoided.

• There is still an open question concerning the formation of a space charge around the probe even in the floating case.

Conclusion


Thank you very much for your attention!

Hechenbergas seen from

our office

Probes for direct determination of the plasma potential What can we do to make the floating potential of a probe identical with the plasma potential?The VINETA Experiment, IPP GreifswaldRecent results of the laser-heated probe �in the VINETA plasmaSome measurements with the ball-pen probe in CASTORConclusion

probes for plasma potential measurements - rsphys -...

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