Diagnostics. Electric probes. Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal

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1 Diagnostics Electric probes Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal

Langmuir probes Simplest diagnostic (1920) conductor immerse into the plasma Data interpretation complicated as probes perturb the plasma Limited to the plasma region were the probes can survive or

2 Langmuir probes Simplest diagnostic (1920) conductor immerse into the plasma Data interpretation complicated as probes perturb the plasma Limited to the plasma region were the probes can survive or do not perturb plasma Allows the determination of a large variety of plasma parameters (some of them only possible with probes) The importance of edge effects resulted in the continued use of probes The most widely used diagnostic techniques for low temperature plasmas, T e < 100 ev 2

3 Plasma Parameters (JET) Core T < 20 kev n ~ 1x10 20 m -3 Edge plasma T < 100 ev n < 1x10 19 m -3 Industrial / Space plasmas T < 10 ev n < 1x10 15 m -3 3

4 Debye Shielding Shielding effect: the free charges move towards a perturbing charge to produce, at a large enough distance D, (almost) a neutralization of the electric field. E D E~0 4

5 Debye Shielding The quantity is called the (electron) Debye length of the plasma The Debye length is a measure of the effective shielding length beyond which the electron motions are shielding charge density fluctuations in the plasma 5

6 Debye Shielding Typical values of the Debye Length under different conditions: Plasma Density n e (m -3 ) Electron temperature T(K) Magnetic field B(T) Debye length λ D (m) Solar core Tokamak Gas discharge Ionosphere Magnetosphe re Solar wind Interstellar medium Intergalactic medium

7 Debye shielding Physics of probes equivalent to that of plasma-wall interaction Electrostatic potentials are shielded within a short distance. Sheath keeps the plasma neutral Sheath Thin: λ D << d (probe dimension, ~mm) Collisionless: l (mean free path, cm - m) >> λ D Not to scale 7

8 Sheath As electrons are more mobile a electric field arises in the sheath so that Γ i = Γ e. Probe rapidly charges up negatively, floating potential. Probe floats below the plasma potential Sheath has a positive charge 8

9 Sheath analysis Space divided quasi-neutral plasma and the sheath (n i n e ) Sheath analyses: Simplest possible case (B = 0, Z = 1, T i = 0, collisionless, plane probe, 1D), all particles absorbed by the probe Aim: estimate parameters at sheath edge (se) Relation density and potential follows Boltzmann factor (Maxwellian) 9

10 Summary A plasma can coexist with a material boundary only if a thin sheath forms, isolating the plasma from the wall In the sheath there is a potential drop (~3 kt e ) which repels electrons from and accelerates ions toward the wall. The sheath drop adjusts itself so that the fluxes of ions and electrons leaving the plasma are equal, so that quasi-neutrality is maintained. There must be a small E in the plasma to accelerate ions to an energy ~ ½kT e toward the sheath edge: c s = (kte/m i ) 1/2 n se = 0.5 n 0, Γ i = 0.5 n 0 c s 10

11 Single probe 11

12 Single probe, I - V characteristic T e, V f and I sat derived from the characteristic (fit to experimental data) and then n estimated from I sat 12

13 I - V characteristic fit 13

I sat non-saturation Sheath expansion When the size of the probe is comparable with the sheath thickness the sheath expansion is important: collection area increases with applied voltage -

14 I sat non-saturation Sheath expansion When the size of the probe is comparable with the sheath thickness the sheath expansion is important: collection area increases with applied voltage - Child-Langmuir relation (V) 3/2 I sat does not saturate but linearly increases for larger negative voltages Must subtract the ion current (fit to the linear region) I =I sat [1-a(V-V f )-exp((v-v f )/T e )] 14

15 Sheath rectification in rf plasmas In rf plasmas, the space potential can fluctuate at the rf frequency is such a way that the circuitry responds incorrectly (I - V characteristic is nonlinear). The I - V curve becomes wider, leading to a falsely high value of T e and shifting the floating potential V f to a more negative value. 15

16 Typical circuit 16

17 Typical I, V signals 17

18 I - V characteristic 18

19 Fixed probes Graphite probes fixed in the plasma facing components (same material as PFCs) do not perturb plasma Study plasma-wall interaction Materials: Graphite, Tungsten Γ wall = I sat /ea p [m -2 s -1 ] q wall =γt e Γ wall [W/m 2 ] 19

20 ISTTOK probe arrays Poloidal array Radial array 20

Diagnostics. Electric probes. Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal http://www.ipfn.ist.utl.

Diagnostics. Electric probes. Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal http://www.ipfn.ist.utl. C. Silva Lisboa, Jan. 2014 IST Diagnostics Electric probes Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal http://www.ipfn.ist.utl.pt Langmuir probes Simplest diagnostic