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

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1 C. Silva Lisboa, Jan IST 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 Physics of probes equivalent to that of plasmawall interaction Electrostatic potentials are shielded within a short distance. Sheath keeps the plasma neutral Sheath dimension 10 λ D ~ 0.1 mm, thin layer (λ D ~ 10-5 m for T e = 20 ev, n = m -3 ). Thin: λ D << d (probe dimension, ~mm) Collisionless: l (mean free path, cm - m) >> λ D Not to scale 4

5 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 5

6 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) 6

7 Sheath analysis Energy conservation in the sheath Combining particle and energy conservation n i > n e sheath: Solve the 1D Poisson s equation 7

8 Bohm criterion y(x) is non-oscillatory if a > 0 V se kt e /2e v se 2 kt e /m i or v se c s with c s = (kt e /m i ) 1/2 This is called the Bohm sheath criterion (1949): Ions must stream into the sheath with c s for a sheath to form (depends on T e ). How can the ions get such a large velocity? 8

9 Pre-sheath There must be a small electric field in the plasma that accelerates ions to an energy ~ ½kT e toward the sheath edge. This region is called the presheath (E sheath kt e /λ D 10 5 E pre-sheath ). The pre-sheath field is weak enough that quasi-neutrality does not have to be violated. The point where (n i n e )/n i becomes significant, corresponding to the plasma sheath interface. The density at the sheath edge cannot be the same as the plasma density in the main plasma. Sheath edge: v se = c s, V se = - ½kT e, therefore n se = n 0 e - ½ 0.6n 0 Total pressure constant 9

10 Summary A plasma can coexist with a material boundary only if a thin sheath forms, isolating the plasma from the boundary. In the sheath there is a potential drop (few times 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 almost exactly equal, so that quasi-neutrality is maintained. 10

11 More general cases T i 0, weak dependence Probe geometry: Results generally applied provided the sheath is thin, probe considered locally planar used with probes of any shape Exact numerical solution with B = 0, T i 0 (Loframboise,1966): approximate analysis adequate, ~10%. Orbit-limited collection: λ D > d 11

12 Probe parameters Ion flux to a surface se = w, no dependence on the sheath potential drop Potential drop between plasma and a floating surface ( se e = sei ) ev f /kt e 3 for T e T i, D plasma V p = V f + 3Te 12

13 Single probe What if the surface is not floating, but electrically biased? 13

14 Single probe, I V characteristic B=0, Z=1, T i =0, Maxwellian distribution, no secondary emission, collisionless, no particle sources, d > λ D Sheath: V se = c s, n se =0.5 n 0 14

15 Single probe, I - V characteristic T e, V f and I sat derived from the characteristic and then n from I sat 15

16 Effect of the magnetic field Particles collected over larger distances Collisionless assumption may not be valid as, l L Typical case: ρ e < d and ρ i d ~ mm (magnetized / strong magnetized) Determination of the effective probe area can be complex Despite theoretical difficulties, Bohm formula is valid and information may be extracted assuming for the area A, limiting the analysis to V ap ~< V f 16

17 Double and triple probes Valid only if no significant gradients exist Double probe Triple probe 17

18 References Principles of Plasma Diagnostics, Hutchinson, Cambridge The Plasma Boundary of Magnetic Fusion Devices, Stangeby, IoP 18

19 Typical circuit 19

20 Typical I, V signals 20

21 I - V characteristic 21

22 Typical applications 22

23 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 ] 23

24 JET divertor probes Essential for the characterization of the divertor particle and heat fluxes Array of embedded probes made of the target material (CFC / W). The probes have heat conduction capabilities that are similar to that of the target plates, and they will erode at a similar rate 24

25 Divertor probes for ITER Successful operation of the divertor relies on achieving a detached divertor regime. Probes give the most direct indication of detachment I sat drops Main problem: For heavily loaded PFCs (10 MWm 3 ), the design of the Langmuir probe becomes as difficult as that of the target Heat conduction capabilities and a erosion rate that are similar to that of the target plates (replaced in parallel with the exchange of the divertor ). 25

26 Reciprocating probes Reduce the probe heat loads (few 100 ms) Pneumatic systems Typical velocity 1 m/s 26

M //, E θ, E r, Γ ExB ) Local measurements (only limited pin size) High temporal

27 JET reciprocating probe head 9 pin probe head (C, BN) Allows determination of a large variety of parameters (some only possible with probes) ( I sat, V f, T e, M //, E θ, E r, Γ ExB ) Local measurements (only limited pin size) High temporal resolution (limited by the data acquisition system) Ideal for turbulence studies 27

28 Reciprocation at JET 28

29 Reciprocation at JET 29

30 ISTTOK probe arrays Poloidal array Radial array 30

31 Gundestrup probe Determination of the poloidal e toroidal plasma rotation M // = 0.4 ln(i satu /I satd ) 31

32 Retarding field energy analyzer Simple Langmuir probes can tell us nothing about T i. (This require measuring the ion current at positive voltages at which the current to the Langmuir probe is dominated by highly mobile electrons.) RFA operation: The slit plate is biased negatively to repel electrons. The transmitted ions are retarded in the electric field created by a swept positive voltage applied to grid 1. The collector measures the ion current. 32

33 Space plasmas One of two Langmuir probes on board ESA's space vehicle Rosetta (intended to study the comet 67P/Churyumov-Gerasimenko). The probe is the spherical part, 50 mm in diameter and made from titanium with a surface coating of titanium nitride. This specific Langmuir probe is on a mission to study the space plasma around the comet. Probes also used in the Cassini mission to measure the inner magnetosphere of Saturn 33

34 Typical results 34

35 Examples of application Heat loads (IR, thermocouples), particle flux, detachment monitor, High temporal resolution, local measurement 35

36 ELM studies Filaments during ELMs ELMs have a complex internal structure (filaments) clearly seen in the far SOL density (also seen in other diagnostics as the fast visible camera) 36

37 SOL profiles at JET 37

38 Edge plasma studies with probes Determination of plasma parameters in different regimes Heat and particle loads Detailed ELM structure and propagation Characterization and control of turbulence 38

39 Turbulence in tokamaks Turbulence is responsible for and increase in the radial transport (anomalous transport) limiting the tokamaks performance SOL transport occurs via intermittent convective bursts (composed of long filaments along B) 39

40 Turbulent particle flux If local density and E fluctuations are in phase then a net time-averaged radial transport exists ~ E v ~ r ~ E B E B ne ~ ~ B 40

41 Fluctuations measurements Temperature fluctuations are often ignored and therefore density and plasma potential fluctuations derived respectively from the ion saturation current and floating potential fluctuations. I sat nt 1/2, V p = V f + 3T Results from emissive and ball pen probes as well as numerical codes indicate that the fluctuation level in V f is larger than in V p and therefore standard Langmuir probes overestimate the turbulent transport 41

42 Typical fluctuations analysis Fluctuations poloidal structure Poloidal correlation C xy x( t ) x y( t) y x( t) x 2 y( t) y 2 42

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. Diagnostics Electric probes Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico Lisbon, Portugal http://www.ipfn.ist.utl.pt Langmuir probes Simplest diagnostic (1920) conductor immerse into