Introduction to the Diagnosis of Magnetically Confined Thermonuclear Plasma

Transcription

1 Introduction to the Diagnosis of Magnetically Confined Thermonuclear Plasma EDGE-SOL II: Plasma Wall Interactions J. Arturo Alonso Laboratorio Nacional de Fusión EURATOM-CIEMAT E6 P2.10 version 0.1 (February 28, 2011) EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

2 Outline 1 Basic Plasma-Wall Interaction General concepts and issues in PWI Material Erosion 2 Divertor physics Divertor regimes 3 Divertor and Wall diagnostics Infrared thermography Divertor bolometry Quartz Microbalance and post-morten tile analysis EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

3 Introduction to PWI As we saw in the introduction, in a steady state plasma, P α + P h = P rad + P κ P κ is transported accross the LCFS into SOL. Once there, the particles and the energy they carry quickly flow towards the divertor plates (or limiter surface) while more slowly diffusing perpendicular to the field lines towards the main wall. Recall that the characteristic SOL thickness is λ n 1cm. It turns out that the typical thickness of the energy flux q TΓ is even narrower meaning that the heat fluxes at the divertor plates can be as high as 10MW/m 2 in present day large tokamaks. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

4 Introduction to PWI In its journey towards the material surface, plasma can still give part of its energy neutrals and impurities (exciting impurities and CXing with neutrals) which then re-distribute it more homogeneously. However, impurities in the core plasma have deleterious effects on the fusion reactions (particularly high Z ones) as they disipate energy through radiation and dilute the reactants. Impurities have this two sides, it is good to have them radiating in the SOL to reduce heat loads to the surfaces but they should be kept out of the main plasma The main topics of current research in PWI can be grouped in Material erosion, Tritium retention and Material migration and mixing. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

5 Issues in PWI Material erosion Caused by energetic plasma particles and charge-exchange neutrals inpinging on the material surface (physical sputtering) and chemical reactions between plasma and wall species that unbind atoms from the surface of the wall (chemical sputtering). The polution of the plasma by sputtered impurities dilutes the fusion fuel and increases the radiated power that can lead to a radiation collapse and a disruption. Extreme transient power fluxes to the walls (like those caused by a plasma disruption or edge localised mode ELM) can heat the materials over their melting point with very deleterious consequences. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

6 Issues in PWI Tritium retention The radioactive hydrogen isotope inventory in a D-T fusion machine has to be kept under a strict control for safety. Tritium can react with some species like Carbon and form stable molecules in the surfaces that are hard to remove. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

7 Issues in PWI Material migration and mixing Eroded material can be redeposited and eroded again so that it is transported to a distant wall region. Different wall materials can then react to form alloys with poorer thermal and mechanical properties. Be-W alloys can reduce the melting point by over a 1000 deg BeO can have a tritium retention comparable to C etc These and others PW issues are or particular relevance for next step fusion devices like ITER (long pulse durations, larger energy contents) Wall erosion, for instance, will experience an increase of from a few µm in today s large devices to the cm scale. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

8 Choice of materials for the ITER wall Be wall for its low T retention and low radiating power (low atomic number Z). The divertor targes (around strike points) are made of Carbon for its good thermal properties, it does not melt and can provide some intrinsic radiation to the divertor. However, C can be a problem for it forms hydrocarbons and T retention. The other parts of the divertor, made of Tungsten are chosen for its high physical sputtering threshold. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

9 Plasma recycling and wall conditioning Charged particles leaving the plasma are neutralised at the wall and returned to the plasma chamber. This re-fueling of the plasma by the wall is knonw as recycling. Recycling affects the fueling of the plasma and therefore its density control. Walls are baked (so that neutrals are themally desorbed and pumped away) and conditioned (coating the walls with materials able to get and retain hydrogen like B or Li so that wall saturation is postponed). EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

10 Physical Sputtering: the effect of the Sheath The mean energy of an ion entering the sheath can be approximated assuming the ion velocity ditribution is a Maxwellian with T i shifted by c s = [(T i + T e )/m i ] 1/2. This gives E s = 5 2 T i mc2 s = 7 2 T, (for T i = T e ). More realistic ion distributions give somewhat lower sheath entrance energies E s 2T (see [2, chapter 2]). From the sheath entance to the surface, ions adquire an extra 3T e energy from the potential drop, therfore E 0 2T i + 3T e 5T, which shows that, for an aproximatelly isothermal plasma, the sheath increases the ion impact energy significantly. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

11 Physical Sputtering Yield Physical sputtering has a strong dependence on the energy of the projectile ion (or neutral form CX). (Figure taken from [2, page 119]) There exists a threshold energy which is larger for larger lattice binding energy and larger substrate atomic mass. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

12 Chemical suttering More complex phenomenon involving the reactivity and kinetics of many chemical processes. It is nearly independent of the incident energy but depends on the flux of ions to the material surface [1] EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

13 Divertor advantages in PWI plasma core LCFS strike point upstream flux expansion target orientation divertor target Diverted plasmas lend themselves to a better control of their interaction with the wall. flux expansion reduces the heat flux entering the divertor by increasing the flux tube cross-section An appropriate orientation of the divertor plates allow to reduce geometrically the power densities to the wall increasing the wetted area EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

14 Divertor advantages in PWI The longer paralel conections lengths (L div 40m, L lim 10m) and the physical separation of the plasma and divertor volumes allow to have larger neutral and impurity densities in the divertor and parallel temperature gradients. lower divertor temperatures reduce the energy of the surface-striking ions and therefore the physical sputtering yield, higher neutral and impurity densities can absorb part of the plasma energy and distribute it more uniformly over the wall in form of radiation. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

15 Parallel temperature gradients in the SOL Let us briefly discuss the conditions for the sustainment of parallel temperature gradients in the SOL. In general heat transport from the upstream region to the divertor plates can be conductive and convective q = q cond + q conv = κ dt/dx + v T Strong parallel convection tends to flaten the temperature along the field line. Needs a conduction dominated heat transport in the SOL to have significant T. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

16 Source location to reduce convective energy transport A way to reduce flows over most of the extension of the SOL is to localise the particle source near the target. This can be seen from ion continuity equation dnv dx = S i(x), where S i is the particle source. Integrating the above equation from the sheat entrance (where v(x S ) = c s ) to a distance x x S and assuming roughly constant density v(x) = c s + 1 n x x S S i (x)dx. The localisation of S i (x) determines how close to the sheath the ions are accelerated to c s : the more distant the source is the longer the fracion of the SOL where ions are at nearly sonic speed and advection is important. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

17 Source location to reduce convective energy transport Close-to-target ionization is easier to achieve in diverted plasmas. Ionization in limiter plasmas tend to occur inside the LCFS so the ion source in the SOL is not localised near the limiter. In the remainder of this section we ll assume a conduction dominated SOL. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

18 The two-point model of the SOL 1D flux tube, next-to-target ionization, conduction dominated [2, sec 5.2] used to relate upstream u and target t conditions 1 Pressure balance: p + nmv 2 = constant, with v t = (2T t /m i ) 1/2 2n t T t = n u T u 2 Power balance: κ = κ 0 T 5/2 κ e 0 T5/2 (Spitzer) Tu 7/2 = T 7/2 t + 7 q L ; q 2 κ = q t = γn t T t c st 0e Considering q and n u the control params (controlled in XPs by P in and n e ), it follows that [see notes] T t q 10/7 /n 2 ul 4/7, n t n 3 u/q 8/7, Γ t n 2 u/q 3/7 L 4/7, (φ recycle Γ t ) EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

19 Sheath-limited and Conduction-limited regimes Sheath-limited divertor convection dominated SOL or conduction dominated with low n u and/or short L T 0 All the power entering the SOL reaches the solid surfaces. The power deposition is highly localised close to the divertor strike points. Conduction-limited (or High recycling) divertor increased n u and φ recycle n 2 u Neutrals ionised in the SOL plasma close to the target removing part of the energy (radiation and CX) T t drops and with it the energy of the ions striking on the surface pressure still remains constant along the field line (T t n t ) EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

20 Sheath-limited and Conduction-limited regimes EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

21 Divertor Detachement With further increase of the plasma density the amount of charged particles that reach the divertor plates falls to negligible levels. As the density is increased more impurities are released by plasma facing components that raise the radiation levels As the temperature in the divertor decreases over a large volume, electrons and ions can recombine to form neutrals volumetrically Neutral friction becomes important (p constant) slowing down the plasma and increasing the odds of particles recombining before the target. Partial detachment is the divertor regime in which the succesful operation of next spet fusion devices relies and is a topical subject of current research. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

22 Divertor regimes: a summary EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

23 Divertor and Wall diagnostics From what we learnt reviewing the basic physics in the divertor and SOL there are obvious things that are interesting to measure like: heat fluxes to the wall (see IR Thermography next) material erosion and migration (see QMB and post-morten analysis next) impurity radiation (see K. McCarthy s lectures on Spectroscopy) radiation levels particularly in the divertor region (see Divertor Bolometry next) upstream/target dentities and temperatures (i.e. with Divertor/wall mounted probes see previous) ect EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

24 Infrared Thermography The IR radiation from the wall can be related to the surface temperature assuming black (or gray) body radiation. (T range 200deg to 2500deg). EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

25 Infrared (IR) Thermography The wall temperature is monitored in current devices by Infrared (IR) cameras. Similar to visible CMOS cameras but semiconductor has a narrower gap to be sensitive to µm wavelength radiation often made of Indium compounds (InSb or InAs). Because of the narrow gap, electron noise at room temperature is untolerable and the detector array need to be cooled down to 70K. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

26 IR Thermography: Heat flux extraction It is posible to inferr the heat flux to the wall by tracking the changes on the surface temperature from the 1D heat equation in the solid T t = κ c p ρ ( 2 ) T x 2 where κ(t), c p (T) and ρ are the thermal conductivity, specific heat capacity and density of the wall material so q = κ T. This equation is integrated numerically with appropriate boundary conditions given by the measured surface temperature and rear cooling temperature. Surface layers (material depositions poorly coupled thermally to the bulk material with low heat capacities) complicate the interpretation of the measurements. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

27 IR Thermogaphy at JET Movie: Courtesy of G. Arnoux CCFE, UK. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

28 Divertor bolometry Provides absolute measurements of total radiation losses of a plasma discharge, regardless of the radiation wavelengths. A bolometer is just a tiny piece of metal with precisely defined thermal properties that heats up due to plasma radiation (more details in coming lectures). EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

29 Divertor bolometry: Tomogaphy The radiation comes through a pinhole that defines a viewing line of each bolometer. A set of suitably positioned viewing lines allows to estimate the radiation emissivity pattern on plasma cross-section by Tomographic reconstruction (last lecture if weather permits). EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

30 QMBs and post-morten tile analysis QMBs are extremelly sensitive balances that measure with time resolution the small mass of deposited material by measuring the frequency change of the Quartz oscillator. Post-morten geological (chemical, microscopic,... ) analysis of tiles removed in technical shutdowns provide information on impurity migration and deposition. EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33

31 J. Roth, R. Preuss, W. Bohmeyer, S. Brezinsek, A. Cambe, E. Casarotto, R. Doerner, E. Gauthier, G. Federici, S. Higashijima, J. Hogan, A. Kallenbach, A. Kirschner, H. Kubo, J.M. Layet, T. Nakano, V. Philipps, A. Pospieszczyk, R. Pugno, R. Ruggieri, B. Schweer, G. Sergienko, and M. Stamp. Flux dependence of carbon chemical erosion by deuterium ions. Nuclear Fusion, 44(11):L21 L25, P. C. Stangeby. The plasma boundary of magnetic fusion devices. Plasma physics series. Bristol: Institute of Physics Pub., EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft / 33