Introduction to the Diagnosis of Magnetically Confined Thermonuclear - - PowerPoint PPT Presentation

Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 arturo.alonso@ciemat.es

version 0.1 (February 21, 2012)

EDGE-SOL II: Plasma Wall Interactions, A. Alonso, copyleft 2010 1 / 34

Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Introduction to PWI

As we saw in the introduction, in a steady state plasma, Pα + Ph = Prad + Pκ Pκ 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 λn ∼ 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/m2 in present day large tokamaks.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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).

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 Ti shifted by cs = [(Ti + Te)/mi]1/2. This gives Es = 5 2Ti + 1 2mc2

s = 7

2T , (for Ti = Te). More realistic ion distributions give somewhat lower sheath entrance energies Es ∼ 2T (see [2, chapter 2]). From the sheath entance to the surface, ions adquire an extra 3Te energy from the potential drop, therfore E0 ≈ 2Ti + 3Te ∼ 5T , which shows that, for an aproximatelly isothermal plasma, the sheath increases the ion impact energy significantly.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Chemical suttering

More complex phenomenon involving the reactivity and kinetics of many chemical processes. It is nearly independent of the incident energy but depends

• n the flux of ions to the material surface [1]

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Divertor advantages in PWI

divertor target strike point LCFS plasma core upstream flux expansion target

• rientation

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

• f the divertor plates allow

to reduce ‘geometrically’ the power densities to the wall increasing the wetted area

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Divertor advantages in PWI

The longer paralel conections lengths (Ldiv

• ≈ 40m, Llim
• 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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 = qcond

• + qconv
• = κdT/dx + vT

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 = Si(x) , where Si is the particle source. Integrating the above equation from the sheat entrance (where v(xS) = cs) to a distance x ≥ xS and assuming roughly constant density v(x) = −cs + 1 n x

xS

Si(x)dx. The localisation of Si(x) determines how close to the sheath the ions are accelerated to cs: the more distant the source is the longer the fracion of the SOL where ions are at nearly sonic speed and advection is important.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 + nmv2 = constant, with vt = (2Tt/mi)1/2

2ntTt = nuTu

2 Power balance: κ = κ0T5/2 ≈ κe 0T5/2 (Spitzer)

T7/2

u

= T7/2

t

+ 7 2 qL κ0e ; q = qt = γntTtcst

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

The two-point model of the SOL

target upstream

Considering q and nu the control params (controlled in XPs by Pin and ¯ ne), it follows that [see notes] Tt ∝ q10/7

• /n2

uL4/7, nt ∝ n3 u/q8/7 , Γt ∝ n2 u/q3/7 L4/7, (φrecycle ∝ Γt)

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Sheath-limited and Conduction-limited regimes

Sheath-limited divertor convection dominated SOL or conduction dominated with low nu 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 nu and φrecycle ∝ n2

u

Neutrals ionised in the SOL plasma close to the target removing part of the energy (radiation and CX) Tt drops and with it the energy of the ions striking on the surface pressure still remains constant along the field line (Tt ↓⇒ nt ↑)

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Sheath-limited and Conduction-limited regimes

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Divertor regimes: a summary

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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).

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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 cpρ ∂2T ∂x2

• where κ(T), cp(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.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

IR Thermogaphy at JET

Movie: Courtesy of G. Arnoux CCFE, UK.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

Divertor bolometry

Provides absolute measurements of total radiation losses

• f 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).

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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).

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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

• scillator.

Post-morten geological (chemical, microscopic,. . . ) analysis of tiles removed in technical shutdowns provide information on impurity migration and deposition.

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Basic Plasma-Wall Interaction Divertor physics Divertor and Wall diagnostics

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• 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, 2004. P . C. Stangeby. The plasma boundary of magnetic fusion devices. Plasma physics series. Bristol: Institute of Physics Pub., 2000.

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