Plasma Materials Interactions

The Dirty Part of Plasma Physics

May 2, 2006

M. UlricksonPresented at GCEP Fusion Workshop

Princeton, NJ

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration

under contract DE-AC04-94AL85000.

MAU 2 5/20/2003

Introduction

• The interaction between fusion plasmas and cold materials is complex

• Design of Plasma Facing Components (PFCs) involves science from a diverse set of skills

• Successful PFCs must be robust and forgiving and operate very near the limit of catastrophic failure

MAU 3 5/20/2003

Introduction

• My topics will include:– Plasma edge characteristics– Plasma materials interactions– Materials selection for PFCs– Off-Normal events – Research opportunities (red titles)

MAU 4 5/20/2003

ITER PFCs

MAU 5 5/20/2003

Complex Forces Act in the Scrape-Off Layer

• Radial Electric fields• Gradients of magnetic

fields• Neutral particle influx from

the PFCs• Impurity generation at the

PFCs• The resulting plasma flows

create the plasma conditions in the divertor and at the first wall

MAU 6 5/20/2003

Example of Plasma Parameters at Divertor

MAU 7 5/20/2003

Plasma Core to SOL and Divertor Coupling

• The divertor needs to be at high density and have some impurities to enter the detached regime where heat loads are reduced.

• Complete detachment leads to flow stagnation and may not be compatible with He pumping.

• But the SOL density needs to be controlled for good core confinement and low wall erosion.

• Long pulses and high power also lead to new regimes for plasma wall interactions and erosion.

• If SOL pumping is extremely high, flat density profiles and high temperature plasma edge may be realized

MAU 8 5/20/2003

Fusion Plasma Materials Interactions

• The core plasma must be kept clean of impurities and He ash

• The plasma facing component surface sees high density and temperature plasma

• Key issues are hydrogen trapping, erosion, and thermal fatigue

• Spans science specialties from ionized gases to materials science

Core Plasma

Boundary Plasma

Plasma Facing Material

20-100 M K 0.1-2 M K 800-3500 K

Energy and particles

Fuel and impurities

Ionization and transport

Trapping

Sputtering Evaporation

MAU 9 5/20/2003

Plasma Surface Interactions

Permeation

Trapping

Plasma conditions

density, T, flux

Impurities & recycling

Conditioning & coatings

Plasma facing material

MAU 10 5/20/2003

Science Involved in PFC Design

• Atomic and molecular physics for ionization, dissociation, and photon radiation of plasma and impurity species

• Surface physics for sputtering, chemical erosion, hydrogen trapping and release, surface segregation

• Materials science for nuclear radiation damage, thermal fatigue, stress corrosion, creep, bonding, and hydrogen trapping

• Engineering science for stress management, heat transfer, and component design

MAU 11 5/20/2003

Materials Selection and PFC Design

• Selection limited by capability for absorbing heat and minimization of plasma contamination

• Refractory materials have an advantage• Plasma interactions can change material

properties• Off-Normal events force compromise in material

selection and PFC design

MAU 12 5/20/2003

Magnetic Fusion Energy Heat Fluxes

10-4 10-3 10-2 10-1 100 101 102 103 104 105 106

Duration (s)

10-1

100

101

102

103

104

105

106

Hea

t Flu

x (M

W/m

2 )

Fusion Divertor

Radiant Flux at Sun SurfaceFast Breeder

Fission Reactor

Fusion First Wall

Fusion Disruption

Fusion ELM

Rocket Nozzle

MAU 13 5/20/2003

Heat Flux Capability

Al Be C PyC Cr Co Cu Au Fe Mg Mo Ni Nb Pt Ag Ta Ti W V ZrMaterial

0

10

20

30

40

50

Lim

iting

Hea

t Flu

x (M

W/m

2 )

Typical Maximum

Normal Operation

Typical Minimum

MAU 14 5/20/2003

Materials Choices for PFCs

• Divertor applications– Only W and C are acceptable for the highest heat

flux (C limited because of T retention and neutron damage)

– With some radiation in the divertor Mo, Ta, and Nb(?) are candidates (Cu is not acceptable because of erosion)

• For first wall applications– Iron alloys (ferritic steel), V (?), Be, and all the

divertor materials

MAU 15 5/20/2003

Carbon Based Plasma Facing Materials

• Very high temperature operation possible• Thermal conduction is primarily by phonons

(radiation damage lowers performance)• Chemical reactions with atomic H are a major

issue (chemical erosion)• Control of tritium inventory in C:H layers is a

critical issue for ITER• Hydrogen is very mobile in C

MAU 16 5/20/2003

Tungsten Plasma Facing Material

• Second only to carbon in thermal performance• Tungsten is brittle, it melts, it is not low

activation, it cannot be welded• Can have low or no physical erosion in high

recycling divertor• H isotope retention is very low• W PFC surfaces must be divided• Blisters may form due to particle irradiation

MAU 17 5/20/2003

Beryllium First Wall PFM

• Very low Z• Very good oxygen getter• Good thermal properties• Low T retention unless thick oxide layers are

present• Low melting point (~1275C) (off-normal events

limit lifetime)

MAU 18 5/20/2003

Mixed Material Issues

• For example consider the case of a Be first wall and C or W divertor PFC system– Be deposition on C by transport through the

scrape-off layer will lower the chemical erosion of C (PISCES at UCSD)

– Be deposition on W can form lower melting point materials (W2Be, WBe, WBe12)

– The operating T for a high power divertor is several hundred to >2000 C making experiments difficult

MAU 19 5/20/2003

Active Cooling

• To realize the maximum potential for magnetic fusion energy devices must operate either long pulse or steady-state

• Steady-state means the thermal time constant of the PFC is much shorter than the plasma duration

• Water cooling is the method of choice now• In magnetic fusion reactors the use of Helium gas

cooling is the leading candidate• Only a few devices are exploring this regime now

MAU 20 5/20/2003

Off-Normal Operation

• The two main classes of off-normal events are:– Plasma current interruptions (often accompanied

by plasma motion)– Instabilities in the plasma edge that release short

bursts of energy and particle to the PFC (Edge Localized Modes or ELMS)

• Both restrict the maximum acceptable heat flux during normal operation because they require thickening of the plasma facing surface to prevent catastrophic failure of the PFC

MAU 21 5/20/2003

Off-Normal Mitigation Progress

• The lifetime of these actively cooled components is governed by disruption and ELM events.

• There has been significant progress on predicting disruptions and mitigating the effect of disruptions. – Neural network prediction of disruptions about 50

ms before they occur with a >90% accuracy– Massive gas puffing to mitigate halo currents,

energy deposition and current decay rates (liquid jets under development)

MAU 22 5/20/2003

Off-Normal Mitigation Progress

• Experiments have shown that ELMS can be mitigated by:– Double null operation– High triangularity operation– High edge density – Active divertor pumping– Coils to perturb the edge

• The full impact on plasma performance remains to be assessed for all options

MAU 23 5/20/2003

Bringing It All Together

• Successful PFC designs have progressed from short pulse devices (fractions of seconds) to tens of seconds of operation (even hours for actively cooled PFCs)

• Plasma contamination due to PFCs is routinely low on well designed devices

• Catastrophic failures are rare and have not occurred on the largest, most advance machines

• Actively cooled devices are rapidly growing in number

MAU 24 5/20/2003

Looking Forward

• Helium gas cooled PFCs are being designed for parts of ITER

• Liquid surface PFCs are at the fore-front of PFC research– May allow simultaneous heat and particle removal– May give access to unique plasma operating

regimes (very low particle recycling, flat density profiles, high edge temperatures)

• Control of off-normal events will allow further optimization of PFC designs and greater performance

MAU 25 5/20/2003

Porous Metal Heat Sinks (He)

• Promising designs have been found for Cu alloys

• Heat removal is approaching water values

• Pressure drop is ok.• Refractory metal research

just starting.• Helium gas purity is a key

issue but there appear to be solutions.

• Refractory alloy development is needed.

Area1Min Mean Max910.3 1,010 1,119

Area1Min Mean Max910.3 1,010 1,119

*>1,233°C

*<160.6°C

200.0

400.0

600.0

800.0

1,000

1,200

Progress in helium cooling

0

5000

10000

15000

20000

25000

30000

35000

40000

1993 1994 1995 1996 1997 1998 1999 2000year

h (W

/m2 K

)

microchannels dual channel pellets single channel pelletstungsten foam tungsten pellets water

MAU 26 5/20/2003

Status of Refractory He Cooled R&D

• Development of irradiation resistant refractory materials is a key issue

• Refractory PFM to refractory heat sink joining techniques remain to be proven

• Demonstration of He gas purity must be done• Requires low T edge plasma because the sputter

erosion can be eliminated.• High temperature gas produced (up to 1100C)

implying high electrical efficiency.

MAU 27 5/20/2003

Impact of ELMS

• ELM energy deposition severely limits the operating heat flux

• The thermal fatigue caused by ELMs has not been experimentally verified (could be very severe)

• ELMs will severely limit the lifetime of PFCs and may reduce reliability.

• Reducing ELM energy to less than 0.5 MJ/m2 is sufficient to increase the margin for failure.

MAU 28 5/20/2003

Liquid Surface PFCs

• Liquids being considered are: Lithium, Gallium, Tin (SnLi mixture)

• Flowing Liquids have high heat flux capability (up to ~50 MW/m2)

• Key Issues are:– Magnetohydrodynamic effects on flowing

conducting liquids (much modeling needed)– Materials compatibility (corrosion)– Practical methods– Particle pumping capability (mostly He)

MAU 29 5/20/2003

Opportunities for Research Summary

• Refractory Materials development including joining to heat sinks

• Gas cooled refractory heat sink inventions• Off-normal event mitigation methods and

detection methods• Liquid metal PFC MHD modeling, methods for

injection and control, and materials development