Magnetic Confinement Fusion: Progress and Recent Developments

Transcription

1 Magnetic Confinement Fusion: Progress and Recent Developments Howard Wilson, Dept Physics, University of York, Heslington, York YO10 5DD With thanks to A Field, K Gibson and A Kirk howard.wilson@york.ac.uk

2 Outline Introduction The tokamak D + T He + n MeV Some of the scientific challenges Confinement The role of the pedestal Exhaust Plasma eruptions (edge localised modes) Disruptions The impact of wall materials Summary

3 The Tokamak The spherical tokamak has a relatively simplest structure: Rod current ~few MA Toroidal component of magnetic field ~ T

4 The Tokamak The spherical tokamak has a relatively simple structure: But we have B=m 0 I rod /2pR orbits are not closed Grad B drift db/dr Rod current ~few MA ions E Toroidal component of magnetic field ~ T

5 The Tokamak The spherical tokamak has a relatively simple structure: Rod current ~few MA E Toroidal component of magnetic field ~ T ExB drift leads to particle loss poor confinement

6 The Tokamak A plasma current provides a poloidal component to the magnetic field, which averages out the effect of the grad-b and curvature drifts: Rod current ~few MA Poloidal component of magnetic field ~ T Toroidal component of magnetic field ~ T and toroidal current ~MA Solenoid current

7 The UK s spherical tokamak, MAST

8 JET: the world s most advanced fusion facility Inside the JET vacuum vessel, with (right) and without (left) a plasma The structure of JET

9 A key role of MAST and JET: preparing for ITER Extrapolations of today s tokamaks predict a device twice the size of JET will yield 10 times more fusion power than that to heat the plasma (Q=10) The multi-national ITER experiment is under construction in the S of France More than half the World s population is represented by ITER: EU, RF, US, Japan, China, India, S Korea cost ~ 15Bn Key features: Integrates plasma, materials, nuclear and technology (+ politics!) Q=10 means heating by a s is 2x external heating

10 The different plasma regions High heating brings core towards fusion conditions Heat diffuses at a rate ~1/(confinement time) Tokamak exhaust system Scrape-off-layer Divertor Target plate takes most of exhaust

11 Confinement: Controls the size of a fusion device The magnetic bottle is not perfect Plasma does leak out Reducing the heat loss reduces the size (and cost!) of a fusion power plant The loss of heat from the plasma is determined by turbulence Quenching turbulence improves confinement

12 But this turbulence can be suppressed the H-mode

13 Laser (Thomson) scattering shows edge transport barrier As heating power is increased through a threshold, turbulence is just suppressed in the edge region Provides a narrow insulating region (transport barrier) called the pedestal Raises the full core pressure, even though the core remains turbulent H-mode: high central pressure Transport barrier L-mode

14 The different plasma regions High heating brings core towards fusion conditions Heat diffuses at a rate ~1/(confinement time) Pedestal region Tokamak exhaust system Scrape-off-layer Divertor Target plate takes most of exhaust

15 Confinement is key Understanding how the pedestal forms and what determines the width is key If we can learn how to broaden the pedestal width, we could achieve fusion conditions in smaller devices Less power to be handled in the exhaust region

16 Suppressing turbulent transport: Flows are believed to play a key role Flows in the plasma are thought to play a key role, tearing turbulence apart Is this the mechanism for the H-mode? Could provide the key to improved confinement and more compact fusion devices

17 Better confinement: Helps manage the exhaust Poor confinement pushes one to larger fusion devices Heating power ~ volume, but exhaust handling ~ surface area (or worse!) If we are pushed to large devices to achieve fusion conditions, power handling becomes a challenge

18 The uncertainty in exhaust power load SOL width determines heat load to the target plates It is given by a balance between Fast transport of heat along magnetic field lines Slow transport across field lines: a turbulent process that is uncertain We want a turbulent SOL for exhaust and a quiet pedestal for confinement SOL width

19 Novel Design solutions No solution yet exists for handling the exhaust in a fusion power plant (DEMO) Two novel designs are being explored: The Super-X divertor on MAST (upgrade scheduled for completion 2015) The Snowflake divertor on TCV

20 Plasma eruptions: an additional challenge for materials Large filamentary plasma eruptions are triggered by the steep edge pressure gradient These are called edge-localised modes, or ELMs A major concern for next generation tokamaks

21 Plasma eruptions: an additional challenge for materials Large filamentary plasma eruptions are triggered by the steep edge pressure gradient These are called edge-localised modes, or ELMs A major concern for ITER Between ELMs During ELMs

22 BES Data from MAST probes ELM physics and plasma turbulence BES observations of ELM eruption available at 2 MHz sample rate BES view location A Field, D Dunai, M Fox, et al

23 BES Data from MAST probes ELM physics and plasma turbulence BES observations of ELM eruption available at 2 MHz sample rate Movie of ELM eruption: #29442 BES view location ms +40 s +10 s +50 s +20 s +60 s +30 s +70 s A Field, D Dunai, M Fox, et al Potential for comparisons with non-linear simulations

24 Degrade the confinement to prevent the ELMs Can we degrade confinement at the plasma edge without affecting the core? A system of coils perturbs the confining magnetic field, reducing pressure gradient DIII-D T. E. Evans, et al., Phys. Plas 13 (2006) Only works under certain conditions: not fully understood The physics of plasma interacting with such magnetic fields in subtle Needs rigorous theoretical models and further tests on other tokamaks

25 ELM suppression is not universal: Understanding the plasma response is key ELM suppression has been studied in depth of MAST n=3, CDN MAST ELM control coils

26 ELM suppression is not universal: Understanding the plasma response is key On MAST, ELMs are mitigated, but not suppressed n=3, CDN Part of the challenge is understanding the response of the plasma to the magnetic perturbations A Kirk, IAEA Fusion Energy Conference,2012

27 Pushing the limits can destabilise the plasma: a disruption Under certain conditions, control of the plasma can be lost Strikes material components and terminates the discharge: a disruption Thermal loads and electromagnetic A Thornton and K Gibson

28 Disruptions Disruptions are a concern for two main reasons The huge thermal load that is placed on the material surfaces A significant current (the halo current) flows from the plasma into the vessel components Can induce ~few MA of current in the structure and in magnetic fields ~1T, this produces forces on the components ~106N: enough to lift a jumbo jet! Infra-red image of the interior of the MAST vacuum vessel following a disruption A Kirk, IAEA Conference, 2004

29 Mitigated Disruption: Inject Ar to radiate energy before plasma strikes wall A Thornton and K Gibson

30 Mitigated Disruption: Density accumulation at rational surface may be key Cooling front propagates through the plasma Thomson scattering data shows the propagation of the front Cooling front = T(Ψ95) During the cooling phase, density build up on rational surfaces Could be trapped within a magnetic island? A Thornton and K Gibson

31 Material choices for handling power loads The highest powers are in the divertor region q plasma d Coolant substrate coolant We need to identify materials with high conductivity that tolerate high temperature Carbon has been the material of choice JET tritium experiments revealed C can trap tritium attention has moved to metals The wall needs to be a low atomic number to minimise losses through radiation Be tiles planned for ITER The divertor needs to withstand more extreme conditions Tungsten planned for ITER Very little tungsten can be tolerated in the plasma core (radiation)

32 The ITER-Like Wall project on JET: A key capability in preparing for ITER Been operating for about a year Replacing C with Be and W is found to affect the plasma characteristics Presently being quantified

33 The ITER-Like Wall project on JET: Confinement is reduced compared to C wall Replacing C with Be and W is found to affect the plasma confinement JET baseline H-mode plasma Greenwald density fraction Beurskens et al EPS 2013

34 The ITER-Like Wall project on JET: Confinement is reduced compared to C wall Replacing C with Be and W is found to affect the plasma confinement Particularly noticeable at higher triangularity JET baseline H-mode plasma JET-ILW High δ Greenwald density fraction Beurskens et al EPS 2013

35 The ITER-Like Wall project on JET: Introducing N gas helps to recover confinement Introducing nitrogen into the edge recovers (in part) the confinement Likely explanation is impact on pedestal structure Plasma physics understanding remains elusive Pedestal top JET-ILW High δ N2 seeding Greenwald density fraction Beurskens et al EPS 2013

36 Summary ITER is designed to address the final science and (many) technology questions to develop fusion power We have considered a number of scientific challenges for fusion: Confinement influences the size of a fusion device Improved confinement enables smaller power plants with lower power loads While steady power loads on ITER are expected to be tolerable, no solution yet exists for a power plant: a combination of materials and plasma science research is required Plasma eruptions (ELMs) are a concern on ITER: promising control techniques are being explored Control schemes for disruptions have been developed and are being optimised The plasma-facing materials have a significant influence on confinement JET and AUG are valuable facilities to explore this in preparation for ITER

37 Thank you!

38 Fusion power loads compared