Gregor Morfill Max-Planck Institut für extraterrestrische Physik IPP-CAS, Hefei, 24/1/2008

1. Dust in Fusion Reactors Gregor Morfill Max-Planck Institut für extraterrestrische Physik IPP-CAS, Hefei, 24/1/2008 Thanks go to my co-authors: S. Ratinskaya, U. de Angelis, C. Castaldo, and to J. Martin, S. Khrapak and M. Horanyi for discussions and further information.

2. contents • I. Introduction: • II. ´Dust Physics´ overview: • III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range • IV. Hi-V dust Particles: Direct measurements • V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? • VI. High velocity (Hi-V) dust particles as a source of neutrals • Conclusion

3. I. Introduction Plasma Fusion is one of the most important topics in safeguarding the world energy needs in the future. Advantages: The ´fuel´ is practically inexhaustible. The energy production per gram is huge. The waste production is low. The environmental effects are minimal. The danger of radioactive accidents is minimal.

4. I. Introduction Plasma Fusion is one of the most important topics in safeguarding the world energy needs in the future. Problems: The technology has proven to be much more challenging than initially assumed. The reactor environment is not ´benign´ by any standards. The plasma-wall interactions cannot be avoided and imply limited operation times before repairs are necessary.

5. I. Introduction: Dust in Tokamaks ITER That dust exists in tokamaks is well known.* That this dust may constitute a hazard for fusion reactors is a concern. What is almost completely unknown is the scope of the problem – other than straightforward linear extrapolations. Consequently there is little (if any) thought given to possible reactor design implications. Parameters: R = 6.2m, r = 2.0m B = 5.3T (on axis) I = 15MA Predicted fusion power of 500MW * Dust production in ITER is estimated at 750 kg/year (Be) and 150kg/year (Cu) Mitsubishi Report (2006)

6. I. Introduction: Dust in Tokamaks Size distribution (from dust collection) peaks around 5-10 μm and falls off smoothly to beyond 100 μm.(Ciattaglia, Rohde, EPS Warsaw, 2007) Propagation direction of ´small´ dust particles mostly along the plasma rotation. (Roquemore, Rudakov, EPS Warsaw, 2007) Observed particle velocities are 10´s of m/sec up to 0.5 km/sec.(Rohde, Rudakov, Hong, Roquemore, EPS Warsaw, 2007) Film provided by F. Lott and G. F. Counsell (2006)

7. I. Introduction: Dust in Tokamaks Particle life times,, in the main chamber are a few msec, in the SOL 100msec (Smirnov, Roquemore, Hong, EPS Warsaw, 2007) ´Rocket force´ acceleration may have been observed? (Interaction with ELMs - Asakura, SOL/Plasma boundary - Rudakov, EPS Warsaw, 2007) Film provided by F. Lott and G. F. Counsell (2006)

8. I. Introduction: Dust in Tokamaks The ´traditional´ concerns: • Dust sputtering could lead to core plasma contamination. • Transport and redeposition of dust can roughen surfaces, reducing the performance. • Dust can block gaps in tiles left for engineering reasons. • Dust could contain beryllium and tritium. • Dust can transport impurities around the scrape-off layer. • Be - dust in the diverter may cause H explosion. Co-deposited material e.g. Winter (1999), Rubel et al. (2001), Martin (2006)

9. I. Introduction: Dust in Tokamaks The ´unconventional´ concerns: • Is there some ´dust physics´ that has not been considered so far? • In what way(s) not considered so far could dust become a hazard? • Could ´dust´ become a design or reactor operation driver? Co-deposited material e.g. Winter (1999), Rubel et al. (2001), Martin (2006)

10. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… Provided by F. Lott and G. F. Counsell (2006) Rudakov (2007, priv. comm.)

11. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… Have we studied the role of dust in Plasma Fusion Reactors sufficiently ? Provided by F. Lott and G. F. Counsell (2006) Rudakov (2007, priv. comm.)

12. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… Fresh dust particles can be produced by: • plasma surface erosion and flaking, • nucleation in cooler regions, • destruction of interior elements. • The divertor is believed to be the main source region. Provided by F. Lott and G. F. Counsell (2006)

13. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… • Charging is rapid mainly by electron and ion impacts. • Secondary emission and photoeffect may also contribute. • The overall particle potential is expected to be a few times the electron energy. e  i

14. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… • Charged particle transport is affected by: • Electric and magnetic fields, • Ion drag forces (including Coulomb drag), • Thermophoretic forces, • Photophoretic forces, • ´Rocket effect´, • Thermoionic emission, • Collective effects. e  i

15. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… • Particle destruction processes: • Heat and evaporation, • Ion and electron sputtering, • Photosputtering, • Thermoionic emission, • Collisions. e  i

16. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… • Above a critical velocity of ~1km/sec impacts become destructive (cratering). • Below this velocity impacts are mainly elastic. v ≤ 1km/sec v ≥ 1km/sec

17. II. ´Dust Physics´ - production, charging, transport, destruction, impacts… • High velocity (Hi-V) particles – if they exist – would be a (wall material) source of: • New particles • Neutral gas • Plasma The possible implications have not been investigated for Plasma fusion reactors so far v ≥ 1km/sec

18. III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range Questions: 1. Why worry about Hi-V particles? Tiny μm-sized dust particles with velocities above a (few) km/sec would not have been detected with current ´direct´ observation programmes (too fast, not bright enough). These particles, if they exist, are particularly troublesome – they produce more ejecta on impact than their own mass! For long operation times Hi-V particles can (in principle) cause a runaway effect.

19. III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range Questions: 2. Could such high velocities be reached before the particles are lost or sputtered away? High velocities of around 0.5 km/sec have been seen for much bigger particles (more than 1000 times more massive – hence with much higher kinetic energies). Other (circumstantial) evidence exists – see later. Numerical simulations, however, indicate that high velocities are unlikely to occur… but has all the physics been considered?

20. III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range Questions: 3. Are there any other possible Hi-V signatures? Look for impact craters, acoustic signatures, plasma clouds, neutral clouds, debris clouds, plasma contamination due to neutrals etc. 4. Could Hi-V particles be generic features of Tokamaks? Accelerated (larger) particles apparently occur in all devices.

21. Hi-V dust Particles: Direct measurements • Optical measurements 1. The size of ´visible´ intrinsic dust particles is not known, but: Injected dust of 6μm was visible at v≤100m/s(Rudakov, EPS Warsaw, 2007) and 40-120 μmparticles were also visible (Granetz, EPS Warsaw, 2007). 2. The observed dust velocities (so far) are v ≤500 m/sec: At 1000f/s the trajectory length per frame is ≤50cm. Hi-V particle velocities are ≥ 1 km/sec. The trajectory length per frame is ≥ 1m, and the brightness per pixel a factor 100 lower. 3. Hi-V particles cannot be detected optically with current technology. Film provided by F. Lott and G. F. Counsell (2006)

22. IV. Hi-V dust Particles: Direct measurementsImpacts on a Langmuir probe in FTU* • The plan was to expose a surface to the plasma and investigate it afterwards using high resolution microscopy. • Three probe positions in the SOL on the axis (r1 – r3) - plus one each at the SOL edge (ru and rb). Impact-like signatures were observed at positions r1 – r3, not at ru and rb. *FTU (Frascati Tokamak Upgrade) is a Tokamak with outer radius of 1m, small radius 33 cm.

23. IV. Hi-V dust Particles: Direct measurementsLangmuir probe surface after exposure in FTU* • The image shows ´craters´, possibly due to micron-sized Hi-V projectiles. • The crater dimensions are compatible with 1μm particles impacting at 10km/sec – or 2μm at 3.5km/sec. • The image also shows a distribution of small Fe particles (≤ 20m), presumably ejected from the steel wall of the tokamak. • Their size spectrum is compatible with high velocity impacts on the walls. *FTU is a Tokamak with outer radius of 1m, small radius 33 cm. Castaldo et al. (2007)

24. IV. Hi-V dust Particles – Direct measurements:Plasma signatures of impacts in FTU • Hi-V particle impacts will also produce a cloud of high velocity plasma, which will (mostly) be captured in the SOL. • The impact-produced plasma cloud should be measurable as a short burst of enhanced plasma density with Langmuir probes. • Wall impacts as well as impacts on the probe might be observable. Castaldo et al. (2007) Castaldo et al. 2007, Ratinskaya et al. 2007

25. IV. Hi-V dust Particles – Direct measurements:Plasma signatures of impacts in FTU • A semi-empirical expression for the impact produced plasma (total charge) is: Ni = 2.8·107aμ3 v3.21 whereaμis in microns and v in km/sec Grün 1981, Burchell et al. 1999

26. IV. Hi-V dust Particles – Direct measurements:Plasma signatures of impacts in FTU • A plasma impact cloud from a1μm particle at 10km/sec – or a 2μm particle at 3.5km/sec – contains about 1011 to 1012 ions (charges) released in a few tens of μsec – as was observed. • Typical event rates as well as plasma, neutral gas and secondary particle production rates in FTU were determined. • These were compatible with the ´impact signatures´on the Langmuir probes. Castaldo et al. 2007, Ratinskaya et al. 2007

27. Langmuir probe signatures • About 100 ´events´ per second. • 1011 – 1012 charges per ´event´. • All ´events´have exponential time profiles. • ´Events´ are more frequent near wall. Ratinskaya et al. (2007), Castaldo et al. (2007)

28. Langmuir probe signatures • ´Events´ nearer the wall are smaller and have shorter duration than those further from the wall. • Compatible with a wall source. • Compatible with high velocity impact plasma clouds. Ratinskaya et al. (2007), Castaldo et al. (2007)

29. IV. Hi-V dust Particles – Direct measurements:Plasma signatures of impacts in FTU Caveats: 1. Are there any other signatures? Look for impact craters, accoustic signatures, plasma clouds, neutral clouds, debris clouds, plasma contamination due to neutrals etc. Impact craters may look like unipolar arc signatures, impact plasma clouds may look like blobs, plasma contamination may be due to other sources (e.g. sputtering) 2. Could such high velocities be reached before the particles are lost or sputtered away? High velocities of around 0.5 km/sec have already been seen for much bigger particles (more than 100 times more massive). More research into efficient acceleration processes is necessary – velocities » 0.5 km/sec would be a concern

30. V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? • There are two concerns: • 1. Runaway wall erosion could imply much shorter operation times, with all the negative cost/effectiveness problems – or would dictate a reactor operation mode far from the optimum. • 2. Associated with impact erosion there will also be neutral gas production. This may penetrate into the core plasma, contaminate it and lead to efficiency losses.

31. V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? • On impact with the walls Hi-V particles will generate new ejecta with a ratio ME/ Mp≈ 5v2 where the impact velocity, v, is in km/sec. • Dependence of impact crater volume, V, on impact angle size, θ: V ≈ V0 cosθ • Ejecta mass distribution is dN/dm = Cm-1.8 with the largest ejecta particle having a mass mL ≈ 0.1 ME θ Burchell et al. 1999, Gault 1963; Dohnanyi 1969; Gault and Wedekind 1969

32. V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? We have: dm/dt = m/ τm – m/τl + δ(t) the time scale of the growth of total ejecta mass, τm , in the absence of losses, would be τm =τ/5v2 , where the factor 5v2 is the ratio of ejecta mass/impact mass (which can be large, if the impact velocity v(km/sec) is high). • Also, τl represents losses (e.g. by sputtering) and δ(t) is an initial source (trigger ) term that becomes irrelevant after a few τm, if τm is less than τl .Then we have m(t) = m0exp[t (τl -τm) / τlτm] • Note: the core plasma contamination by neutrals grows at the same rate, i.e. S0(t) ~ exp[t (τl -τm) / τlτm] Morfill et al. 2007

33. V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? We had: S0(t) ~ exp[t (τl -τm) / τlτm] with τm =τ /5v2 It is easy to see that provided τl ≥ τm we obtain a ´runaway´ effect, with the reactor contamination and wall erosion growing exponentially. Losses are due to e.g. particle destruction or deposition, removal of plasma contaminants in the SOL, neutral deposition. • Let us take particle destruction by sputtering as the dominant loss process, i.e. τl = τsput • There are three conditions for starting an erosion and contamination chain reaction: 1. the (particle life) time before wall impact τ≤ τsput 2.the acceleration time to the critical velocity vcrit must be ≤ τ 3. the largest ejecta particle must have a mass mL≥ Mp +Msput Morfill et al. 2007

34. Hi-V dust particle critical velocity –sputter losses:the asymptotic limit for vcritis 1.4 km/secacceleration to velocities above the limits indicated by the lines would lead to an erosion + contami-nation chain reaction. 8 (km/sec) 7 6 5 vcrit 4 Rsputτ = 2μm Rsputτ = 1μm 3 Rsput τ = 0.5μm 2 1 0 1 2 3 4 5 6 ap(μm)

35. Summary of the impact physics investigation: • Hi-V dust particles (if they exist) present a particular hazard for continuous reactor safety and operation – i.e. for τl – τm≥ 0 runaway growth of erosion and contamination becomes unavoidable: • Wall erosion will grow exponentially MErosion ~exp[t (τl -τm) / τlτm] The core plasma contamination by neutrals grows at the same rate S0(t) ~ exp[t (τl -τm) / τlτm] Morfill et al. 2007

36. VI. High velocity (Hi-V) dust particles as a source of neutrals • Neutral gas production Hi-V particles will also produce a cloud of high velocity (up to the free expansion speed) neutral gas (wall material). The estimates vary greatly, practically no measurements exist: MG/ Mp≈ 1 – 10 The neutrals may enter the core plasma - by direct injection - up to some characteristic distance, the ionisation length (e.g. by electron impact). Morfill et al. 1983, Morfill et al. 2007

37. VI. High velocity (Hi-V) dust particles as a source of neutrals • The penetration depth into the plasma depends on the neutral gas velocity, the ejection direction and the plasma density n(x,T). It is typically (ionisation cross section for Fe by ~ 20eV electrons is ≈ 5·10-16cm2):  ≈ 1 – 20 cm • Further core plasma contamination is then by diffusion (for a quick estimate use a linear model, x=0 to x=2R) ∂ni/∂t – D∂2ni/∂x2 = ∂/∂x{S0(t) exp(-x/)} X=0 X=2R Pindzola et al., 1995, Morfill et al. 1983, Morfill et al. 2007

38. VI. High velocity (Hi-V) dust particles as a source of neutrals • The source term for neutral gas contamination is ∂/∂x{S0(t) exp(-x/)} the exp(-x/) represents the ionisation profile with ionisation length  . • The time dependent term S0(t) represents the temporal evolution of the source. If this is due to wall impact production by Hi-V particles (if they exist), it can be written as S0(t)  m(t) where m(t) is given from dm/dt = m/ τm – m/τl + δ(t), i.e. the rate of production of fresh Hi-V particles, dm/dt, is proportional to the impacting population, with total mass m(t). The appropriate time scale is the (mean) dust particle life time until impact, τ. (τ≥τacc the acceleration time.) Morfill et al. 2007

39. VI. High velocity (Hi-V) dust particles as a source of neutrals • Results for FTU were a surprise – they are completely compatible with the Hi-V particle impact physics estimates: • Using the measured impact rates from the ´crater counts´ on the Langmuir probe and the plasma cloud production rates as the source term for neutral gas contamination yields a core Fe ion density of ~ 1010cm-3. • The estimates based on UV spectroscopy also gave 1010cm-3. • In addition, the measured core concentration of Ni was a factor 2 lower than the Fe concentration. Under normal conditions (with inconal poloidal limiter) it is expected that sputtering should produce a higher Ni concentration than Fe – not lower. • This, too, suggests a wall source (the walls are stainless steel), but not from sputtering, which should be too small. Morfill et al. 2007

40. Summary: Evidence for Hi-V particles in Tokamaks • Inferred evidence from impact craters on Langmuir probes in FTU. • Inferred evidence from impact generated plasma clouds in FTU. • Inferred evidence from core plasma contamination in FTU. • Theoretical investigations of particle acceleration are not conclusive: • - critical velocities of ~2 km/sec • have not been obtained in the model • calculations • - there is a question whether the life • times of the particles may be • underestimated (use of SVP not • appropriate in a plasma environment)

41. Conclusion: Hi-V particles in Tokamaks • FTU measurements have provided some initial evidence that dust in tokamaks may be highly accelerated. • If this is generally so, it presents a particular hazard - because on impact with the walls each ´Hi-V particle´ may generate 100 – 1000 times more dust. • This dust, in continuous reactor operation, will also be accelerated, impact the walls - and so on… • Impact-produced neutral high velocity gaseous (wall) material will grow accordingly. It may pass through the SOL, enter the core plasma directly and spread by diffusive transport once it has been ionised. • This scenario inevitably leads to an exponential growth in reactor contamination and wall erosion… • …unless steps are taken to overcome this problem.

42. Conclusion: Hi-V particles in Tokamaks • To identify the scope of this possible Hi-V problem, the following steps should be taken: • Study the physics of dust production, transport, acceleration, destruction and impacts – both experimentally and theoretically. • Measure dust in different Tokamaks (e.g. by acoustic sensors and by direct capture using aerogels – or whatever works). • Develop solutions for reducing the dust production in critical areas in the reactor. • Include the Hi-V dust issue in the reactor design considerations… …and start well before the ITER design freeze… • Because we already have visual observations of ~km/sec dust and cannot assume that a magic barrier exists. Certainly we cannot afford to ignore this effect, if we have not even understood it.

43. Thank you for your attention Rudakov (2007, priv. comm.)

44. IV. Return to the original question: Could there be high velocity (Hi-V) dust particles? • To answer this question let us summarise the evidence: • 1. Direct measurements – what are Hi-V particle signatures? Can these particles be seen/captured? • 2. Trajectory calculations – can high velocities be reached? Can we scale the results to different Tokamaks – i.e. how relevant is this for ITER? • 3. Rocket effect – what is the role of this process? ? Morfill et al. 2007

45. 2. Hi-V dust Particles: Trajectory calculations.Solve the ´Equation of Motion´ taking into account all electromagnetic and plasma drag forces, e.g.: Lorentz force Gravity (Geometrical) Flow Pressure +… etc. Scale the results to different Tokomaks

46. 2. Hi-V dust Particles: Trajectory calculations – Plasma and field model • Use B2-solps5.0: Standard European code to build plasma profiles for the SOL. B2 is a dual fluid code with Braginskii Transport. Either fluid neutrals or EIRENE Monte-Carlo code. • Can build up profiles for many tokamaks worldwide, including ITER From James Martin (2006)

47. 2. Hi-V dust Particles: Trajectory calculations – B2-solps5.0 plasma and field model - scaling MAST ITER From James Martin (2006)

48. 2. Hi-V dust Particles: Trajectory calculations • Comparison of forces for a typical trajectory where the dust particle (1 m) evaporates: • (Geometrical) flow pressure is the most important force • E and vxB become important as the grain evaporates • Coulomb collisions were not included. This can increase the drag cross section (by a factor 10 – 100) • Rocket effect not included From James Martin (2006)

49. THANK YOU FOR YOUR ATTENTION

50. II. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range • Tiny dust particles with velocities above a few km/sec would not have been detected with current ´direct´ observation programmes (too fast, not bright enough). • Questions: 1. Can plasma drag or other processes be sufficiently effective to accelerate dust particles to velocities in the Hi-V range (above ~1 km/sec)? 2. Could such high velocities be reached before the particles are lost or sputtered away? 3. What happens when Hi-V particles impact the reactor walls? Hi-V dust particles present a potential hazard for reactor safety and operation that has not been taken into account so far.