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1 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,

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Presentation on theme: "1 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,"— Presentation transcript:

1 1 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. Dust in Fusion Reactors

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

3 3 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. I. Introduction

4 4 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. I. Introduction

5 5 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. ITER Parameters: 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) is estimated at 750 kg/year (Be) and 150kg/year (Cu) and 150kg/year (Cu) Mitsubishi Report (2006) Mitsubishi Report (2006) I. Introduction: Dust in Tokamaks

6 6 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) I. Introduction: Dust in Tokamaks Film provided by F. Lott and G. F. Counsell (2006)

7 7 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) I. Introduction: Dust in Tokamaks Film provided by F. Lott and G. F. Counsell (2006)

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

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

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

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

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

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

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

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

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

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

18 18 Questions: 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). 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! 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. For long operation times Hi-V particles can (in principle) cause a runaway effect. III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range

19 19 Questions: 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). 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. Other (circumstantial) evidence exists – see later. Numerical simulations, however, indicate that high velocities are unlikely to occur… but has all the physics been considered? Numerical simulations, however, indicate that high velocities are unlikely to occur… but has all the physics been considered? III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range

20 20 Questions: 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. 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. Accelerated (larger) particles apparently occur in all devices. III. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range

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

22 22 The plan was to expose a surface to the plasma and investigate it afterwards using high resolution microscopy. 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 (r 1 – r 3 ) - plus one each at the SOL edge (r u and r b ). Three probe positions in the SOL on the axis (r 1 – r 3 ) - plus one each at the SOL edge (r u and r b ). IV. Hi-V dust Particles: Direct measurements Impacts on a Langmuir probe in FTU* *FTU (Frascati Tokamak Upgrade) is a Tokamak with outer radius of 1m, small radius 33 cm. Impact-like signatures were observed at positions r 1 – r 3, not at r u and r b. Impact-like signatures were observed at positions r 1 – r 3, not at r u and r b.

23 23 The image shows ´craters´, possibly due to micron-sized Hi-V projectiles. 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 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. 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. 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. radius of 1m, small radius 33 cm. IV. Hi-V dust Particles: Direct measurements Langmuir probe surface after exposure in FTU* Castaldo et al. (2007)

24 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. 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. 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. Wall impacts as well as impacts on the probe might be observable. Castaldo et al. 2007, Ratinskaya et al Castaldo et al. (2007)

25 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: A semi-empirical expression for the impact produced plasma (total charge) is: N i = 2.8·10 7  a μ 3 v 3.21 N i = 2.8·10 7  a μ 3 v 3.21 where a μ is in microns and v in km/sec where a μ is in microns and v in km/sec Grün 1981, Burchell et al. 1999

26 26 IV. Hi-V dust Particles – Direct measurements: Plasma signatures of impacts in FTU A plasma impact cloud from a 1μm particle at 10km/sec – or a 2μm particle at 3.5km/sec – contains about to ions (charges) released in a few tens of μsec – as was observed. A plasma impact cloud from a 1μm particle at 10km/sec – or a 2μm particle at 3.5km/sec – contains about to 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. 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. These were compatible with the ´impact signatures´on the Langmuir probes. Castaldo et al. 2007, Ratinskaya et al. 2007

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

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

29 29 Caveats: Caveats: 1. Are there any other signatures? Look for impact craters, accoustic signatures, plasma clouds, neutral clouds, Look for impact craters, accoustic signatures, plasma clouds, neutral clouds, debris clouds, plasma contamination due to neutrals etc. 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) 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 High velocities of around 0.5 km/sec have already been seen for much bigger particles (more than 100 times more massive). particles (more than 100 times more massive). More research into efficient acceleration processes is necessary – velocities More research into efficient acceleration processes is necessary – velocities » 0.5 km/sec would be a concern » 0.5 km/sec would be a concern IV. Hi-V dust Particles – Direct measurements: Plasma signatures of impacts in FTU

30 30 There are two concerns: 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. 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. 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. V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect?

31 31 On impact with the walls Hi-V particles will generate new ejecta with a ratio On impact with the walls Hi-V particles will generate new ejecta with a ratio M E / M p ≈ 5v 2 M E / M p ≈ 5v 2 where the impact velocity, v, is in km/sec. where the impact velocity, v, is in km/sec. Dependence of impact crater volume, V, on impact angle size, θ : Dependence of impact crater volume, V, on impact angle size, θ : V ≈ V 0 cosθ V ≈ V 0 cosθ Ejecta mass distribution is Ejecta mass distribution is dN/dm = Cm -1.8 dN/dm = Cm -1.8 with the largest ejecta particle having a mass m L ≈ 0.1 M E with the largest ejecta particle having a mass m L ≈ 0.1 M E Burchell et al. 1999, Gault 1963; Dohnanyi 1969; Gault and Wedekind 1969 V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect? θ

32 32 We have: dm/dt = m/ τ m – m/ τ l + δ (t) the time scale of the growth of total ejecta mass, τ m, in the absence of losses, would be τ m = τ /5v 2, where the factor 5v 2 is the ratio of ejecta mass/impact mass (which can be large, if the impact velocity v(km/sec) is high). the time scale of the growth of total ejecta mass, τ m, in the absence of losses, would be τ m = τ /5v 2, where the factor 5v 2 is the ratio of ejecta mass/impact mass (which can be large, if the impact velocity v(km/sec) is high). Also, τ l represents losses (e.g. by sputtering) and δ (t) is an initial source (trigger ) term that becomes irrelevant after a few τ m, if τ m is less than τ l.Then we have Also, τ l represents losses (e.g. by sputtering) and δ (t) is an initial source (trigger ) term that becomes irrelevant after a few τ m, if τ m is less than τ l.Then we have m(t) = m 0 exp[t ( τ l - τ m ) / τ l τ m ] m(t) = m 0 exp[t ( τ l - τ m ) / τ l τ m ] Note: the core plasma contamination by neutrals grows at the same rate, i.e. Note: the core plasma contamination by neutrals grows at the same rate, i.e. S 0 (t) ~ exp[t ( τ l - τ m ) / τ l τ m ] S 0 (t) ~ exp[t ( τ l - τ m ) / τ l τ m ] Morfill et al V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect?

33 33 We had: S 0 (t) ~ exp[t ( τ l - τ m ) / τ l τ m ] with τ m = τ /5v 2 It is easy to see that provided τ l ≥ τ m 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. It is easy to see that provided τ l ≥ τ m 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. τ l = τ sput Let us take particle destruction by sputtering as the dominant loss process, i.e. τ l = τ sput There are three conditions for starting an erosion and contamination chain reaction: There are three conditions for starting an erosion and contamination chain reaction: 1. the (particle life) time before wall impact τ ≤ τ sput 1. the (particle life) time before wall impact τ ≤ τ sput 2. the acceleration time to the critical velocity v crit must be ≤ τ 2. the acceleration time to the critical velocity v crit must be ≤ τ 3. the largest ejecta particle must have a mass m L ≥ M p + M sput 3. the largest ejecta particle must have a mass m L ≥ M p + M sput Morfill et al V. Could high velocity (Hi-V) dust particle impacts lead to a runaway effect?

34 34 Hi-V dust particle critical velocity – sputter losses: the asymptotic limit for v crit is 1.4 km/sec acceleration to velocities above the limits indicated by the lines would lead to an erosion + contami- nation chain reaction a p (μm) a p (μm) 8 (km/sec) v crit R sput τ = 1μm R sput τ = 2μm R sput τ = 0.5μm 7 1

35 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 τ l – τ m ≥ 0 runaway growth of erosion and contamination becomes unavoidable: Hi-V dust particles (if they exist) present a particular hazard for continuous reactor safety and operation – i.e. for τ l – τ m ≥ 0 runaway growth of erosion and contamination becomes unavoidable: Wall erosion will grow exponentially Wall erosion will grow exponentially M Erosion ~ exp[t ( τ l - τ m ) / τ l τ m ] M Erosion ~ exp[t ( τ l - τ m ) / τ l τ m ] The core plasma contamination by neutrals grows at the same rate The core plasma contamination by neutrals grows at the same rate S 0 (t) ~ exp[t ( τ l - τ m ) / τ l τ m ] S 0 (t) ~ exp[t ( τ l - τ m ) / τ l τ m ] Morfill et al. 2007

36 36 Neutral gas production 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: 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: M G / M p ≈ 1 – 10 M G / M p ≈ 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). 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 Morfill et al. 1983, Morfill et al VI. High velocity (Hi-V) dust particles as a source of neutrals

37 37 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· cm 2 ): 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· cm 2 ): ≈ 1 – 20 cm ≈ 1 – 20 cm Further core plasma contamination is then by diffusion (for a quick estimate use a linear model, x=0 to x=2R) Further core plasma contamination is then by diffusion (for a quick estimate use a linear model, x=0 to x=2R) ∂ n i / ∂ t – D ∂ 2 n i / ∂ x 2 = ∂ / ∂ x{S 0 (t) exp(-x/ )} ∂ n i / ∂ t – D ∂ 2 n i / ∂ x 2 = ∂ / ∂ x{S 0 (t) exp(-x/ )} Pindzola et al., 1995, Morfill et al. 1983, Morfill et al VI. High velocity (Hi-V) dust particles as a source of neutrals X=0 X=2R

38 38 The source term for neutral gas contamination is The source term for neutral gas contamination is ∂ / ∂ x{S 0 (t) exp(-x/ )} ∂ / ∂ x{S 0 (t) exp(-x/ )} the exp(-x/ ) represents the ionisation profile with ionisation length. the exp(-x/ ) represents the ionisation profile with ionisation length. The time dependent term S 0 (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 The time dependent term S 0 (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 S 0 (t)  m(t) S 0 (t)  m(t) where m(t) is given from dm/dt = m/ τ m – m/ τ l + δ (t), where m(t) is given from dm/dt = m/ τ m – m/ τ l + 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 ≥ τ acc the acceleration time.) 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 ≥ τ acc the acceleration time.) Morfill et al VI. High velocity (Hi-V) dust particles as a source of neutrals

39 39 Results for FTU were a surprise – they are completely compatible with the Hi-V particle impact physics estimates: 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 ~ cm -3. 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 ~ cm -3. The estimates based on UV spectroscopy also gave cm -3. The estimates based on UV spectroscopy also gave cm -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. 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. This, too, suggests a wall source (the walls are stainless steel), but not from sputtering, which should be too small. Morfill et al VI. High velocity (Hi-V) dust particles as a source of neutrals

40 40 Summary: Evidence for Hi-V particles in Tokamaks 1.Inferred evidence from impact craters on Langmuir probes in FTU. 2.Inferred evidence from impact generated plasma clouds in FTU. 3.Inferred evidence from core plasma contamination in FTU. 4.Theoretical investigations of particle acceleration are not conclusive: - critical velocities of ~ 2 km/sec - critical velocities of ~ 2 km/sec have not been obtained in the model have not been obtained in the model calculations calculations - there is a question whether the life - there is a question whether the life times of the particles may be times of the particles may be underestimated (use of SVP not underestimated (use of SVP not appropriate in a plasma environment) appropriate in a plasma environment)

41 41 Conclusion: Hi-V particles in Tokamaks Conclusion: Hi-V particles in Tokamaks FTU measurements have provided some initial evidence that dust in tokamaks may be highly accelerated. 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. 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… 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. 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… This scenario inevitably leads to an exponential growth in reactor contamination and wall erosion… …unless steps are taken to overcome this problem. …unless steps are taken to overcome this problem.

42 42 To identify the scope of this possible Hi-V problem, the following steps should be taken: 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. 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). 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. Develop solutions for reducing the dust production in critical areas in the reactor. Include the Hi-V dust issue in the reactor design considerations… Include the Hi-V dust issue in the reactor design considerations… …and start well before the ITER design freeze… …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. 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. Conclusion: Hi-V particles in Tokamaks

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

44 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: To answer this question let us summarise the evidence: 1. Direct measurements – what are Hi-V particle signatures? Can these particles be seen/captured? 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? 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? 3. Rocket effect – what is the role of this process? Morfill et al ?

45 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 (Geometrical) Flow Pressure +…+…+…+… etc. Scale the results to different Tokomaks

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

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

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

49 49 THANK YOU FOR YOUR ATTENTION

50 50 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). 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: 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. II. ´Dust Physics´ - High velocity (Hi-V) dust particles in the km/sec range

51 51 2. Hi-V dust particles: The gravity constraint A constraint: In order to achieve dust acceleration to velocities v 0 ≥ 1 km/s the azimuthal acceleration, ώ, must be sufficiently large so that gravity does not remove the particle first*: A constraint: In order to achieve dust acceleration to velocities v 0 ≥ 1 km/s the azimuthal acceleration, ώ, must be sufficiently large so that gravity does not remove the particle first*: v = Rώt ≥ v 0 and Δz = ½ gt 2 ≤ ΔH v = Rώt ≥ v 0 and Δz = ½ gt 2 ≤ ΔH Rώ/g ≥ v 0 /√2gΔH Rώ/g ≥ v 0 /√2gΔH For v 0 = 1 km/s and ΔH= 50cm this gives: For v 0 = 1 km/s and ΔH= 50cm this gives: Rώ/g ≥ 300, t ≤ 0.3 s Rώ/g ≥ 300, t ≤ 0.3 s Morfill et al ΔHΔHΔHΔH *Possible electrostatic, photophoretic, thermophoretic etc. forces not yet considered.

52 52 2. Hi-V dust Particles: Trajectory calculations – Plasma Plots (electron temperature - eV) MASTITER From James Martin (2006)

53 53 4. Hi-V dust particles present a particular hazard for reactor safety and operation – sputter losses: A short numerical example: A short numerical example: We use a typical sputter loss rate R sput of 10μm/sec*. We use a typical sputter loss rate R sput of 10μm/sec*. From condition 3 we require that the largest ejecta particle mass is: From condition 3 we require that the largest ejecta particle mass is: m L ≈ 0.1 M E ≥ M p + M sput m L ≈ 0.1 M E ≥ M p + M sput From the empirical relation** M E / M p ≈ 5v 2 (v in km/sec) From the empirical relation** M E / M p ≈ 5v 2 (v in km/sec) we get after some algebra m L / M E = 0.5v 2 ≥ (1+ R sput τ /a p ) 3 we get after some algebra m L / M E = 0.5v 2 ≥ (1+ R sput τ /a p ) 3 For e.g. R sput τ = 1μm and a p = 1μm we obtain the minimum condition for self-sustained erosion growth (and plasma contamination) For e.g. R sput τ = 1μm and a p = 1μm we obtain the minimum condition for self-sustained erosion growth (and plasma contamination) v crit = 4.0 km/sec v crit = 4.0 km/sec provided the acceleration time is shorter than τ (= 0.1 sec). provided the acceleration time is shorter than τ (= 0.1 sec). *J. Martin, 2007, ** Gault 1963; Dohnanyi 1969; Gault and Wedekind 1969

54 54 Summary – FTU measurements and impact physics calculations Conservative estimates for the growth time, τ m (FTU), are – sec. This is shorter than the sputter loss (or life) time for a 1μm particle (typically 100msec in the SOL). Conservative estimates for the growth time, τ m (FTU), are – sec. This is shorter than the sputter loss (or life) time for a 1μm particle (typically 100msec in the SOL). These same conservative estimates have shown that for a 1 second FTU reactor operation, the particle (and neutral gas) contamination increases by a factor These same conservative estimates have shown that for a 1 second FTU reactor operation, the particle (and neutral gas) contamination increases by a factor This is based on the impact rate measurements deduced using Langmuir probe measurements and crater counts*. This is based on the impact rate measurements deduced using Langmuir probe measurements and crater counts*. This result is consistent with the typical measured Fe contamination of the main plasma in FTU**. This result is consistent with the typical measured Fe contamination of the main plasma in FTU**. *Castaldo et al, 2007, **Ratinskaya, priv. comm., Morfill et al, 2007

55 TeTeTeTe M(t)/M Hi-V dust Particles: The ´Rocket Effect´ veveveve vpvpvpvp Condition that ejecta mass is larger than initial particle mass, M 0, is satisfied provided sufficiently high velocities are reached (ie. Provided enough mass has been ´evaporated´). M E ≥M 0 Fe particles Fe particles

56 56 The ´rocket effect´ is seen in the data. Particles are accelerated away from ´hot´ regions due to photosputtering, photophoresis, anisotropic impact sputtering (in steep gradients) etc. This effect tends to confine particles to the cooler SOL. The ´rocket effect´ is seen in the data. Particles are accelerated away from ´hot´ regions due to photosputtering, photophoresis, anisotropic impact sputtering (in steep gradients) etc. This effect tends to confine particles to the cooler SOL. The equations to be solved are: The equations to be solved are: dp/dt = v p dM/dt + Mdv p /dt = v e dM/dt and dM/dt = -R 0 (M/M 0 ) α Am p dp/dt = v p dM/dt + Mdv p /dt = v e dM/dt and dM/dt = -R 0 (M/M 0 ) α Am p with α = 2/3 and v e = (2kT e /Am p ) ½ with α = 2/3 and v e = (2kT e /Am p ) ½ R 0 is the initial atomic erosion rate (particle mass Am p ) R 0 is the initial atomic erosion rate (particle mass Am p ) 3. Hi-V dust Particles: The ´Rocket Effect´ veveveve vpvpvpvp

57 57 The solution of these equations yield: The solution of these equations yield: Particle velocity evolution: v p = v e [M 0 /M(t)-1] Particle velocity evolution: v p = v e [M 0 /M(t)-1] Characteristic time scale:  = M 0 /[R 0 Am p (1- α )] Characteristic time scale:  = M 0 /[R 0 Am p (1- α )] Characteristic distance: x = 2v e M 0 /[R 0 Am p ] Characteristic distance: x = 2v e M 0 /[R 0 Am p ] Condition that on impact M E ≥ M 0 : M 0 /M(t) + M(t)/M 0 ≥ 2 + 1/(5v e 2 ) Condition that on impact M E ≥ M 0 : M 0 /M(t) + M(t)/M 0 ≥ 2 + 1/(5v e 2 ) with v e in km/sec. with v e in km/sec. 3. Hi-V dust Particles: The ´Rocket Effect´ veveveve vpvpvpvp

58 v p /v e 7 M(t)/M Hi-V dust Particles: The ´Rocket Effect´ veveveve vpvpvpvp v e corresponds typically to ´sputter temperatures´ of a few eV (for Fe particles a few km/sec).

59 59 Could there be high velocity (Hi-V) dust particles? To answer this question we proceed in the following way: To answer this question we proceed in the following way: 1. Direct measurements – what are Hi-V particle signatures? Can they be captured? 1. Direct measurements – what are Hi-V particle signatures? Can they be captured? 2. Trajectory calculations – can high velocities be reached? Where? How fast? Before the particle is sputtered away? What size particles are involved? 2. Trajectory calculations – can high velocities be reached? Where? How fast? Before the particle is sputtered away? What size particles are involved? Morfill et al ΔHΔHΔHΔH

60 60 Could there be high velocity (Hi-V) dust particles? Dust particles in the 10´s km/sec range would not have been detected with current observation programmes. Dust particles in the 10´s km/sec range would not have been detected with current observation programmes. Hi-V dust particles present a particular hazard for reactor safety and operation. Hi-V dust particles present a particular hazard for reactor safety and operation. Plasma azimuthal flow velocities are in this range. Plasma azimuthal flow velocities are in this range. If plasma drag were sufficiently effective, could dust particles be accelerated to corotation with the plasma and possibly reach 10s of km/sec? If plasma drag were sufficiently effective, could dust particles be accelerated to corotation with the plasma and possibly reach 10s of km/sec?

61 61 Could there be high velocity (Hi-V) dust particles? In order to achieve dust acceleration to velocities v 0 ~ 10 km/s, the azimuthal acceleration, ώ, must be sufficiently large so that gravity does not remove the particle first: In order to achieve dust acceleration to velocities v 0 ~ 10 km/s, the azimuthal acceleration, ώ, must be sufficiently large so that gravity does not remove the particle first: v = Rώt ≥ v 0 and Δz = ½ gt 2 ≤ ΔH v = Rώt ≥ v 0 and Δz = ½ gt 2 ≤ ΔH Rώ/g ≥ v 0 /√2gΔH Rώ/g ≥ v 0 /√2gΔH For v 0 = 10 km/s and ΔH= 50cm this gives: For v 0 = 10 km/s and ΔH= 50cm this gives: Rώ/g ≥ 3000, t ≤ 0.3 s Rώ/g ≥ 3000, t ≤ 0.3 s Morfill et al ΔHΔHΔHΔH

62 62 IR camera measurements in ´MAST´ show that the dust mostly (co)rotates in the direction of plasma rotation. There is also visual evidence of the ´rocket effect´ acceleration. The dust particles may achieve high speeds – particle velocities in this movie are 10´s of m/sec. Provided by F. Lott and G. F. Counsell (2006) I. Introduction: Dust in Tokamaks

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67 67 Dust particle acceleration in Tokamaks – conclusions so far From James Martin (2006) 1.Particle acceleration to azimuthal velocities of km/sec seems possible (with ion drag Coulomb enhanced). 2.Particularly if the injection into the SOL has a substantial Rω already. 3.Particle life times against sputtering can be larger than the required acceleration times (a ≥ 2μm).

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70 70 MAST Parameters R = 1.0m, r = 0.5–0.65m B = 0.5T (on axis) I = 2MA (max)


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