Mikhail M Tsventoukh Lebedev Physical Institute of the Russian Academy of Science INITIATION of EXPLOSIVE ELECTRON EMISSION PULSES ECTONS as INITIATION of VACUUM DISCHARGE STAGES the BREAKDOWN, the SPARK, and the ARC Gennady A Mesyats and Sergey A Barengolts Lebedev and Prokhorov Institutes of the RAS Vacuum discharge phenomenology Explosive electron emission properties Explosive emission initiation at a clean and contaminated film-like surface Rsum OUTLINE that there is no media (e.g. gas) between the electrodes that suffice to transfer desirable current VACUUM DISCHARGE means anode cathode e normal emission VACUUM BREAKDOWN explosive electron emission VACUUM SPARK VACUUM ARC The formation of plasma from the electrode material 1 A 1 kA 10 kV 100 kV 0.1 1 A 10 kV 1 MV 1 A 10 MA 10 100 V Cathode flare Anode flare Very first photos of VACUUM DISCHARGE Typical photos of the luminescence in a vacuum gap taken at different stages of breakdown G. A. Mesyats and D. I. Proskurovsky Pulsed Electrical Discharge in Vacuum 1989 (Berlin: Springer Verlag) ARC break spark Explosive Electron Emission (EEE) Pulse from the Explosive Center produces the electron bunch ECTON Why the Explosive e-emission is needed ? Common electron emission (thermo-, field-, photo-, secondary, etc.) VAPORIZATION Explosive electron emission BOILING nonstationary, local character There is a positive feed-back for the Joule energy release leads to the explosion EEE Ignition due to: Needle electrical explosion Needle electrical explosion Dielectric film breakdown (triple point) Dielectric film breakdown (triple point) Micro-particle impact Micro-particle impact Plasma and particle flux Plasma and particle flux Energy flux (laser) Energy flux (laser) etc etc General requirement energy concentration (~10 kJ/g) in microvolumes at the surface explosive electron emission issues Pulsed power, generators of e-beams, x-rays, microwaves Physical mechanism of the vacuum discharge the breakdown, the spark, and the arc non-steady-state and local character of electronic emission from the cathode, which occurs in the form of collective strong emission pulses ectons [G. A. Mesyats Cathode Phenomena in a Vacuum Discharge Moscow, Nauka (2000) ; A. Anders Cathodic Arcs Springer (2008) ] A considerable advance in the development of ns and ps pulsed power supplies [G. A. Mesyats Pulsed Power, New York: Kluwer Academic/Plenum (2004) ] U(t)U(t) I(t)I(t)t delay A C power Prebreakdown current density The delay of the explosive electron emission The current growth due to the plasma expansion The principal scheme of the EEE DEVICES bc ef g a d ih Microprotrusions (a); dielectric inclusions (b); oxide and other inorganic dielectric films (c); adsorbed gas layers (d); grain boundaries emerged at the surface (e); microparticles (f); oil vapor cracking products (g); edges of craters formed upon breakdowns (h); pores and cracks (i) THE EMISSION CENTERS ELECTRIC FIELD ENHANCEMENT Ellipsoid Sphere on a tip The enhancement factor up to few hundreds is achievable => films breakdown or microprotrusions explosion Geometrical enhancement at the protrusion tips Effective enhancement in the dielectric films Electric field strength enhancement E = E 0 valence band conduction band band gap eV Energy spectrum FIELD-EMISSION EXPLOSIVE TRANSITION Field Emission 9.23 kV 9.83 kV 10 ns t expl 0 0 Explosive Emission Pulse 1 A +600 V I(t)I(t) Most well interpreted (both theoretically and experimentally) is the field-emission explosive-emission transition 1) Potential applied to the needle cathode (field emission starts) 2) Ohmic heating by emission current (thermofieldemission starts) 3) Explosion of a needle (plasma formation, emission from plasma) EXPLOSIVE ELECTRON EMISSION as ideal plasma means From the electrical wire-explosion physics the explosion time: t expl h/j 2 j 2 t expl const For most of metals (experimental) h expl (1 4) 10 9 A 2 cm -4 s h expl (1 4) 10 9 A 2 cm -4 s j e,pl (10 14 cm -3,10 eV) ~ 0.8 kA/cm 2 ~ j em,Therm (3.6 kK, 4.5 eV) ~1 Electron emission from plasma is much more effective j, A/cm 2 E, 10 8 V/cm t expl, ns Total current >1 A, size ~1 m, hence j ~ 1 /1 2 = 10 8 A/cm 2 j ~ 1 /1 m 2 = 10 8 A/cm 2 EXPLOSIVE EMISSION CENTER PLASMA THE PLASMA DENSITY n ~ cm -3 THE TEMPERATURE T e ~ T i ~ 2-4 eV P/n > 20 eV, ( plasma expansion velocity v pl > 10 6 cm/s) 15 V 100 ns THE TIMESCALE t ~ 10 ns Anders et al, 1992 IEEE Trans. Plasma Sci Puchkarev and Murzakaev 1990 JPhysD ApplPhys 23 26 NEEDLE EROSION AT EXPLOSIVE EMISSION Emission zone Liquid phase Solid phase If the explosion destruction of the needle material slows down as the destruction rate falls down to thermal diffusivity velocity: the crater radius, r e, the burning time, t e eroded mass, M e ; the charge, q e ; and the specific erosion rate: and one can derive: ~ 10 4 cm ~ 10 ns ~ 0.1 pg ~ e EXPLOSIVE EMISSION CENTER PLASMA New explosion occurs within the tens of ns Therefore, we have fast explosive pulse of electron emission from the emission center ECTON (as Explosive Cycle) The ecton charge q e =it e ~10 11 electrons Ignition due to energy concentration in microvolume Extinction due to plasma splashing (acceleration) and thermal conductivity E D ~ 19 85 MV/cm E wall = E D (eU/T e ) 1/4 ~ ~ 30 150 MV/cm j i,Bohm > 1 10 M/ 2 q pl > (20eV) j i,Bohm ~ ~ 20 200 MW/cm 2 Then such plasma of explosive center touches the surface INITIATION BY PLASMA ACTION Numerical modeling has been performed for the plasma action onto the wall having a microprotrusion with taking into account -thermo-field-emission, -2D thermal balance, -heating by incident plasma, -sheath properties [ ] [ Uimanov 2003 IEEE Trans Plas Sci ; Barengolts, Mesyats, Tsventoukh 2008 JETP ] For plasma: cm -3, 4 eV new explosion within t ~ 10 ns has been shown numerically INITIATION BY PLASMA ACTION explosive overheating of a surface microprotrusion by volume Joule energy release, whereas the surface fluxes likely being balanced INITIATION BY PLASMA ACTION The incident energy flux, q > ~200 MW/cm 2, has been found to be the threshold for the explosive overheating The ecton plasma density and temperature 10 21 cm -3 24 eV gives the energy flux above the required for the explosion threshold INITIATION by PLASMA ACTION j 3/2 => j M (2) Emission current density being restricted by emitted electron space charge at j em < j M For initiation the dense plasma is required: > cm -3 (3) Large incident energy flux q > 200 MW/cm 2 produces the dense erosion plasma within the short time, hence provides the explosion (1) Explosive Joule overheating of inhomogenity by emission current : d 2 T/dt 2 >0, dJ/dt>0, at exceeding of the energy-flux threshold q > 200 MW/cm 2 ON THE EMISSION AND PLASMA SHEATH Mackeown eq. (1929) gives a near-wall electric field: Generalized Mackeown-like eq. [JETP (2008)] plasmaemission K e = [0, T surf. ] This means that the stable current density emitted into the plasma cannot exceed that for the plasma electrons, j e,pl results in 1) consistent calculation of E and j em 2) the restriction of current density by a space charge (the analogy of 3/2 law) ON THE EMISSION AND PLASMA SHEATH The energy flux from the plasma to the surface from U/T e Electrons heat surface much more intensely and space-charge restriction at U/T e ~ 2 isnt too strong ARCING IN POWERFUL EXPERIMENTS Intense energy flux experiments (10 7 10 9 W/cm 2 ): A. Maitland, 1961 Journal of Appl. Phys K. Vogel and P. Backlund 1965 Journal of Appl. Physics G. A. Mesyats and V. I. Eshkenazi, 1968 Izv. Vyssh. Uchebn. Zaved., Fiz J. K. Tien, N. F. Panayotov, R. D. Stevenson, R.D. Stevenson and R.A. Gross 1978 J. Nucl. Mater. 76 F. R. Schwirzke and R. J. Taylor, 1980 Journal of Nuclear Materials F. R. Schwirzke, 1991 IEEE Trans. Plasma Sci Initiation of arcing in tokamaks by dust particle impact S. I. Krasheninnikov, 2007 private communication Barengolts, Mesyats, Tsventoukh 2008 JETP Power threshold 200 MW/cm 2 agrees with experiments 10 m SOME FEATURES of the VACUUM DISCHARGE THAT NATURALLY BELONGS to the EXPLOSIVE ELECTRON EMISSION (EEE) SOME FEATURES of the VACUUM DISCHARGE THAT NATURALLY BELONGS to the EXPLOSIVE ELECTRON EMISSION (EEE) THERE ARE THE CATHODE SPOT CELLS High speed photos of arc cathode spot (Cu, 30 A). Juttner B J. Phys. D.: Appl. Phys. 34 R103 (2001) Arc trace of cathode spot of first and second type [Bochkarev and Murzakaev, Proc. XVIIIth ISDEIV 244 (1988)] 2 nd type, W, 23 A 1 st type, Au, 5 A ANOMALOUS IONS FROM CATHODE SPOT Tanberg (1930) has found the particles flux of about tens eV Plutto et al (1960) have found them to be the plasma flux Velocity distribution functions for a vacuum arc with Al (a) and Bi (b) cathodes. A.S. Bugaev et al. Zh. Tech. Fiz (2000) For all materials regardless to the ion charge (+1, +2, +5), to the current, etc., the velocity is in the range 10 20 km/cm The ions W + (1), W 2+ (2), W 3+ (3), W 4+ (4), W 5+ (5), W 6+ (6), W 7+ (7), + (8), + (9), 2+ (11), 3+ (12) ANOMALOUS accelerated IONS from spark Plutto et al 1960s Additional acceleration at cathode flare plasma U, kV I, kA t, 200 ns There are the cathode spot cell CYCLES Spectrum of the arc current that exhibits a maxima at some tenth of GHz [Andr Anders and Efim Oks 2006 J. Appl. Phys ] Waveforms of arc voltage, current and light intensity for tungsten cathode [Maxim Bochkarev and Igor Uimanov, Proceedings of XXth ISDEIV] 10 ns UaUa U icic ec ii 100 V 50 ns 2 A arc on tungsten; arc voltage waveforms [V.F. Puchkarev, A.M. Murzakaev J. Phys. D.: Appl. Phys (1990)] There are the cathode spot cell CYCLES MOTION OF CELLS arc v drift ~ 10 4 cm/s 10 5 cm/s (for clean and contaminated surf.) The direction of motion is B J spark v drift ~ 10 6 cm/s The direction of motion is J B ROLE OF EASILY ERODING MICROSTRUCTURE UNIPOLAR ARCS A common phenomenon for fusion devices [ V. Rohde et al., th Int. Conf. on Plasma Surface Interactions ] Asdex-U Divertor plates The current circulates between the external plasma and the wall Robson and Tonemann 1959 it is necessary U fl > U c ~ V (T e > 5 eV) McCracken and Goodall Nucl. Fus Cathode Anode e Cathode spot Wall Plasma e Cathode spot e i e UNIPOLAR ARCS VACUUM ARCS there is little information on unipolar arcs cathode processes are the same as in vacuum arc [McCracken and Stott 1979 Nucl. Fusion ] Uncertainties in characterizing the nature and quantity of surface protrusions makes predictive modeling of (unipolar) arcs quite difficult [Federici et al Nucl. Fusion ] there are no reliable experimental data on the discharge conditions leading to arcing or on the frequency of arc occurrence. [Loarte et al., Progress in the ITER Physics Basis. Chapter 4 2007 Nucl. Fusion 47 S203] UNIPOLAR ARC => VACUUM ARC VACUUM ARC => UNIPOLAR ARC Unipolar arc cathode spot functioning by explosive electron emission, as in vacuum arc cathode spot [Mesyats 1984 J. Nucl. Mater. 128& ] Micro-craters; retrograde motion; plasma MHD-activity and relief influence on initiation etc RECENT INTEREST FOR ARCING IN FUSION Two reasons for recent interest in arcing and, in general, in collective plasma-surface interactions Large transient energy flux ( ) due to ELMs Large transient energy flux (~1 MW/cm 2 ) due to ELMs (edge localized modes) MAST (megaamp. spherical tokamak) Surface fine structure, i.e. W-deposited films (ASDEX-Upgrade tiles) Layers of W-fuzz Liquid Li films on a capillary structure ELMs ARCING OBSERVATION in ASDEX-U The ELMs arcing correlation has been observed in novel experiments at AUG tokamak ( with both temporal and spatial resolutions ) [ ] [ ASDEX Upgrade Team, 2011 J. Nucl. Mater. 415 S46 ] The surface a few m-layer of W, deposited onto the carbon tiles Arcing erosion of the W-film PROMPT IGNITION OF A UNIPOLAR ARC Unipolar arc ignition at tungsten network-like nanostructured layer (W-fuzz) under the ELM modeling laser action [NAGDIS- II] [ S Kajita S Takamura N Ohno 2009 Nucl. Fusion ] 1)Positive drop of potential for duration 2.8 ms >> t laser = 0.6 ms 2)W-atom glow motion 3)Arcing traces ECTON MECHANISM OF UNIPOLAR ARC Arc ignition and burning at W-fuzz according the ecton model Arc ignition and burning at W-fuzz according the ecton model [ Barengolts, Mesyats, Tsventoukh 2010 Nucl. Fusion ] a) b) d) c) W-plasma He-plasma sheath B Laser He + e Bulk W l d ~1m j r ex l d e He-plasma sheath B He + e j He-plasma sheath B Laser He + e r ex (t) e He-plasma sheath B He + e j e dcdc IGNITIONEXPLOSIONIGNITIONEXPLOSION Crater diameter Cell time a n-w = 510 3 cm 2 /s 22 [cm -3 ]) 3-body recombination rate (being significant for lgn e >22 [cm -3 ]) One can simplify charged particles balance as One can estimate the ionization time t ion to be ~10 ns for (n e2 +n pl ) ~10 16 cm -3 2) Intense electron emission buildup Plasma near-wall field E D 10 7 V/cm; E wall 210 7 V/cm (plate potential U = 60 V) Emission current density space charge limited value in the erosion plasma being much larger than in background plasma (10 13 cm -3, 10 eV) : Emission current density space charge limited value in the erosion plasma being much larger than in background plasma (10 13 cm -3, 10 eV) : j M,He-pl ~10 2 A/cm 2 Incident energy flux q ~ 1 MW/cm 2 (ELM type-I) leads to j M larger than ~10100 MA/cm 2 Fast heating of W-plasma electrons by the emitted electron beam is possible [ similar feature has been revealed in a vacuum spark modeling: ] Fast heating of W-plasma electrons by the emitted electron beam is possible [ similar feature has been revealed in a vacuum spark modeling: Shmelev, Barengolts 2009 IEEE TPS ] Space-charge limiting current density 3) Joule explosion of nanowires layer Energy flux from incident ELM plasma (laser) and from the erosion one is sufficient to heat up the rest of nanowires up to ~ kK within tens of ns Tenfold enhancement of j, up to ~ 0.1 1 GA/cm 2, leads to the fast Ohmic explosions of W-nanowires nodes within t expl ~ h/j 2 ~ 1 100 ns that means a fast explosive erosion buildup W plasma The corresponding emission current density ( according to Richardson-Schottky ) will be of about ~10 100 MA/cm 2 j M The corresponding emission current density ( according to Richardson-Schottky ) will be of about ~10 100 MA/cm 2 j M For a network-like structure geometric enhancement of a flowing current density arises e FINE-STRUCTURED SURFACE (W-fuzz) 1)Dense erosion plasma producing 2)Intense electron emission buildup 3)Joule explosion of nanowires layer [ ] [ Barengolts, Mesyats, Tsventoukh 2011 IEEE Trans. Plas. Sci ] Threshold energy flux becomes lower at least tenfold in contrast to the clean surface A surface fine-structure (film-like) promotes a dense erosion plasma production, and, hence. explosive electron emission ignition However, such a film burns-out faster Analogy cathode spots of 1 st type (at a contaminations) TRANSITION of SPOT CELLS 1 ST to 2 ND TYPE Jakubka and Juttner 1981 JNM st type 2 nd type 1 st type FINE-STRUCTURE EFFECT on ARCING A new power threshold q threshold,1 q 0 ~200 MW/cm 2 RSUM Intense plasma action onto the surface (in form of the transient power flux) as well as fine structure of surface (such as W-fuzz layers and other film-like structures) have been found to promote ignition of the explosive electron emission (ectons) Intense plasma action onto the surface (in form of the transient power flux) as well as fine structure of surface (such as W-fuzz layers and other film-like structures) have been found to promote ignition of the explosive electron emission (ectons) Easy erosion of such a film structures and the readiness of the explosive electron emission on them, in turn, indicates probably a lower specific erosion due to the arcing (e.g. less melting, and droplets) in comparison with the solid targets Easy erosion of such a film structures and the readiness of the explosive electron emission on them, in turn, indicates probably a lower specific erosion due to the arcing (e.g. less melting, and droplets) in comparison with the solid targets FAST IGNITION implies FAST BURNING OUT THANK YOU VERY MUCH FOR YOUR ATTENTION M.B. Bochkarev, A.M. Murzakaev, Proc. XVIIIth ISDEIV 244 (1988)