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Mikhail M Tsventoukh

INITIATION of EXPLOSIVE ELECTRON EMISSION PULSES – ECTONS as INITIATION of VACUUM DISCHARGE STAGES – the BREAKDOWN, the SPARK, and the ARC. Mikhail M Tsventoukh. Lebedev Physical Institute of the Russian Academy of Science. Gennady A Mesyats and Sergey A Barengolts.

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Mikhail M Tsventoukh

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  1. INITIATION of EXPLOSIVE ELECTRON EMISSION PULSES – ECTONS as INITIATION of VACUUM DISCHARGE STAGES – the BREAKDOWN, the SPARK, and the ARC Mikhail M Tsventoukh Lebedev Physical Institute of the Russian Academy of Science Gennady A Mesyats and Sergey A Barengolts Lebedev and Prokhorov Institutes of the RAS

  2. OUTLINE • Vacuum discharge phenomenology • Explosive electron emission properties • Explosive emission initiation at a ‘clean’ and ‘contaminated’ – film-like surface • Résumé

  3. VACUUM DISCHARGE means… …that there is no media (e.g. gas) between the electrodes that suffice to transfer desirable current The formation of plasma from the electrode material ‘ explosive ’ electron emission ‘ normal ’ emission anode Anode flare e Cathode flare VACUUM BREAKDOWN VACUUM SPARK VACUUM ARC cathode 1 A – 1 kA 10 kV – 100 kV 1 A – 10 MA 10 – 100 V 0.1 – 1 A 10 kV – 1 MV

  4. 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 ARC spark

  5. Why the Explosive e-emission ‘is needed’ ? Common electron emission (thermo-, field-, photo-, secondary, etc.) – VAPORIZATION Explosive electron emission – BOILING nonstationary, local character Explosive Electron Emission (EEE) Pulse from the Explosive Center produces the electron bunch – ECTON There is a positive feed-back for the Joule energy release leads to the explosion • EEE Ignition due to: • Needle electrical explosion • Dielectric film breakdown (triple point) • Micro-particle impact • Plasmaand particle flux • Energy flux (laser) • etc… General requirement – energy concentration (~10 kJ/g) in microvolumes at the surface

  6. explosive electron emission issues… • Pulsed power, generatorsof e-beams, x-rays, microwaves A considerable advance in the development of ns and ps pulsed power supplies [G. A. Mesyats “Pulsed Power”, New York: Kluwer Academic/Plenum (2004)] • 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)]

  7. The principal scheme of the EEE DEVICES U(t) A power C I(t) tdelay Prebreakdown current density The delay of the explosive electron emission The current growth due to the plasma expansion

  8. THE EMISSION CENTERS a b c d e f g h i 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)

  9. Sphere on a tip Ellipsoid ELECTRIC FIELD «ENHANCEMENT» Electric field strength enhancement E = E0 Geometrical enhancement at the protrusion tips Effective enhancement in the dielectric films conduction band eV band gap Energy spectrum valence band The enhancement factor up to few hundreds is achievable=>films breakdown or microprotrusions explosion

  10. 1 A FIELD-EMISSION – EXPLOSIVE TRANSITION 9.23 kV I(t) Field Emission Most well interpreted (both theoretically and experimentally) is the field-emission – explosive-emission transition 0 +600 V Explosive Emission Pulse 10 ns 9.83 kV texpl 0 2) Ohmic heating by emission current (thermofieldemission starts) 1) Potential applied to the needle cathode (field emission starts) 3) Explosion of a needle (plasma formation, emission from plasma)

  11. EXPLOSIVE ELECTRON EMISSION Electron emission “from plasma” is much more effective as ideal plasma means ~1 je,pl (1014cm-3,10eV) ~ 0.8kA/cm2 ~ jem,Therm(3.6kK, 4.5eV) From the electrical wire-explosion physics the explosion time: texpl h/j2 j, A/cm2 j2texplconst texpl, ns For most of metals (experimental) hexpl(1 – 4) 109 A2 cm-4 s E, 108 V/cm

  12. EXPLOSIVE EMISSION CENTER PLASMA THE PLASMA DENSITY n ~ 1020 – 1021cm-3 THE TEMPERATURE Te ~ Ti ~ 2-4eV Anders et al, 1992 IEEE Trans. Plasma Sci.20 466-472 Total current >1 A, size ~1m,hence j ~ 1А/1m2 =108 A/cm2 P/n > 20 eV, (plasma expansion velocity vpl > 106 cm/s) THE TIMESCALE 100 ns 15 V t ~ 10 ns Puchkarev and Murzakaev1990 JPhysDApplPhys23 26

  13. NEEDLE EROSIONAT EXPLOSIVE EMISSION Emission zone If the explosion destruction of the needle material slows down as the destruction rate falls down to thermal diffusivity velocity: Liquid phase Solid phase and one can derive: the crater radius, re, the burning time, te ~ 10ns ~ 10–4cm eroded mass, Me; the charge, qe; and the specific erosion rate: ~ 0.1pg ~ 1011e

  14. EXPLOSIVE EMISSION CENTER PLASMA Then such plasma of explosive center touches the surface ED~ 19 – 85 MV/cm Ewall = ED(eU/Te)1/4 ~ ~ 30 – 150 MV/cm ji,Bohm > 1– 10 MА/см2 qpl > (20eV)  ji,Bohm ~ ~ 20 – 200 MW/cm2 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) Ignition – due to energy concentration in microvolume Extinction – due to plasma splashing (acceleration) and thermal conductivity The ecton “charge” qe=i∙te ~1011 electrons

  15. INITIATION BY PLASMA ACTION For plasma: 1020cm-3, 4eVnew explosion withint~10 ns has been shown numerically • 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 PlasSci31 822; Barengolts, Mesyats, Tsventoukh 2008 JETP107 1039]

  16. INITIATION BY PLASMA ACTION explosive overheating of a surface microprotrusion by volume Joule energy release, whereas the surface fluxes likely being balanced

  17. INITIATION BY PLASMA ACTION The incident energy flux, q > ~200 MW/cm2, has been found to be the threshold for the explosive overheating The ecton plasma density and temperature 1020–1021cm-32–4eV gives the energy flux above the required for the explosion threshold

  18. INITIATION by PLASMA ACTION (1) Explosive Joule overheating of inhomogenity by emission current : d2T/dt2>0, dJ/dt>0,at exceeding of the energy-flux threshold q > 200 MW/cm2 (2) Emission current density being restricted by emitted electron space charge at jem < jM j3/2 =>jM • For initiation the dense plasma is required: • > 1018 cm-3 (3) Large incident energy flux q> 200 MW/cm2 produces the dense erosion plasma within the short time, hence provides the explosion

  19. ON THE EMISSION AND PLASMA SHEATH Mackeown eq. (1929)gives a near-wall electric field: Generalized Mackeown-like eq.[JETP107 1039 (2008)] results in 1) consistent calculation of E and jem plasmaemission Ke= [0, Tsurf.] 2) the restriction of current density by a space charge (the analogy of 3/2 law) This means that the stable current density emitted into the plasma cannot exceed that for the plasma electrons, je,pl

  20. ON THE EMISSION AND PLASMA SHEATH The energy flux from the plasma to the surface from U/Te Electrons heat surface much more intensely and space-charge restriction at U/Te ~ 2 isn’t too strong

  21. ARCING IN POWERFUL EXPERIMENTS Power threshold 200 MW/cm2 agrees with experiments Intense energy flux experiments (107 – 109 W/cm2): A. Maitland, 1961 Journal of Appl. Phys.32 2399 K. Vogel and P. Backlund 1965 Journal of Appl. Physics36 3697 G. A. Mesyats and V. I. Eshkenazi, 1968 Izv. Vyssh. Uchebn. Zaved., Fiz.2 123 J. K. Tien, N. F. Panayotov, R. D. Stevenson, R.D. Stevenson and R.A. Gross 1978 J. Nucl. Mater.76–77 481 F.R. Schwirzke and R.J. Taylor, 1980 Journal of Nuclear Materials93-94 780 F.R. Schwirzke, 1991 IEEE Trans. Plasma Sci. 19 690 10m Initiation of arcing in tokamaks by dust particle impact S.I.Krasheninnikov, 2007 private communication Barengolts, Mesyats, Tsventoukh 2008 JETP107 1039

  22. SOME FEATURES of the VACUUM DISCHARGE THAT NATURALLY BELONGS to the EXPLOSIVE ELECTRON EMISSION (EEE)

  23. THERE ARE THE CATHODE SPOT CELLS Arc trace of cathode spot of first and second type [Bochkarev and Murzakaev, Proc. XVIIIth ISDEIV244(1988)] 1st type, Au, 5 A 2nd type, W, 23 A High speed photos of arc cathode spot (Cu, 30 A). Juttner B J. Phys. D.: Appl. Phys.34 R103 (2001)

  24. ANOMALOUS IONS FROM CATHODE SPOT Tanberg (1930) has found the particles flux of about tens eV Pluttoet 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. 45 1135 (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

  25. ANOMALOUS accelerated IONS from spark Pluttoet al1960s U, kV I, kA t, 200 ns The ions W+(1), W2+ (2), W3+ (3), W4+ (4), W5+ (5), W6+ (6), W7+ (7), О+ (8), С+ (9), С2+ (11), С3+ (12) Additional acceleration at cathode flare plasma

  26. There are the cathode spot cell CYCLES 10 ns Spectrum of the arc current that exhibits a maxima at some tenth of GHz [André Anders and Efim Oks 2006 J. Appl. Phys. 99 103301] Waveforms of arc voltage, current and light intensity for tungsten cathode [Maxim Bochkarev and Igor Uimanov, Proceedings of XXth ISDEIV]

  27. There are the cathode spot cell CYCLES U ic Ua ec i 100 V 50 ns 2 A arc on tungsten; arc voltage waveforms [V.F. Puchkarev, A.M. Murzakaev J. Phys. D.: Appl. Phys. 23 26 (1990)]

  28. ‘MOTION’ OF CELLS spark vdrift ~ 106 cm/s The direction of motion is JB arc vdrift ~ 104 cm/s – 105 cm/s (for clean and contaminated surf.) The direction of motion is BJ

  29. ROLE OF EASILY ERODING MICROSTRUCTURE

  30. Plasma Anode e i e i e e e Cathode spot Cathode spot Wall Cathode UNIPOLAR ARCS The current circulates between the external plasma and the wall Robson and Tonemann 1959 it is necessary Ufl> Uc~10-30 V (Te > 5 eV) A common phenomenon for fusion devices [V. Rohde et al., 2010 19th Int. Conf. on Plasma Surface Interactions] Asdex-U Divertor plates McCracken and Goodall Nucl. Fus. 1978

  31. UNIPOLAR ARCS –VACUUM ARCS “…there is little information on unipolar arcs …cathode processes are the same as in vacuum arc” [McCracken and Stott 1979Nucl. Fusion19 889] “Uncertainties in characterizing the nature and quantity of surface protrusions makes predictive modeling of (unipolar) arcs quite difficult” [Federici et al. 2001Nucl. Fusion41 1968] “…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” 2007Nucl. Fusion47 S203] Micro-craters; retrograde motion; plasma MHD-activity and relief influence on initiation etc… UNIPOLAR ARC => VACUUM ARC Unipolar arc cathode spot functioning by explosive electron emission, as in vacuum arc cathode spot [Mesyats 1984J. Nucl. Mater.128&129 618] VACUUM ARC => UNIPOLAR ARC

  32. 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 (~1 MW/cm2) due to ELMs (edge localized modes) • Surface fine structure, i.e. • W-deposited films (ASDEX-Upgrade tiles) • Layers of W-fuzz • Liquid Li films on a capillary structure MAST (megaamp. spherical tokamak)

  33. ELMs – ARCING OBSERVATION in ASDEX-U The ELMs – arcing correlation has been observed in novel experiments at AUGtokamak(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

  34. 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. Fusion49 032002] • Positive drop of potential for duration 2.8ms >> tlaser = 0.6 ms • W-atom glow motion • Arcing traces

  35. Laser Laser He-plasma sheath He-plasma sheath He-plasma sheath He-plasma sheath He+ e He+ e He+ e He+ e B B B B j j j ECTON MECHANISM OF UNIPOLAR ARC… Arc ignition and burning at W-fuzz according the ecton model[Barengolts, Mesyats, Tsventoukh 2010 Nucl. Fusion50 125004] IGNITION EXPLOSION IGNITION EXPLOSION a) b) d) c) e e W-plasma e ld ~1μm Bulk W rex(t) rex≈ld dc Arc burning within the W-fuzz layer, (similarly to the film cathodes) Crater diameter Cell time an-w= 5∙10–3 cm2/s << a0 = 0.64 cm2/s

  36. UNIPOLAR ARCING at W-FUZZ in LHD Unipolar ignition at W-fuzz surface by plasma action onto the Large Helical Device (LHD) divertor plates[M Tokitani S Kajita, S Masuzaki, Y Hirahata, N Ohno, T Tanabe and LHD Experiment Group 2011 Nucl. Fusion51 102001] Arc burning within the W-fuzz layer [Barengolts, Mesyats, Tsventoukh 2010 Nucl. Fusion50 125004]

  37. ARCING AT LIQUID Li Capillary-porous structure (e.g.W,Mo, SS), filled by a liquid… …gallium (Ga, Ga-In)– advanced explosive emission cathodes …lithium(Li)– advanced surface for a fusion devices first wall S V Mirnov et al 2006 Plasma Phys. Control. Fusion48821 D I Proskurovsky2008 «Dyke Award» Lecture23rdISDEIV • Only liquid metal film erosion • No craters as the surface is a liquid

  38. Laser Laser He-plasma sheath He-plasma sheath He-plasma sheath He-plasma sheath He+ e He+ e He+ e He+ e B B B B j j j Ufl ic Ua ec i IGNITION of ECTONs Ignition – initiation of explosive electron emission at the W-fuzz ignition explosion ignition explosion a) b) d) c) e e W-plasma e ld ~1μm Bulk W rex(t) rex≈ld dc

  39. FINE-STRUCTURED SURFACE (W-FUZZ) • Dense erosion plasma producing • Intense electron emission buildup • Joule explosion of nanowires layer • [Barengolts, Mesyats, Tsventoukh 2010 Nucl. Fusion50 125004; 2011 IEEE Trans. Plas. Sci.39 1900] W plasma

  40. 1) Dense erosion plasma producing Intense fast erosion of W-nanowires under the plasma (laser) qv = qincident Wire cooling by vaporization balanced with incident flux: one gives specific temperature, the wire-erosion time and the atomic density <ve·ion>~10-8 cm3/s Charged particles balance with ionization by secondary electrons Ionization rate 3-body recombination rate (being significant for lgne >22 [cm-3]) rec ~ 2∙10–30·ne cm3/s One can simplify charged particles balance as One can estimate the ionization time tion to be ~10 ns for (ne2+npl) ~1016 cm-3 The vapor- and erosion W-plasma density substantially exceeds the “incident” plasma density

  41. 2) Intense electron emission buildup Plasma near-wall field Space-charge limiting current density Incident energy flux q ~ 1 MW/cm2(ELM type-I)leads to ED≈ 107 V/cm; Ewall≈ 2·107 V/cm (plate potential U = 60 V) jMlarger than ~10–100 MA/cm2 Emission current density space charge limited value in the erosion plasma being much larger than in background plasma (1013 cm-3, 10 eV) : jM,He-pl ~102 A/cm2 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 TPS37 1375]

  42. 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 ~5.5-7.7 kK within tens of ns The corresponding emission current density (according to Richardson-Schottky) will be of about ~10 – 100 MA/cm2≤jM For a network-like structure geometric enhancement of a flowing current density arises W plasma e Tenfold enhancement of j, up to ~ 0.1 – 1 GA/cm2, leads to the fast Ohmic explosions of W-nanowires nodes within texpl ~ h/j2 ~ 1 – 100 ns that means a fast explosive erosion buildup

  43. FINE-STRUCTURED SURFACE (‘W-fuzz’) • Dense erosion plasma producing • Intense electron emission buildup • Joule explosion of nanowires layer • [Barengolts, Mesyats, Tsventoukh 2011 IEEE Trans. Plas. Sci.39 1900] • 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 spotsof 1st type (at a contaminations)

  44. TRANSITION of SPOT CELLS 1ST to 2ND TYPE 2nd type 2nd type 1st type 1st type Jakubka and Juttner 1981 JNM 102 259-266

  45. ‘FINE-STRUCTURE’ EFFECT on ARCING A new power threshold qthreshold,1 << q0 q > q0 ~200 MW/cm2 • Film-structure of surface absorbs the incident energy (positive) • The condition for explosive electron emission (arcing) arise at a lower power threshold (negative)

  46. RÉSUMÉ • 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 • FAST IGNITION implies FAST BURNING OUT

  47. THANK YOU VERY MUCH FOR YOUR ATTENTION M.B. Bochkarev, A.M. Murzakaev, Proc. XVIIIth ISDEIV 244 (1988)

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