We have been extending and refining our model of vacuum breakdown and gradient limits and describe recent developments. The model considers a large number of mechanisms, but finds that vacuum arcs can be described fairly simply and self-consistently, however simulations of individual mechanisms can be involved, in some cases. Although based on accelerator rf data, we believe our model of vacuum arcs should have general applicability. The paper explores breakdown in plasmas, and self-sputtering and damage by parasitic arcs

Insepov, Z.

Moretti, A.

Norem, J.

English

Nazarbayev University Repository

VACUUM ARCS AND GRADIENT LIMITS∗Z. Insepov, J. Norem† , ANL, Argonne, IL 60439, USAA. Moretti, Fermilab, Batavia IL, 60615 USAAbstractWe have been extending and refining our model of vac-uum breakdown and gradient limits and describe recent de-velopments. The model considers a large number of mech-anisms, but finds that vacuum arcs can be described fairlysimply and self-consistently, however simulations of indi-vidual mechanisms can be involved, in some cases. Al-though based on accelerator rf data, we believe our modelof vacuum arcs should have general applicability. The pa-per explores breakdown in plasmas, and self-sputtering anddamage by parasitic arcs.OUTLINE OF THE MODELThe model we are describing assumes that breakdownevents are triggered by electric fields on the order of 10GV/m mechanically tearing asperities. Fragments fromthis event are then ionized by field-emitted beams, produc-ing a plasma. As the plasma density builds up, the Debyelength becomes smaller, causing increased surface field un-der the plasma, increasing the field emission and plasmaenergy and density. The plasma produces electrons that aredriven away by the driving field, leaving ions that rapidlystream away from where they were created. These ionsstream in all directions, producing high local heat fluxesthat melt the surface and produce a variety of surface dam-age .A general summary of the model with some details hasbeen presented in LINAC10 [1].This paper will cover extending this model to some as-pects of superconducting rf, and a variety of surface dam-age mechanisms not previously considered.E-BEAM WELDING PITSThe origin of small circular pits in the Heat AffectedZone (HAZ) of full penetration e beam welds has not beenidentified, although these pits can be a gradient limit in Su-perconducting RF (SRF) systems. We consider the possi-bility that these pits are caused by unipolar arcs that de-velop during the full penetration weld around the equatorof the rf structure.As shown by Schwirzke and Taylor [2], laser inducedplasmas on the surface of a metal can locally produce smallpits in the surface far away from the initial point of plasmaproduction. We have shown that the early evolution of avacuum arc seems to involve small, dense plasmas whosesheath potential is determined by particle kinematics, andwhose Debye length decreases to very small values [1].∗Work supported by USDOE/Office of High Energy Physics† norem@anl.govThe combination of a roughly constant sheath potential anda decreasing Debye length requires that the surface electricfield under the plasma increases to very high values. Webelieve that the environment around full penetration ebeamwelding, with a high flux of electrons leaving the surface,dense plasma and gas near the surface and high surfaceroughness due to particulates, could also produce sponta-neous unipolar arcs that would rapidly increase in densityand plasma pressure until they were able to produce the pitsseen in SRF structures.Figure 1: Pits in the heat affected zone of a full penetrationweld of niobium plate, from [3]. False color shows depth.The threshold for initiation of a self sustaining unipolararc driven by field emission currents can be derived fromthe condition for vacuum breakdown, Elocal 10 GV/m,in this case giving an expression for the required enhance-ment factor, β ∼ 10 GV/m / (φ/λD(n)), in terms of thesheath potential, φ, and Debye length, λD, and ion density,n. Assuming φ ∼ 75 V, and the dependence of the Debyelength as a function of plasma density, gives a relation be-tween the plasma density and local enhancement factors,β2n ∼ 2× 1024 m−3, that would produce a self sustainingunipolar arc driven by field emission.The evolution of these arcs would closely follow thatof normal vacuum arcs, with increasing density and sur-face electric field until the surface melted and began tobe released as particulates [1]. The plasma pressure onewould expect from these cold, dense plasmas is given by.p = nkT , where p, n, k and T are the pressure in Pa, den-sity in m−3, Boltzmanns constant, and the temperature indegrees K. For sheath potentials on the order of 100 eVand densities from 1024 m−3 this would produce pressuresin the range 10 MPa or larger, enough to produce craters ofthe required size in liquid metals.We plan to continue a modest experimental study ofProceedings of 2011 Particle Accelerator Conference, New York, NY, USA TUP049Accelerator TechnologyTech 28: Subsystems and Technology (Other) 895 Copyrightc ○2011byPAC’11OC/IEEE—ccCreativeCommonsAttribution3.0(CCBY3.0)these arcs.SELF-SPUTTERING: DEPENDENCE ONGRAIN ORIENTATIONIn order to understand the plasma material interactionit is necessary to understand the mechanisms of sputteringand melting at the atomic level. Since the sputtering yieldcoefficients are the atomistic characteristics of the plasmaion interaction with the surface, they can be calculated di-rectly by either Monte Carlo or molecular dynamics meth-ods. Experimental measurements show distinctly differentsputtering yields for Cu surfaces with different crystallo-graphic planes, when bombarded by Ar+ ions. This differ-ence cannot be explained by simply referring to differentbinding energies of the crystal planes since the binding en-ergy (and the density) of (100) is higher that that of a (110)plane, and the latter is higher than the energy of a Cu (111)plane.Figure 2: self-sputtering yield of copper low-index faces atlow energies.The sheath potential in unipolar arcs is on the order of10s of eV, depending on the geometry and other param-eters. In order to determine the dependence low energysputtering on crystal orientation, we have modeled the sput-tering process on copper for a variety of grain orienta-tions with molecular dynamics (MD). The evolution of theatomic system was determined by the solution of the set ofthe classical equations of motion for all the atoms in thesystem. The copper target was represented by crystalline(100), or other, substrates.Our MD simulation results, shown in Fig. 2 and 3, showthe scale of the dependence on grain orientation. We be-lieve that self sputtering could be a significant componentof surface damage away from the immediate center of thearc, where damage sometimes appears to be dependent onindividual grain properties. A variety of data seems to showFigure 3: Ratio of self-sputtering yields for Cu bombardedwith Ar and Cu ions.structures consistent with significant mass removal due tosputtering [4].TEMPERATURE OF BREAKDOWN SITESThe geometrical origin of field enhancements has beendebated for many years. As described in an earlier pa-per [1], we find that sharp corners associated with crackjunctions, and cone shaped asperities can produce the highenhancement factors seen in experimental measurements.Field emission would thus be produced only a very smallsources. Since the heated volume would be small ( nm3)and the volume of metal heat sinked, in a ns, to the emitteris on the order of 0.1 μm3, the time constant for coolingof the emitter would be on the order of a few 10 fs. Thus,these emitters would be difficult to heat.TYPES OF ARC DAMAGEArcs are produced in a large number of environmentsand a wide variety of arc damage has always been seen,complicating the understanding the mechanisms that areresponsible. In rf systems we find that arc behavior fallsbetween two extremes which we call, a) killer arcs, that areable to quickly short out the cavity energy and eliminatethe fields that drive them and, b) parasitic arcs, that exist inlow field regions of the structure and are able to operate forlong periods of time with minimal (perhaps undetectable)interactions with cavity operation. Examples of killer arcswould include the small pits seen in the iris region of Xband structures [ ] and parasitic arcs, which can operatefor a longer time can have the chicken track appearance ofclassic unipolar arcs commonly seen in tokamaks [2] Wefind that rf structures can produce killer arcs in an environ-ment with no magnetic field or with parallel E and B fields,but if the electric and magnetic field are perpendicular, thearcs are qualitatively different, producing much more visi-ble light and lasting for a longer time.We have also been able to identify what we believe to bedamage due to entirely parasitic arcs in the coupling unitTUP049 Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA896Copyrightc ○2011byPAC’11OC/IEEE—ccCreativeCommonsAttribution3.0(CCBY3.0)Accelerator TechnologyTech 28: Subsystems and Technology (Other)Figure 4: Plots of visible light emitted during a breakdownevent, a) with no magnetic field, and b) with a magneticfield perpendicular to the electric field. The perpendicularmagnetic field prevents the arc from discharging the cavity.for the 201 MHz rf cavity. This cavity operates with a pulselength on the order of 300 μs. These tracks, which wereproduced in a weak (0.3 T) magnetic field, are very similarto classic unipolar arc tracks in tokamaks, produced withhigher fields and longer pulses.Figure 5: Tracks of arc damage, far from the high fieldregion of the cavity. These tracks are 1 - 2 cm long.Examining this damage at high magnifications Sec-ondary Emission Microscope (SEM) shows considerablestructure at high magnification, however at low magnifica-tions there seemed to be little visible structure or contrast.The arc damage was, in fact, more visible by naked eyethen in low power SEM images.CONCLUSIONSWe continue to extend our model of breakdown and gra-dient limits to other damage mechanisms and types of arcs.We find that unipolar arcs can be produced by electronbeams in the absence of any external electric field. We areFigure 6: SEM images of arc damage (enlargement of Fig.5). The surface was dominated by structure on the scaleof a few hundred nm, and at lower magnifications showedcomparatively little structure or contrast.exploring sputtering as an explanation for many of the dam-age types in rf structures. We can identify a range of arcsfrom killer arcs, that short out their driving fields, to par-asitic arcs, that burn without affecting the overall system.We describe the damage seen in these parasitic arcs.ACKNOWLEDGEMENTSWe thank MAP program and the staff of the Accelera-tor and Technical Divisions at Fermilab for supporting andmaintaining the MAP experimental program in the MTAexperimental area. Bob Sommer of ANL/Central Shopshas been helpful. The work at Argonne is supported by theU.S. Department of Energy Office of High Energy Physics,by Contract No. DE-AC02-06CH11357.REFERENCES[1] Z Insepov, J. Norem, D. Huang, S. Mahalingam, S. A.Veitzer, Proceedings of LINAC10, Sept 12 - 17,Tsukuba,Japan (2010).[2] F. Schwirzke and R. Taylor, J. of Nucl. Mat. 93&94, 780(1980).[3] G. Wu, M. Ge, P. Kneisel, K. Zhao, L. Cooley, J. Ozelis,D. Sergatskov and C. Cooper, Fermilab Technical NoteFERMILAB-CONF-10-430-TD (2010)[4] M. Aicheler, International Workshop on Linear Colliders2010 Oct. 18 - 22, CERN, Geneva, Switzerland, (2010).Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA TUP049Accelerator TechnologyTech 28: Subsystems and Technology (Other) 897 Copyrightc ○2011byPAC’11OC/IEEE—ccCreativeCommonsAttribution3.0(CCBY3.0)

Vacuum arcs and gradient limits

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Vacuum arcs and gradient limits

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