DC breakdown measurements Sergio Calatroni Present team: Jan Kovermann, Chiara Pasquino, Rocio Santiago Kern, Helga Timko, Mauro Taborelli, Walter Wuensch

Outline Experimental setup Typical measurements Materials and surface preparations Time delays before breakdown Gas released during breakdown Evolution of  and Eb Effect of spark energy Future HelsinkiSergio Calatroni

Experimental set-up : ‘‘ the spark system ’’ HelsinkiSergio Calatroni V power supply (up to 15 kV) C (28 nF typical) vacuum chamber (UHV mbar) anode (rounded tip, Ø 2 mm) cathode (plane) HV switch spark  -displacement gap  m ( ± 1  m) 20  m typically Two similar systems are running in parallel Types of measurements : 1)Field Emission (   ) 2)Conditioning (  breakdown field E b ) 3)Breakdown Rate (  BDR vs E) 3

Experimental set-up : diagnostics HelsinkiSergio Calatroni V V HV probe to scope current probe vacuum gauges V gas analyzer optical fibre photomultiplier or spectrometer 4

Field emission -  measurement An I-V scan is performed at limited current, fitting the data to the classical Fowler-Nordheim formula, where [j FE ] = A/m 2, [E] = MV/m and [φ] = eV (usually 4.5 eV). HelsinkiSergio Calatroni  is extracted from the slope

Conditioning – average breakdown field MolybdenumCopper Conditioning phase: 40 sparks Deconditioning 1-5 sparks or no conditioning Helsinki6Sergio Calatroni Eb

Surface damage (Mo) HelsinkiSergio Calatroni

Conditioning curves of pure metals HelsinkiSergio Calatroni Selection of new materials for RF structure fabrication was the original purpose of the experiment

Breakdown field of materials (after conditioning) HelsinkiSergio Calatroni In addition to other properties, also importance of crystal structure? reminder : Cu < W < Mo  same ranking as in RF tests (30 GHz) hcp bcc fcc fcc : face-centered cubic bcc : body-centered cubic hcp : hexagonal closest packing 9

Surface treatments of Cu HelsinkiSergio Calatroni Surface treatments on Cu only affects the very first breakdowns rolled sheet / heat treatm. millingSubuelectro-polishing  before 1 st spark ~ ~ 20~ ~ st brkd field [MV/m]~ ~ ~ ~ After a few sparks: ~ 170 MV/m,  ~ 70 for every samples The first sparks destroy rapidly the benefit of a good surface preparation and result in deconditioning. This might be the intrinsinc property of copper surface In RF, sparks are distributed over a much larger surface, and conditioning is seen. Might be due to extrinsic properties. More foreseen in the near future to assess the effect of etching, brazing, etc. 10

Oxidized copper HelsinkiSergio Calatroni Cu 2 O is a p-type semiconductor, with a higher work function than Cu : 5.37 eV instead of 4.65 eV 1. Cu oxidized at 125°C for 48h in oven (air): purple surface ↔ Cu 2 O layer ~15 nm 2. Cu oxidized at 200°C for 72h in oven (air):  BDR = 1 for standard 300 MV/m  BDR = – for oxidized 300 MV/m, but last only a few sparks

Breakdown rate experiments A target field value is selected and applied repeatedly for 2 seconds BDR is as usual: #BD / total attempts Breakdown do often appear in clusters (a simple statistical approach can account for this) HelsinkiSergio Calatroni

Breakdown Rate : DC & RF (30 GHz)  DCRF Cu Mo BDR ~ E  Same trend in DC and in RF, difficult to compare ‘slopes’ HelsinkiSergio Calatroni

Time delays before breakdown HelsinkiSergio Calatroni delay Voltage rising time : ~ 100 ns Delay before spark : variable Spark duration : ~ 2  s 14

Time delays with different materials HelsinkiSergio Calatroni CuTaMoSS R = fraction of delayed breakdowns (excluding conditioning phase, where imediated breakdowns dominate) R = 0.07R = 0.29R = 0.76R = 0.83 R increases with average breakdown field E b = 170 MV/mE b = 300 MV/mE b = 430 MV/mE b = 900 MV/m (but why ?!?) 15

Correlation pre-current & delays HelsinkiSergio Calatroni From: Kartsev et al. SovPhys-Doklady15(1970)475 With our simple thermal model (based on Williams & Williams J. Appl. Phys. D 5 (1972) 280) which lead to the establishment of the scaling quantity “Sc” Phys.Rev.ST- AB 12 (2009)

Gas released during a breakdown HelsinkiSergio Calatroni J / spark 0.95 J / spark Same gases released, with similar ratios  Outgassing probably dominated by Electron Stimulated Desorption (ESD) Slight decrease due to preliminary heat treatment Data used for estimates of dynamic vacuum in CLIC strucures (heat treatment: ex-situ, 815°C, 2h, UHV) 17

H 2 outgassing in Breakdown Rate mode (Cu) HelsinkiSergio Calatroni ‘quiet’ period consecutive breakdowns Outgassing peaks at breakdowns Slight outgassing during ‘quiet’ periods  ESD with FE e - at the anode No visible increase in outgassing just before a breakdown 18

Local field:  · Eb (Cu) HelsinkiSergio Calatroni Measurements of  after each sparks (Cu electrodes)  ↔ next E b correlation  · E b = const  ↔ previous E b no correlation  · E b is the constant parameter (cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964)) 19

Evolution of  & Eb during conditioning experiments HelsinkiSergio Calatroni (± 32%) (± 36%) E b = 159 MV/m  = 77 Local field = const = 10.8 GV/m for Cu (± 16%)  · E b = 10.8 GV/m conditioning ? good surface state 20

Evolution of  during BDR measurements (Cu) HelsinkiSergio Calatroni Spark General pattern : clusters of consecutive breakdowns / quiet periods (BDR = 0.11 in this case)  slightly increases during a quiet period if E is sufficiently high The surface is modified by the presence of the field (are « tips » pulled?) No spark 21

Evolution of  during BDR measurements (Cu) HelsinkiSergio Calatroni Breakdown as soon as  > 48 ( ↔  · 225 MV/m > 10.8 GV/m) Consecutive breakdowns as long as  >  threshold length and occurence of breakdown clusters ↔ evolution of   ·E = 10.8 GV/m Spark No spark 22

Effect of spark energy - Cu HelsinkiSergio Calatroni E BRD increases with lower energy (less deconditioning is possible) Local breakdown field remains constant

Effect of spark energy - Mo HelsinkiSergio Calatroni E BRD seems to increase with higher energy (better conditioning possible) Local breakdown field remains constant However, we have doubts on representative the  measurement is in this case

The diameter of the damaged area depends on the energy available – Area mostly determined by the conditioning phase – Decreases with decreasing energy; saturates below a given threshold HelsinkiSergio Calatroni CuMo 25 Energy scaling of the spot size

The future Ongoing work: – Finalise the work on the effect of spark energy on BD field, and understand the beta measurements for Mo – Trying to understand “worms”( “flowers”?) HelsinkiSergio Calatroni

The future Effect of temperature on  evolution and other properties – To verify the hypothesis and the dynamic of dislocation Fast electronics – Defined pulse shape and duration, without and during breakdowns – Repetition rate up to 1 kHz (10 7 pulses -> less than 3 hours) HelsinkiSergio Calatroni

Fast electronics HelsinkiSergio Calatroni Mike Barnes Rudi Henrique Cavaleiro Soares

The future Effect of surface treatment and in general of the fabrication process on BD – To study the influence of etching and its link with machining (preferential etching at dislocations, field enhancement or suppression, smoothening etc.) – To study the influence of H 2 bonding (faceting, etc) – (In parallel, ESD studies on the same samples) HelsinkiSergio Calatroni

HelsinkiSergio Calatroni

HelsinkiSergio Calatroni st oxidation 30 sparks 2 nd oxidation O element%= O element% = O element% = O element% = st oxidation 150 sparks 2 nd oxidation 180 sparks O element% = 3.8 O element% = 1.0 O element% = A sparked (damaged) surface was reoxidised by heating and was sparked then again – Was not able to recover the initial high E BRD – Oxidised, smooth surface  high E BRD – Oxidised, sparked surface  no improvement – Connection to the oxidation process? Reoxidation

Field profile on the cathode surface Tip – plane geometry dependence with the gap distance x E Helsinki32Sergio Calatroni

 · E b is the constant parameter (cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964)) Gap dependence of E b,  and  · E b (Cu) HelsinkiSergio Calatroni

Surface damage (Cu) Helsinki34Sergio Calatroni

Gap measurement damage HelsinkiSergio Calatroni

HelsinkiSergio Calatroni CLIC Breakdown WorkshopSergio Calatroni TS/MME36 Heating of tips by field emission - I The tip has a   height/radius (field enhancement factor) For a given value of applied E the Fowler-Nordheim law gives a current density J(E)=A*(  E) 2 *exp(-B/  E) This current produces a power dissipation by Joule effect in each element dz of the tip, equal to dP = (J  r 2 ) 2  (z) dz /  r 2 The total dissipated power results in a temperature increase of the tip (the base is assumed fixed at 300 K). The resistivity itself if temperature dependent Using the equations we can, for example, find out for a given  what is the field that brings the “tip of the tip” up to the melting point, and in what time. Used for “Sc” derivation Phys.Rev.ST-AB 12 (2009) J(E) h=height r=radius dz T = 300K T = T melting

HelsinkiSergio Calatroni CLIC Breakdown WorkshopSergio Calatroni TS/MME37 Heating of tips by field emission - II If the resistivity is considered temperature-independent, a stable temperature is always achieved [Chatterton Proc. Roy. Soc. 88 (1966) 231] If the resistivity (other material parameters play a lesser role) is temperature dependent, then its increase produces a larger power dissipation, resulting in a further temperature increase and so on [Williams & Williams J. Appl. Phys. D 5 (1972) 280] Below a certain current threshold, a stable regime is reached Above the threshold, a runaway regime is demonstrated The time dependence of the temperature can be calculated.

Sergio Calatroni - CERNHigh Gradient Workshop Simulation for Mo cone: diameter 20 nm, beta = 30 E=378 MV/mE=374 MV/m

HelsinkiSergio Calatroni CLIC Breakdown WorkshopSergio Calatroni TS/MME39 Time constant to reach the copper melting point (cylinders,  =30) The tips which are of interest for us are extremely tiny, <100 nm (i.e. almost invisible even with an electron microscope)

HelsinkiSergio Calatroni CLIC Breakdown WorkshopSergio Calatroni TS/MME40 Power density at the copper melting point (cylinders,  =30) Power density (power flow) during the pulse is a key issue. See talk by A. Grudiev for RF structures scaling based on Poyinting vector