Recent DC spark results

1 / 14RF structures team meeting - 24.9.08

Recent DC spark results

Antoine Descoeudres, Yngve Levinsen,

Sergio Calatroni, Mauro Taborelli


Outline

1. Surface treatments of Cu samples

2. Evolution of during conditioning and BDR measurements


Outline


• rolled sheet (reference)• heat treatment• milling• Subu• Electro-polishing


Cu standard sample (rolled sheet)

• First 2 – 3 sparks : Eb = 350 – 400 MV/m, = 15 – 20

• After a few sparks: ~ 170 MV/m, ~ 50

• The delay before the first spark is usually very long (ms) ‘‘de-conditioning’’ due to spark damages (observed only with Cu)


Cu + heat treatment at 875°C

• Both electrodes treated in a brazing cycle (2h)

• Same results as non-heated Cu (Eb, , BDR…)

slightly less, but within error bars

• Quantity of gas released during a spark :


Cu + milling

• Sample milled in the CERN workshop (no idea what the roughness may be…)

good surface state at the start, like rolled sheet

• Standard behaviour :


Cu (milled) + Subu treatment

• Subu : ~ 20 m removed

• The surface state at the start is not as good :

Eb = 200 MV/m, = 20 – 35


Cu (milled) + Electro-polishing

• Electro-polishing: ~ 20 m removed

• Also a good surface state, comparable to standard Cu


Surface treatments of Cu : summary

• Surface treatments on Cu only affects the very first breakdowns

rolled sheet / heat treatm.

milling Subu electro-polishing

before 1st spark ~ 15 - 20 ~ 20 ~ 25 - 30 ~ 15 - 20

1st brkd field [MV/m] ~ 200 - 400 ~ 300 - 500 ~ 150 - 200 ~ 300 - 400

• After a few sparks: ~ 170 MV/m, ~ 50 for every samples

The first sparks destroy rapidly the benefit of a good surface preparation

And in RF ? Sparks are distributed over a much larger surface…Treatments are maybe still useful


Outline


2. Evolution of during conditioning and BDR measurements


Evolution of during conditioning measurements

• In the 2nd DC spark setup: automatization of measurements

between each spark



• Correlation between and the next breakdown field

• No correlation between and the previous breakdown field

· Eb = cst



• · Eb is much less dispersed than Eb

Local field : · Eb = cst = 9.8 GV/m (± 0.2) for Cu


Evolution of during BDR measurements

spark

• ‘calm’ period low

• seems to increase (a few %) during a calm period if E is sufficiently high

Are small tips pulled by the field? (we need more data)

Recent DC spark results

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