RF Breakdown and MAP Introduction Current Understanding Field - PowerPoint PPT Presentation

RF Breakdown and MAP Daniel Bowring RF Breakdown and MAP Introduction Current Understanding Field Emission Daniel Bowring Physics MAP-Specific Issues Lawrence Berkeley National Laboratory, Muon Accelerator Program Conclusions Bibliography March 4, 2012 Supplemental Slides

A statement of the problem RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides RF cavities in cooling channel conditions are limited by breakdown phenomena.

Strong magnetic fields limit cavity gradient. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Figure: Similar phenomenon Supplemental Figure: Maximum achievable Slides observable during button tests gradient affected by magnetic field [Huang et al., 2007]. Coupler strength [Palmer et al., 2009]. problems?

A few words of caution before we begin. RF Breakdown and MAP Daniel RF breakdown is a very interesting problem. Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides

A few words of caution before we begin. RF Breakdown and MAP Daniel RF breakdown is a very interesting problem. Bowring Introduction Current Understanding RF breakdown is a very old problem. Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides

A few words of caution before we begin. RF Breakdown and MAP Daniel RF breakdown is a very interesting problem. Bowring Introduction Current Understanding RF breakdown is a very old problem. Field Emission Physics MAP-Specific Issues There is very likely no “magic bullet” solution. Conclusions Bibliography Supplemental Slides

A few words of caution before we begin. RF Breakdown and MAP Daniel RF breakdown is a very interesting problem. Bowring Introduction Current Understanding RF breakdown is a very old problem. Field Emission Physics MAP-Specific Issues There is very likely no “magic bullet” solution. Conclusions Bibliography Supplemental Slides Our priority is a functioning cooling channel.

RF Breakdown and MAP Daniel Bowring Introduction Current Understanding A General Picture Of Breakdown Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides

The Conventional Picture RF Breakdown Microscopic E -field and MAP Daniel enhanced to GV/m levels. Bowring Local F-N field emission Introduction currents approach Current 10 11 A/m. Understanding Field Emission Joule heating vaporizes Physics surface features. MAP-Specific Issues Cu particles ionized by Conclusions emitted e − . Bibliography Sheath forms, enables Supplemental Slides further emission. Explosion, melting, craters Figure: Cartoon of the emission process [Mesyats, 1983]. [Loew and Wang, 1999].

There are problems with the conventional picture. RF Breakdown and MAP Empirical observation of Daniel Bowring frequency-dependence. Introduction 5 < β < 8 measured. Current 40 < β < 60 required by theory Understanding [Wang and Loew, 1989]. β > 50 Field Emission Physics not observed MAP-Specific [Descoeudres, 2009]. Issues Conclusions Geometric β ∼ h / r . Hard to Figure: Damage area from Bibliography measure directly open-cell 805 MHz cavity Supplemental [Norem et al., 2003]. Slides [Norem et al., 2003]. Measuring � j FN � also imprecise.

Things get very complicated, very quickly. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography NOT FROM A CAVITY. Supplemental Slides Cu nanowires grown, � β � = 245 from FESM. Form factor predicts a factor of 3 lower. AND only 6% of them are strong emitters [Maurer et al., 2006].

A priori models are difficult. RF Breakdown and MAP Daniel Test geometry-dependence Bowring of 11.242 GHz accelerating Introduction structures Current Understanding [Dolgashev et al., 2010]. Field Emission BD rate independent of Physics fabricating lab, Cu type MAP-Specific Issues Figure: Vary geometry, study rf (OFHC, etc.). Conclusions properties Surface treatment did not Bibliography [Dolgashev et al., 2010]. affect BD rate . Did Supplemental Slides improve conditioning time.

Correlation of geometry with RF properties (1) RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Figure: Vary geometry, study rf Conclusions properties [Dolgashev et al., 2010]. Bibliography Figure: Gradient correlation with Supplemental BD probability. Slides

Correlation of geometry with RF properties (2) RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Figure: Vary geometry, study rf Conclusions properties [Dolgashev et al., 2010]. Bibliography Figure: Peak electric field Supplemental correlation with BD probability. Slides

Correlation of geometry with RF properties (3) RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Figure: Vary geometry, study rf Issues properties Conclusions [Dolgashev et al., 2010]. Figure: Peak magnetic field Bibliography correlation with BD probability. Supplemental Slides NB: It is not correct to say “magnetic field causes breakdown”!

Contribution of pulse length is also studied. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Varying pulse length shows strong correlation between BD Slides probability and pulsed heating [Dolgashev et al., 2010].

Very recent work on pulsed heating looks promising. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides Figure: TE 011 cavity has no surface electric fields, applies magnetic fields to small, removable samples [Laurent et al., 2011].

Pulsed heating experiments show material behavior. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides [Laurent et al., 2011].

Mushroom cavity results RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides Figure: Results from [Laurent et al., 2011].

Mushroom cavity results RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides Figure: Results from [Laurent et al., 2011].

Mushroom cavity results RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides Figure: Results from [Laurent et al., 2011]. NB: This tells us nothing about field emission!

In Summary RF Breakdown and MAP Daniel Bowring Introduction Even without strong magnetic fields, BD is difficult to Current understand. Understanding It’s generally accepted that field emission plays a role in Field Emission Physics triggering breakdown events. MAP-Specific Issues Many cavities tested over many years, and still very little Conclusions definitive knowledge of BD physics. Bibliography Supplemental Slides

It’s even harder for low-frequency cavities. RF Breakdown and MAP Daniel Bowring Introduction Current An observation: 201 MHz cavities are large and therefore Understanding expensive. How can we hope to approach this level of Field Emission Physics statistical understanding? MAP-Specific Issues Conclusions Bibliography Supplemental Slides

RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides

Field Emission RF Breakdown Considering the Fowler-Nordheim equation: and MAP Daniel � j � = 5 . 7 × 10 − 12 · 10 4 . 52 φ − 0 . 5 − 6 . 53 × 10 9 · φ 1 . 5 Bowring � � ( β E s ) 2 . 5 exp φ 1 . 75 β E s Introduction Current Understanding φ is the work function of the metal, measured in eV. Field Emission Physics It is usually taken as a constant. MAP-Specific Issues φ is not constant. It changes depending on grain Conclusions orientation [Smoluchowski, 1941], and also depending on Bibliography the local crystal strain [Chow and Tiller, 1984]. Supplemental Slides An examination of variations in φ may resolve some of the inconsistencies involved in β -oriented measurements and calculations.

φ changes with surface structure. RF Breakdown and MAP Daniel Bowring Introduction Current Understanding Field Emission Physics MAP-Specific Issues Conclusions Bibliography Supplemental Slides Figure: A qualitative argument that tips alter the surface dipole layer [Chow and Tiller, 1984]. (See paper for a quantitative argument.)