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HDS 60 (cells) copper was processed from both sides - PowerPoint PPT Presentation


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Low Vg a/ λ =0.16. High Vg a/ λ =0.19. So why?. The first cell puzzle….

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Presentation Transcript
slide2

Low Vg

a/λ=0.16

High Vg

a/λ=0.19

So why?

The first cell puzzle…

Very often we do observe, that after accelerating structure processing the most of the surface modifications take place in a few first cells. Also the number of cells involved is correlated with the group velocity, the less the Vg the fewer cells modified.

As one of the conventional explanation one could expect the statistical distribution of the events in a chain model. However with adopted processing strategy (trip rate ~10-3) the event probability and normalized to that damage distribution is calculated to be very flat.

HDS 60 (cells) copper was processed from both sides

HDS 60

HDS 11 titanium

slide3

- The breakdown initiation

  • The RF pulse dependence
  • The damage mechanism
  • and …

will not be addressed

What do we certainly know, the breakdown ignition is a very fast process: 0.1 -10 ns. If so, one can propose the main difference between the “first” and “second” cell is accessible bandwidth.

And the lower group velocity the more the difference.

The first cell, if breakdown occurs is loaded by the input coupler/waveguide and is very specific in terms of bandwidth.

Other words, the first cell can accept “more” energy during breakdown initiation then consequent ones.

Worse to mention that we do not know the exact transient behavior of the breakdown and the structure bandwidth could play important role.

slide4

Breakdown ‘naive’ modeling in HFSS

Structure: 2pi/3 aperture 3.5 mm (Vg=4.5%)

Radiation spectra

(breakdown in cell#1 )

Missing energy plot

To the input coupler

Pout

RF current

source Iejwt

To the output coupler

Pin = 50MW

Ib, kA/mm2

F, GHz

slide5

Configuration #1 (breakdown resonant fuse) :

Resonant cavity with reduced electric surface field (HO1) is located between structure and waveguide.

Standing wave

S-parameters, dB

F, GHz

Traveling wave

slide6

1-st cell

Configurations #...

Inline taper with increased stored energy

Vg=PLc/W

Stored Energy/cell

P/W

NLC inline taper

Cell #

Cell #

V. Dolgashev (May 2002)

Inline taper with a “speed” bump

Stored Energy/cell

Vg=PLc/W

Cell #

Cell #

R. Zennaro (June 2007)

slide8

TWO FREQUENCY STRUCTURE

Type 1

Type 2

Transversely “0”

Transversely “pi”