June 2007, CERN

June 2007, CERN

HDS 60 (cells) copper was processed from both sides

Low Vga/λ=0.16

High Vga/λ=0.19

HDS 11 titanium

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.

0 20 40 600

0.5

1

1.5

Cell no

Nor

mal

ized

tota

l dam

age

HDS 60

The first cell puzzle…

So why?

June 2007, CERN

- The breakdown initiation- The RF pulse dependence- The damage mechanism- and …

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.

will not be addressed

June 2007, CERN

RF current source Iejwt

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

Ib, kA/mm2

Pout

Pin = 50MW

28.5 29 29.5 30 30.5 310.01

0.1

1

F, GHz

Breakdown ‘naive’ modeling in HFSS

Radiation spectra(breakdown in cell#1 )

To the output coupler

To the input coupler

Missing energy plot

June 2007, CERN

29 29.5 30 30.5 3140

30

20

10

0

F, GHz

S-p

ara

mete

rs,

dB

Traveling wave

Configuration #1 (breakdown resonant fuse) :Resonant cavity with reduced electric surface field (HO1) is located between structure and waveguide.

Standing wave

June 2007, CERN

0 2 4 6 80.9

1

1.1

1.2

1.3

1.4

R. Zennaro (June 2007)

0 2 4 6 83

3.5

4

4.5

5

Vg=PLc/W

P/W

Stored Energy/cell

Cell #Cell #

0 2 4 6 80

1

2

3

4

Vg=PLc/W

0 2 4 6 80.2

0.4

0.6

0.8

1

1.2

Stored Energy/cell

Cell #Cell #

NLC inline taper

V. Dolgashev (May 2002)

Inline taper with increased stored energy

Inline taper with a “speed” bump

1-st cell

Configurations #...

June 2007, CERN

June 2007, CERNTWO FREQUENCY STRUCTURE

Transversely “0”

Transversely “pi”

Type 1 Type 2

June 2007, CERN

1

0

E x1

12

z0999

0 x0 10 20

0

0.5

11

0

0.505

0.651

E x 0( )

E_11i

z0999

0 x z1i

z0500

z1500

0 10 20

0

0.5

11

0

E x1

6

z0999

0 x0 10 20

0

0.5

11

0

E x3

12

z0999

0 x0 10 20

0

0.5

11

0

E x4

12

z0999

0 x0 10 20

0

0.5

1

E x5

12

x0 10 20

0

0.5

1

1

0.650887

Re E z0500

tt

30 tt0 0.5 1 1.5 2 2.5 3

1

0

1

1

4.871054 107

0.504

0.705

E x 0( )

E_11i

z0999

0 x z1i

z0500

z1500

0 10 20

0

0.5

11

8.769268 107

E x1

12

z0999

0 x0 10 20

0

0.5

1

E x1

6

x0 10 20

0

0.5

1

E x1

12

x0 10 20

0

0.5

11

1.432727 106

E x1

6

z0999

0 x0 10 20

0

0.5

1

0.307 0.395( )

0.505

2

20.966

1

1.826586 106

E x3

12

z0999

0 x0 10 20

0

0.5

11

1.96676 106

E x4

12

z0999

0 x0 10 20

0

0.5

1

E x5

12

x0 10 20

0

0.5

1

1

0.704153

Re E z0500

tt

30 tt0 0.5 1 1.5 2 2.5 3

1

0

1

Type 1

Type 1

Transverse Ez (t) distributions