Overview of the CLIC RF Structure Development Program

W. Wuensch

Second Collaboration Meeting on X-band Structures

KEK, 13-5-2008

Outline

• Introduction to CLIC structures: Accelerating and PETS

• High-power rf constraints

• The ‘other’ technical challenges (briefly)

• The 2010 CLIC feasibility goal and beyond

Main Beam ~1 ADrive Beam 100 A

Accelerating structure

+100 MV/m, 64 MW, 229 mm

PETS

-6.5 MV/m, 136 MW, 210 mm

CLIC module

rf distribution

PETS CLIC_G

Aperture [mm] 23 6.15-4.7

Phase advance π/2 2π/3

Vg/c 0.45 .017-.012

R’/Q [kΩ/m] 2.2 15-16

Length [mm] 210 229

Pmax [MW] 136 64

Esurfmax [MV/m] 56 245

Pin/CtPp

1/3[MW/mm ns1/3]* 13 18

ΔT (ºK) * 2 56

Allowable breakdown rate O(10-7 to10-6) O(10-7)

moding overmoded single

Fundamental mode properties

* pulse length 240 ns High-power constraints

Typical full structure properties

Power flow

PETS Accelerating structure

linear field increase

Pulse shape

Comparison of high-power behaviors should be very enlightening.

Higher-order mode properties

The drive beam has single cycle spacing and the main beam has six cycle spacing.

Damping mechanism

PETS: Heavy damping through eight symmetry slots

Accelerating structure: Heavy damping with detuning. Heavy damping from four cutoff waveguides in CLIC_G. Option of addition of damping slots if pulsed surface heating too strong a constraint.

cell first middle last

Q1 10 7.7 6.3

A1 [V/pC/mm/m] 117 140 156

f1 [GHz] 16.74 17.21 17.67

PETS

Acc

eler

atin

g st

ruct

ure

Higher order mode properties continued

Wakefields computed using GDFDL and HFSS.

Benchmark from old ASSET experiment.

Power couplersE max (135 MW)=41.7 MV/m H max (135 MW)=0.07 MA/m

PETS couplers are extremely challenging due to over-moding.

Acc

ele

ratin

g s

tru

ctu

res

Mode launchers are used in current generation of test structures for simplified interpretation of results.

Compact damped couplers will be added once structures are working OK.

That was all stuff we can compute. Now look what happens when we turn on high power. How well can we predict the phenomena which limit gradient?

Gradient versus pulse length at BDR=10-6

y = 264.8x-0.2025

y = 364.73x-0.2666

y = 150.24x-0.1466 y = 182.05x-0.1731

y = 114.03x-0.1572

y = 220.55x-0.2516

y = 96.223x-0.1785

0

20

40

60

80

100

120

0 100 200 300 400 500

pulse length [ns]

av

era

ge

gra

die

nt

[MV

/m]

T53VG3MC

H90VG3

H75VG4S18

H60VG4R17-2

HDX11-Cu

2pi/3

HDS60L

Power (T53VG3MC)

Power (H75VG4S18)

Power (H90VG3)

Power (H60VG4R17-2)

Power (HDX11-Cu)

Power (2pi/3)

Power (HDS60L)

Sqrt(Sc) for different geoemtries at 100ns,BDR=1e-6

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20

geometry number

Sq

rt(S

c) [

sqrt

(MW

)/m

m]

As important as any given result we have right now is our quantitative knowledge of high power constraints - the ability

to predict the gradient a structure will achieve based on its geometry (assuming reproducible preparation and testing).

The immediate impact is on the efficiency-gradient tradeoff in the CLIC base-line design.

It is also guides our experimental program – low efficiency 100 MV/m has been ‘demonstrated’ we need to move to a

better efficiency.

And of course it is the unequivocal test of whether or not we know what we are doing.

What are the constraints as we see them now?

Field

Electric: Esurface < 250 MV/mMagnetic: Pulsed surface heating less than 56 K

Power flow

Real power flow: Pt1/3/C<18 MW·ns1/3/mmComplex power flow: Sc=Re(S)+0.2Im(S)<2.5 MW/mm2 at 100 ns. In the process of replacing electric+P/C.

Emitted and/or captured current, multipactoring

role not yet clear

Other critical issues,

tolerances

technology

system integration: vacuum (both for high-gradient and beam stability), cooling (especially the effect on

tolerances), active alignment, assembly etc.

MUCH more information will be presented in this meeting than I

will show here!

Tolerances

PETS: 10 μm precision so not a feasibility issue.

Accelerating structure: Shape and transverse-alignment tolerances 5 μm. Good progress towards this goal on test pieces, disks and quadrants. Going as low as 1 to 2 μm for the bookshelf but this is not obvious.

Accelerating structure

± 2.5 μm bandPETS

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

98 99 100 101 102 103 104 105 106 107 108 109 110 111 112

[mm]

[mm

]

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

[mm]

[mm

]

-16

-15.5

-15

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

180 181 182 183 184 185 186 187 188 189 190 191 192 193 194

[mm]

[mm

]

2 μm shape error with ‘banana’ removed

Technology

PETS: Milled octants in copper, forget about disks. Copper can be heat treated if necessary due to low pulsed surface heating.

Accelerating structure: Baseline test-structure assembly is disks in copper that have diamond turned irises with milled damping waveguides, then brazed/bonded (NLC/JLC and very old CLIC). BUT soft copper may not be tolerable so CuZr alloy milled quadrants may be required. Also this might be the better mass production solution anyway… And no heating cycle after machining best for maintaining tolerances. And the PETS are milled.

PETS

Accelerating structures

2010

At CERN we have a mandate (directive?) from our DG to demonstrate the key feasibility issues of CLIC by 2010. Success will allow us to pursue CLIC further and at a higher level if LHC results available by then indicate that it is an interesting option.

One of our most important feasibility demonstrations is operating a fully featured accelerating structure at 100 MV/m at the full pulse length with an appropriate breakdown rate (we target 10-7).

What does “fully featured” mean? We have some influence on this… Damping features certainly. Compact couplers probably. Long-lifetime capability (pulsed surface heating) maybe. Certainly not optimized for mass production (although we don’t want to be inconsistent with mass production and large scale integration in CLIC).

The three technical parts to our experimental program to get there:

Demonstrate prototype: Steps of steadily increasing efficiency and completeness.

Determine the most important dependencies: Aperture dependence, effect of damping, pulsed surface heating, basic breakdown physics etc. Essentially specialized tests to better determine high-power constraints.

Explore ways to get to a higher gradient-efficiency tradeoff: Standing wave, alternative materials, preparation techniques, alternative damping mechanisms, structure/linac optimization.

The other part to our experimental program to get there:

Collaboration!!!

It is clear that CERN cannot demonstrate CLIC feasibility alone. We do not yet have an X-band power source to give but a single example.

BUT

There is a substantial group of people who believe that normal conducting technology can be made to work in the neighborhood of 100 MV/m and thus be the better choice for a higher energy linear collider.

And there is an enormous expertise which was built up in the NLC/JLC days.

So it is pretty obvious what to do and that is why we are all here together. I would like to extend my person thanks and also thanks on behalf of my CERN colleagues to KEK for hosting this workshop.

Towards demonstrating feasibility in a prototype accelerating structure by 2010

- Establish reproducible fabrication and testing (achieved by adopting NLC/JLC technology)

- Design and test a new structure with a predicted gradient in the neighborhood of 100 MV/m. (CLIC_vg1)

- Determine the effect of damping features. This may expose either fundamental or technical problems (damping features will oblige us to deviate from established fabrication techniques).

- Move to an optimized geometry giving good efficiency in CLIC.

- Declare success!

The steps which follow

- Add compact damped couplers

- Address lifetime (pulsed surface heating) in test structures. This may mean we are obliged to use copper alloys, different assembly techniques, perhaps quadrants.

In parallel of course we have the investigative tests – C10s and single cell structures for example.

And I look forward to a creative competition for new and improved structures.

But I make a sincere plea – could we please make a concerted effort to give priority for the moment to a 100 MV/m CLIC-like structure demonstration. The continuity of the CLIC study can contribute a lot the general health of normal conducting development.

The general state of affairs

No shortage of well justified ideas and proposals for testing.

Production capacity is almost keeping up with requirements and can be (and is being) improved with relatively modest resources.

Existing testing capacity is being fully used (?) but is much more expensive to expand…More is on its way but when will it be on line?