Introduction to CLIC and breakdown in rf structures W. Wuensch Breakdown physics workshop 6-5-2010

Introduction to CLIC and

breakdown in rf structures

W. WuenschBreakdown physics workshop

6-5-2010

Walter Wuensch Breakdown physics workshop 6-5-2010

Outline

• Introduction: TeV colliders, CLIC and 100 MV/m accelerating gradient• High-gradients, high-powers and breakdown in rf structures• The CLIC R&D program on breakdown


TeV energy colliders

The most powerful particle collider in the world, the LHC here at CERN, has just begun probing the new energy range of 7 TeV center of mass energy, going to 14 TeV in the coming years, through proton-proton collisions. Upgrades in the coming years (decades?) are already being discussed.

The future of frontier high-energy physics facilities in a few slides...


Many new physics discoveries are hoped for from the LHC - Higgs, super symmetry, dark matter – but which the physics community will need to study in detail using the simpler experimental environment provided in lepton-lepton collisions.

The leading candidate for a collider for lepton-lepton physics is an electron-positron linear collider operating in the range of 0.5 to 3 TeV. The lower energy compared to the LHC is that it’s only the energy of individual constituent quarks and gluons of the protons, six in total, that actually contribute to the relevant interaction.

TeV energy physics


There are two main approaches currently formalized as projects: the superconducting 31 MV/m ILC and the normal conducting 100 MV/m CLIC. Each has different strengths and weaknesses so both are being developed in parallel while waiting for the physics horizon to clarified by LHC results.

The idealized chain of events is that in the next few years LHC discoveries give a consensus on the collision energy that a linear collider should provide, agreement is found on which machine is best adapted to provide that energy, get funding, build, run, discover secrets of nature, award prizes etc.

The other main path towards TeV range lepton physics that people talk about is a muon collider. Oddly enough for the purposes of this workshop they too face high-gradient issues and we have been collaborating for many years.

In the mean time, more about the CLIC approach...

More information can be found at http://clic-study.web.cern.ch/CLIC-Study/

TeV energy linear colliders


CLIC and 100 MV/m acceleration

The CLIC study has been developing technology for a e+e- collider with energy reach all the way up to 3 TeV.

The broad constraints colliders face are to provide this collision energy in a cost effective and energy (mains power) efficient way.

For the latter, the important issue is that we must also provide sufficient luminosity for the physics experiments. Physics cross sections generally go down with increasing collision energy so the necessary accelerated beam power is dramatically high, 10’s of MW average power in linear colliders. With energy conversion efficiencies and losses included these facilities will use a few hundreds of MW.

An important aspect of cost is length, which itself is inversely proportional to gradient.

Quick calculation: 3 TeV with 100 MV/m is already 30 km of acceleration, so when you add all the rest you need you get a facility of around 50 km of densely packed high-tech equipment. This is not gonna be cheap…


CLIC complex at 3 TeV


CLIC and 100 MV/m acceleration

The central idea of the CLIC approach is to reduce cost/acceleration by going to a high accelerating gradient. Our target is 100 MV/m. However 100 MV/m is a very ambitious target in large part because of

BREAKDOWN

A typical, high, accelerating gradient is 20 MV/m. Getting to 100 MV/m is one of the highest priority advances in the state of the art the CLIC study must make.

In my personal opinion, achieving our target gradient, and ultimately being confident enough to build 40 km of equipment based on it, will require a deep and quantitative understanding of the physics of breakdown.

It also clear that other applications would gain from high rf accelerating gradients and/or from a detailed understanding of breakdown.

Hence – this ↓ workshop!


CLIC accelerating structures

Frequency 12 GHz

Average accelerating gradient 100 MV/m

Peak surface electric field 243 MV/m

Input power 64 MW

Pulse length (flat top) 240 (156) ns

Peak pulsed surface temperature rise 50 °C

Maximum breakdown rate 10-7 /pulse

The parameters for our accelerator come out of a complex optimization which includes beam dynamics effects (the vertical size of the beams at collision are below a nm) and high gradient and high power limits in the accelerating structures. See for example “Optimum Frequency and Gradient for the CLIC main linac accelerating structure,” Proceedings LINAC08.

The main parameters are:

These structures were discussed in detail during the X-band workshop earlier this week:http://indico.cern.ch/conferenceDisplay.py?confId=75374.


Accelerating structure features

HOM damping waveguides

Magnetic field concentration –pulsed surface heating

High electric field and powerflow region - breakdown

Short range wakefields

Cooling Vacuum pumping

Alignment

Beam and rf

11.994 GHz, 2π/3, 8.332 mm period


Accelerating structure features

Hs/EaEs/EaSc/Ea

2

0 2 4 6 8 10 12 14 16 180

50

100

150

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250

iris number

P [

MW

] (b

lack

), E

s (

gre

en),

Ea (

red

) [M

V/m

],

T

[K

] (b

lue)

, S

c*50

[MW

/mm

2 ] (m

agen

ta)

53.7

86.6

165

242

3.7

5.0

84

129

65.6

39.1

Pinload = 65.6 MW, P

outload = 29.0 MW

Eff = 16.0 % tr = 22.2 ns, t

f = 38.7 ns, t

p = 278.6 ns

Profiles of crucial high-gradient quantities of the TD18 test structure.

Surface field quantities

Baseline procedure

G. Riddone, 5th CLIC-ACE, 02/02/2010 12

Diamond machining (sealed

structures)

H2 diffusion bonding/brazing at

~ 1000 ˚C

Cleaning with light etch

Vacuum baking650 ˚C > 10 days

J. Wuang

J. Wuang

G. Riddone, J. Wang, X-band workshop, Tuesday afternoon

Walter Wuensch Breakdown physics workshop 6-5-201012 October 2009

CERN/KEK/SLAC high-power test structuresT18 - undamped TD18 - damped

Under test

Done


High Power Test begin at 12/03/2009 15:00

0 100 200 300 400 500 600 700 800 900 10000

20

40

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120

Accumulated rf process time (hr)

BDR (1/hr)

<G> for regular cell (MV/m)

Pulse Width (divided by 10 ns)

TD-18 Faya Wang, SLAC


CERN/KEK/SLAC T18 structure tests

SLAC 1

SLAC 2

KEK

Lines are E30/BDR=const

rf test results have been presented in detail at the X-band workshop, Monday morning, with a summary by S. Doebert on Wednesday morning.


Why exactly do we get these performances?

There are three main areas we can identify and would like to quantify,

Material, preparation and assembly: We seem to be doing quite well now with the technology developed at KEK and SLAC for the NLC/JLC project based on pure copper and exposure to 1050 °C hydrogen atmosphere. Alternative material studies are also being followed. V. Dolgashev and Y. Higashi on Tuesday.

Structure geometry: To illustrate how we approach this dependence, I will describe the other main structure that we need to develop – the PETS (Power generation and Transfer Structure).

Pulse time-structure: Shorter pulses give higher gradients roughly as Eτ1/6=const. But things get trickier if you start to consider pulse shape...


Power generating structures - PETS

The PETS is used to decelerate a high-current relativistic beam to generate the power necessary for the accelerating structures. It is a key element in our CLIC two-beam acceleration scheme. One PETS drives two accelerating structures.

Accelerating structure PETS

Frequency 12 GHz 12 GHz

Average accelerating gradient 100 MV/m 6.3 MV/m (decelerating)

Peak surface electric field 243 MV/m 56 MV/m

Input power 64 MW 136 MW (output)

Pulse length (flat top) 240 (156) ns 240 (156) ns

Peak pulsed surface temperature rise

50 °C 1.8 °C

Maximum breakdown rate 10-7 /pulse 10-7 /pulse

group velocity/c 0.017 to 0.0083 (tapered) 0.45

aperture radius 3.15 to 2.35 mm tapered 11.5 mm


PETS

pictures of PETSEnergy

Average power

Peak power

CLIC target

CLIC target

#pulses


So what drives the geometrical dependence?

We have considered field-based limits and power-flow based limits and analyzed compiled data from a large number of X-band and 30 GHz experiments, both travelling wave and standing wave. Please see “New local field quantity describing the high gradient limit of accelerating structures,” Phys. Rev. Spec. Top. Accel. Beams 12 (2009) 102001

0

50

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300

350

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450

0 5 10 15 20 25 30

H

H4rt(f/f0)

Surf

ace

mag

netic

fiel

d [A

/m]

Achieved values normalized to 100 ns pulse length and 10-6 breakdown rate. Black is X-band, blue is 30 GHz and red is standing wave.

http://cdsweb.cern.ch/ejournals.py?publication=Phys.+Rev.+Spec.+Top.+Accel.+Beams&volume=12&year=2009&page=102001




C L I CC L I C

CLIC2008, 16 Oct. 2008Alexej Grudiev, Pulse shape dependence.

0 500 1000 1500 20000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Sub pulse width and average unloaded gradient

Main pulse width and average unloaded gradient

Time: hr BKD Events BKD Rate(1/pulse)

No sub pulse 100ns@119MV/m (*150MV/m) 19 16 3.9e-6

100ns@81MV/m 100ns@119MV/m (*150MV/m) 16 6 1.74e-6

100ns@97MV/m 100ns@119MV/m (*150MV/m) 21 8 1.76e-6

100ns@111MV/m 100ns@119MV/m (*150MV/m) 20 81 1.88e-5

100ns@119MV/m 100ns@119MV/m (*150MV/m) 21 68 1.5e-5

sub pulse

main pulse

after pulse

0 20 40 60 80 100 12010

-6

10-5

Unloaded gradient for sub pulse: MV/m

Bre

akdow

n r

ate

: 1/p

uls

e

0 20 40 60 80 100 1200

20

40

60

80

100

Unloaded gradient for sub pulse: MV/m

Perc

ent

of

bre

akdow

n e

vents

Sub pulse breakdown

Main pulse breakdown

After pulse breakdown

SLED output pulse

*:Max gradient in the structure

Staircase pulse shape experiment

Chris, Faya

T18 structure – low pulsed surface heating

Pulse Heating Test BDR Test measured at SLAC

0 500 1000 1500 2000-10

0

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Time (ns)

Po

we

r to

Str

uct

ure

(M

W)

150ns - normal150ns - pre heating

0 500 1000 1500 20000

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Time (ns)

Pe

ak

Pu

lse

He

atin

g in

La

st C

ell

(K)

100 MV/m, 200 ns100 MV/m, 150 ns100 MV/m, 150 ns pre-heating

60 65 70 750.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Peak Pulse Heating in Last Cell (K)

BD

R (

1/h

ou

r)

100MV/m@150ns100MV/m@150ns + pre-heating100MV/m@200ns

TD18 structure – high pulsed surface heating

Faya Wang and Chris Adolphsen – presentation 14:00 tomorrow


The next level of understanding

The scaling laws, in fact even just the process of trying to understand them, have already played a major role in pushing the gradient of NLC/JLC structures from 50 MV/m range to the current 100 MV/m range. This occurred predicatively, designs based on power flow scaling laws actually did run higher than previous structures.

But so far we have been restricted to developing our scaling laws, understanding time dependence and developing fabrication technology only by general arguments.

Going beyond this, in my personal opinion, requires developing high-gradient, high-power limits from first principles.

This is part of the theoretical and experimental program to study the details of breakdown ongoing in the CLIC study.


Overview of the CLIC R&D program on breakdown

The core of our high-gradient program is testing prototype rf structures.

This is done mainly at klystron based test facilities at KEK (Nextef) and SLAC (NLCTA and ASTA) and in the coming year at CERN. In addition we have testing capability, especially for PETS, using two-beam based power generation in CTF3.

In order to separate out specific dependencies, such as iris aperture, we have a program of special simplified accelerating structures, the so-called C10 series. There is also a single-cell test series at SLAC which has produced significant amounts of data.

We also try to benefit from medical accelerator and X-FEL structure tests where, through collaboration, we help developing the structures and data is taken during tests in a way that is relevant for us. S. Verdu Andres on Monday.

NEXTEF at KEK


In order to increase our experimental capacity (and constrain speculation) we have also invested in two dc spark systems.

Advantages: The systems and samples are far cheaper than for rf. Easier to introduce alternative materials, new diagnostics, test ideas like temperature dependence etc. Easier to geometry to think about and to simulate.

Overview of the CLIC R&D program on breakdown continued

But aren’t rf and dc sparks “different?” Mostly not and where they are - the total voltage, single polarity – the differences are telling us a lot.

Haven’t lots of people done dc tests before? Yes, but we have many specific questions especially, what is the breakdown rate vs. field dependence and where does it come from? Also practical stuff like: What is our copper like or what effect does this surface treatment have? Sergio Calatroni, 14:00 today


But to finally move from breakdown from the qualitative “I know everything that’s going on but I can’t predict anything” and image-rich “Then there’s this plasma growing above the surface” and technological best practice “I know what works but I don’t why” in which it has languished requires a robust theory effort backed up by a dedicated experimental program.

Overview of the CLIC R&D program on breakdown continued

We have formal collaboration agreements with the Helsinki Institute of Physics, SLAC, IAP SUMY Ukraine, The TERA foundation and the University of Uppsala We also have PhD students from University of Aachen and University of Bochum.

We hope very much to expand and reinforce this effort with this workshop.


General comments

It is with particularly great pleasure to see so many non-accelerator researchers here at this workshop. It has been clear that expertise from many fields will be necessary to solve the breakdown problem but not obvious how to draw everyone together.

CERN’s primary motivation in doing breakdown R&D is to advance our linear collider study in order to do TeV physics. To promote this, we are trying to help provide a framework for breakdown studies. This includes investing in experimental infrastructure, hosting a formal collaboration structure and to a limited extent providing resources, the CERN PhD program for example, for support. We would be happy to collaborate/cooperate with other efforts.

The primary objectives for us in organizing workshop are advancing our understanding of breakdown and advancing the collaboration through discussion, new ideas for common projects and new contacts between researchers.

I wish you a productive, and enjoyable, workshop!


“Electric flowers” by Kazue Yokoyama and Patrick Alknes. dc spark on copper.

The original artwork

Documents

Introduction to CLIC and breakdown in rf structures W. Wuensch Breakdown physics workshop 6-5-2010