High gradient acceleration

Kyrre N. Sjøbæk*

FYS 4550 / FYS 9550 – Experimental high energy physicsUniversity of Oslo, 26/9/2013

* k.n.sjobak(at)fys.uio.noCERN & University of Oslo

Outline

Uses of high gradient acceleration Particle physics Other uses

Techniques for high gradient acceleration Superconducting Plasma-wakefield Normal-conducting RF

The next big particle physics machines

If the LHC discovers something new(apart from the Higgs) heavy & low cross section => high energy

Want a precision experiment Electron accelerator

Circular machines limited bysynchrotron radiation Use a linear accelerator

P∝ E4

m4R2

X-ra

ys

CLIC and ILC

Two projects to build the next big particle physics machine TeV-scale electron-

positron colliders ILC superconducting,

CLIC normal conducting Share detectors etc.

CLIC ILC

Energy [GeV] 500 – 3000 200 – 500

Luminocity [1034 cm-2 s-1]

Peak 1% :1.4 – 2.0

full spectrum:2.3 – 5.9

2

Accelerating gradient [MV/m]

80 – 100 31.5

Bunches/train 234 – 312 1000 – 5400

Particles/bunch [1010] 6.8 – 3.7 10 – 20

Bunch spacing [ns] 0.5 180 – 500

Wall-plug power [MW] 272 – 589 230

Site length [km] 13.2 – 48.3 31

Data from ILC Reference Design Reportand CLIC Conceptual Design Report

√s

High energy and high gradient

In a linear accelerator:Energy = L * q * E

z * f

f is the “fill factor”, fraction of L which are accelerating structures

Need to reach very high energies,O(energy) = 1 TeV Assuming f = 1, 1 TeV:

Ez

= 20 MV/m (SLC) => L = 50 kmE

z= 30 MV/m (ILC) => L = 33 km

Ez

= 100 MV/m (CLIC)=> L = 10 km

Gradient is critical!

CLIC – how compact is it?

CLIC gradient = 100 MV/mILC superconducting ~30 MV/m

“Laser straight” tunnel

Other usesfor high gradient

Free electron lasers:

Very bright UV/X-ray lasers with defined time structure Used for research into

materials Captures “snapshots” of

atomic structure with ~ 1nm and ~10fs resolution (LCLS)

Multiple proposals to build with “CLIC” technology:More compact & cheaper

Hadron therapy Smaller & cheaper

Superconducting high gradient

Superconducting cavities have extremely large Q-factors ~1010

Can store field for a long time Ideal for circular accelerators

Superconductivity breaks at high magnetic field Peak surface field proportional to

gradient Design cavities to minimize constant

of proportionality Even with optimal cavities, gradient

is limited to ~40 MV/m Need to keep at cryo temperatures,

liquid Helium necessary

Hc (T)

T

Tc

H0

Meissner state

(superconducting)

H

Normal state

Plasma-wakefield acceleration

Capable of extremely high gradients, ~100'000 MV/m

Drive beam or laser pushes away electrons in plasma Steep charge density

gradient => huge fields Still relatively unproven

technique

Normal-conductinghigh gradient structures

Able to reach ~100 MV/m Gradient limited by vacuum arc “breakdowns”

Breakdown probability determined bygradient, pulse length, material,and structure shape

Breakdown phenomenanot completely understood

Baseline for CLIC

Normal conducting breakdowns Breakdown = vacuum

arc Spontaneous formation

of plasma on the surface Breakdowns are bad:

Surface craters Deflects the beam Reflects RF power

Scanning E

lectron Microscope

image by M

arkus Aicheler

Measurem

ent byA

ndrea Palaia

BeamKicked

beam

Normal

“Standard model of vacuum arcs”Phases:

1.Field emission from tip

2.Ionization of neutrals

3.Creation of plasma sheath

=> Enhanced emission

4.Sputtering of neutrals

5.Growth

6.Saturation of energy supply

7.Extinction

Extremely high current densities on the order of106 A/cm^2 ≈ 1025 ions/cm^2/s

From pA to kA and Ångstrøm -> 100 µm in a few ns Creating plasma densities on the order of 1020 ions / cm2

Particle- and density plots

Potential & field

Scaling laws Tested large number of structures with different designs Approximate scaling law: Constant differs between structures...

E z30 t5

BDR=const

Electric field

Scaling law – predicting the constant

local complex power flow

Sc=ℜ( S⃗)+ 16

ℑ(S⃗)constC

P

global power flow

Different designs have different field patterns Field magnitude proportional to gradient Surface fields proportional to gradient Scale Esurf, sqrt(S

c), and sqrt(P/C) to same gradient and pulse

length

Find that sqrt(Sc) and sqrt(P/C)

clustered above some limitfor all structures

Structure design

Vary the shape of the structure

In each geometry, calculate field pattern

Minimize ratios of surface fields/gradient Avoid concentrating fields

Use scaling laws to predict which design yields the lowest BDR for any gradient

Summary

High gradient acceleration technology needed for next big particle physics machines Useful for other projects as well

Normal conducting technology can reach~100 MV/m accelerating gradient

Gradient limited by vacuum arcs Avoiding these is an ongoing research topic

Backup

TLEP

Circumference = 80 kmEnergy = 350 GeV(LEP: 209 GeV)

Normal conducting breakdowns

Simulation #1:Particle- and particle density plots

Phases visible: Emission Ignition Spreading

Also see powerful oscillations which some ions “surf” Electrostatic oscillations May be a numerical instability...

Simulation #1: Potential and field plots