Accelerator Science and Technology David Holder The Cockcroft Institute, and the University of Liverpool Department of Physics. HEP Christmas Meeting,

Accelerator Science and Technology

David HolderThe Cockcroft Institute,and the University of Liverpool Department of Physics.

HEP Christmas Meeting ,Barkla Lecture Theatre,20th December 2010

Outline• Headlines:

– First operation of ALICE FEL;– Circulating beam around EMMA;

• Other projects:– Beam dynamics for low-emittance rings;– Tools and techniques for accelerator modelling and optimisation– Tomography measurements at ALICE and PITZ;– Quasar group;

• Microbunching.

First lasing of ALICE Free-Electron Laser

ALICE (Accelerators and Lasers In Combined Experiments)• Medium-energy, superconducting linac, energy recovery-based electron accelerator at Daresbury;• Third ER-based FEL in the world;• Produces very short (~1ps), intense pulses of mid-IR radiation;• Lasing medium is the electron beam.

First lasing of ALICE Free-Electron Laser

Electrons pass through wiggler and emit synchrotron radiation. Electrons dumped after recovering their energy

Photons make many circuits of the optical cavity.Cavity length chosen so photon pulses are on top of subsequent electron bunches as they pass through the wiggler.

First circulating beam in EMMA

Electron protoype ns-FFAG:• Rapid acceleration (10 to 20

MeV in 10 turns)• Energy-independent magnet

strength• Large (distributed) RF gradient • Simple & reliable

EMMA (Electron Model with Many Applications)

Applications of EMMARapid acceleration for: • Muon acceleration/Neutrino factory

High power proton accelerators for:• Accelerator-Driven Sub-critical Reactor (ADSR)

High reliability & tunable energy for:• Oncology (proton therapy)

Beam dynamics for low-emittance rings

Circumference 6476 mBeam energy 5 GeVAverage current 400 mATransverse damping time 21 msNatural emittance 6.4 mm

Lattice design for ILC damping rings is now complete.

Satisfies strong layout constraints:– identical lattices for e+ and e- rings;– e+ and e- beams in opposite directions;– e+ dipoles directly above e- dipoles;– Injection/extraction lines in shared tunnels.

Modelling SR power loads in the ILC damping ring wigglers

In order to obtain an exceptional low emittance beam, many techniques must be implemented.

For example there are 88 wiggler modules (2.45 m, 1.6 T peak field) each producing 40 kW SR power in the ILC wiggler section. This power must be handled.

Impedance modellingWake potentials are calculated using CST Particle Studio:

BPM insertion:

loss factor = 0.043 V/pC.

power load = 53 W.

Photon absorber:

loss factor = 0.03 V/pC.

power load = 37 W.

Development of techniques for low-emittance tuning at CesrTA

Accurate measurement of vertical dispersion is complicated by the fact that the BPMs have some tilt and gain errors that are difficult to determine.

This technique is fast, appropriate for large rings, and insensitive to BPM coupling errors.

CesrTA emittance tuning simulation, without BPM calibration

CesrTA emittance tuning simulation, with normal mode BPM calibration

Modelling SR from long undulatorsConstruct an analytical representation (via mode decomposition) of the field in the undulator.

In cylindrical polar coordinates, the field modes are expressed in terms of simple trigonometric and Bessel functions.

The mode amplitudes can be obtained from data on a cylindrical surface:

Inside the cylinder, residuals decrease exponentially; but outside the surface, errors grow exponentially.

ILC helical undulator prototype

Phase space tomography in the EMMA injection line (and PITZ)

Tomography system in EMMA injection line was originally designed to used data from three screens.However, using a quad scan produces an image of the phase space that has much better quality.

Simulated tomographic reconstruction of horizontal phase space in the EMMA injection line

using 3-screen and quad scan techniques:

3 screens quad scan

Experimental results:

Facility for Low-energy Antiproton and Ion Research*

*at the Facility for Antiproton and Ion Research

Development of diagnostics for very low energy (keV) anti-proton storage ring:• Faraday cup charge measurement;• Screens and gas jet beam profile monitors• Fibre-optic Cherenkov radiation-based loss monitors• Screens for beam profile monitoring• Beam position monitorsAre the measured properties of antiproton exactly the same as protons?

Development of Diagnostics for Low- Energy Antiproton Beams

ALICE Microbunching Project

Co-conspiritors:• Steve Jamison• Bruno Muratori• David Newton• Andy Wolski

Ez seen by an on-axis reference particle at terahertz focus

ALICE Microbunching Project• Aim – to produce evidence of a interaction of an EM wave

with a electron beam in free space;

• By – using a longitudinally polarised terahertz beam to

produce energy modulation in ALICE electron beam;

• How - terahertz radiation generated by converting the mid-

IR multi- 10 terawatt laser into terahertz radiation using a

semiconductor wafer-based antenna;

• Observe - the result of the radiation-electron bunch

interaction in a high-dispersion region of ALICE.

Generation of Terahertz Radiation• Semiconductor wafer with one earth and one high-voltage (e.g.

several 10s of kV, DC or pulsed) electrode;• Short-pulse IR laser used to make wafer break down briefly

(electrons promoted to the conduction band from valence band);• During extremely fast rise in current radiation is emitted:

electron beam Antenna

Terahertz Antenna – 3” GaAs Wafer

Third-angle projection

Outer electrode

Inner electrode

Wafer

Terahertz Antenna

• Gallium arsenide (GaAs);

• 75 mm diameter;

• 300 to 500 µm thick;

• Dielectric constant 13;

• Intrinsic (i.e. with no doping) resistivity 106 m;

Laser beam with idealised top-hat transverse profile fills wafer aperture.

Sudden promotion of electrons from valence band to conduction band.

Large voltage across wafer causes large current surge.

Radiation emitted in both forward and reverse directions as current rises.

Opera 2D Electrostatic Modelling

Axi-symmetric model (R, z) with:

1. Inner and outer electrode radii varied;2. Outer electrode grounded and inner electrode at 30 kV.

R

z

0.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

2.5E+04

3.0E+04

3.5E+04

0.0075 0.0125 0.0175 0.0225 0.0275 0.0325 0.0375

Pot (

V)

R position (m)

Wafer_110

Theoretical (infinite coax)

Terahertz Antenna (new)

Longitudinally Polarised Radiation

phase later →

Status• Intensity of terahertz radiation measured off-

line laboratory at 40 kV antenna voltage:– Modified to increase voltage – further testing after

Christmas;• Parallel experimental set-up under construction

in ALICE experimental hall;• Final ALICE hardware being installed during

current shutdown;• Experiments with electron beam next year.

The EndThank YouQuestions?