towards a next generation sr facility bob cywinski school of applied sciences ffag’08,...

Towards a Towards a Next Generation Next Generation

SR FacilitySR Facility

Bob CywinskiBob CywinskiSchool of Applied SciencesSchool of Applied Sciences

FFAG’08, Manchester, September 2008

Towards a next generation muon source, FFAG’08

SRSR is a uniquely powerful technique for studying magnetic field distributions and dynamics in condensed matter.

SR is a universal acronym which stands for: Muon Spin Rotation

Muon Spin Relaxation

Muon Spin Resonance

The coherent precession of the muon spin in a transverse applied magnetic field

The depolarisation of muon spins in zero or longitudinal applied field

The response of the muon spin to pulsed RF fields

The technique uses predominantly low energy (keV-MeV) positive muons

Physics

MagnetismSuperconductivitySurfacesFundamental physics

Materials

PolymersSemiconductorsHydrogen in metals

ChemistryMolecular dynamicsOxidesMuonium

BiologyBiologyProteins

There are currently ~300-500 muon beam users worldwide


Typical SR experiments

Spin dynamics and spin fluctuations Heavy fermion and small moment magnetism Slow spin dynamics, spin glasses and frustration Organic magnets Superconductors Vortex states and the flux line lattice

Diffusion, tunneling, and localisation Semiconductor physics Muonium formation and dynamics Molecular dynamics Muon chemistry Low energy muons - surface/near surface studies Muon catalysed fusion

Fundamental Physics (1S-2S etc)


Muon production

High energy proton

C or Benuclei

neutrino

pionMuon(~4MeV)

+

+

p pS S

S =0

= 26 nsm140MeV

= 2.2 s

ISIS

PSI


Muon implantation

~1-3

mm

Implantation is rapid and occurs without loss of muon polarisation

A “cold” muon technique utilising cryogenically moderated muons has also been developed to probe surface and near surface effects:

E(keV) R(nm) R(nm)

0.010 0.5 0.3

0.100 2.1 1.3

1.0 13.1 5.4

10.0 75.0 18.0

30.0 244.0 36.0


Muon decay

Gyromagnetic ratio: 1.355342x108 x2 s-1T-1

Mass 0.1126xMp

Lifetime: 2.19714sCharge + (-)Decay asymmetry: W() = 1+a0cos

ao~0.25

pμ


Muon precession

)t(B)t(F)t(B)t(F

)t(R


Muon spin rotation

)tcos()t(Ga)t(B)t(F)t(B)t(F

)t(R Lxox

FFBB

Bx >0Bx >0


Muon spin rotation – an exampleStudies of the flux line lattice in Type II superconductors

The time evolution of the muon polarisation in a transverse field B is

)tcos()t(Ga)t(P Lxox

where L=B

Gx(t) is the Fourier transform of the field distribution averaged over all muon sites.

=500nm = 29nmTC=1.7K


Muon facilities worldwide


Parasitic or symbiotic?

PSI - a continuous (CW) muon source ISIS - a 50Hz pulsed muon source


Graphite targets at PSI

Target E4cm graphite

“necessary component to expand proton beam before SINQ”

Target MThin graphite (few mm)


PSI muon facilities


ISIS muon facilities

argus RIKEN-RAL

MuSRemu


Pulsed or CW?

Tw(ns)

25

50

100

0 20 40 10080600.0

0.5

1.0

Rel

ativ

e µ

SR

asy

mm

etry

Transverse field, mT

The finite proton pulse width (80ns at ISIS) limits the dynamic response of a SR spectrometer at a pulsed source. There are no such limitations in CW

Synchrotron operation at 50Hz is inefficient – it provides a measuring window of 20ms whilst only 20s (ie 10) is needed.


Pulsed or CW?The finite proton pulse width (80ns at ISIS) limits the dynamic response of a SR spectrometer at a pulsed source. There are no such limitations in CW

Synchrotron operation at 50Hz is inefficient – it provides a measuring window of 20ms whilst only 20s (ie 10) is needed.

At a CW source only one muon can be allowed in the sample at a time. PSI spectrometers already count at 25-40KHz – this is the maximum rate possible with CW operation.

At a pulsed source the positron count rate is limited only by detector deadtime. Significant increases in countrate can be achieved by increasing source intensity

Muons on request (MORE) at PSI


A next generation source

Question: What do muon beam users want ?

Answer: Orders of magnitude more muon intensity and smaller muon beam dimensions

Why?

At current positron count-rates (up to 40kHz) a typical spectrum from a typical sample (of a few cm2) will take ~30min to collect with reasonable statistics

Parametric studies (as a function of temperature, field and/or sample concentration) can take days

Studies of small (mm2) sample (eg single crystals) can take even longer

Low energy muon studies of surfaces can take weeks

At existing facilities, muon provision is thus a sub-optimal compromise defined by other users of the proton driver


Low energy muons

Muons are cryogenically moderated and energy-selected (MeV→eV) to tune localisation depth within the sample:

Incident muon rates remain relatively low (~500µ/s)

Cooling efficiency ~ 10-5


An EU funded workshop on Future Developments of Muon Sources in November 2006 concluded that a stand-alone muon facility may be the best way forward – if economically viable

Physics World, December 2006

A stand-alone facility?

It has been suggested that high intensity muon facilities could be incorporated into the SNS and ESS designs as they have been at J-PARC

This would be yet another compromise both for neutron and muon beam users – and would also be expensive


....and the follow up

....an nmi3 funded workshop at the Cockcroft Institute, April 2008


nmi3 workshop summary

Muon production rates

Substantial (two orders of magnitude) gain in muon intensity can only be achieved in pulsed source operation

Gains of 30 could be achieved at ISIS x3 by increasing target thicknessx10 by increasing solid angle acceptance of muons from pion target

Both are unacceptable by the neutron users of the facility

A gain of 100 could be achieved by making the above optimisations, and increasing the power of the proton driver by a factor of 3

This in turn implies that a fully optimised muon source requires a pulsed proton driver of at least 500kW, ie 1mA at 500meV or 0.5mA at 1GeV

For optimal efficiency the pulse repetition rate should approach 25kHz


nmi3 workshop summaryPulse width considerations

For pulsed surface muons, the pion lifetime (26ns) itself sets a lower intrinsic time resolution

kicker chops within a pulse

However, narrower pulses can be achieved by electrostatically trimming the pion decay tail, and even substantially wider pulses (80ns) can be shaped with muon pulse conditioning post-production, as demonstrated at ISIS

Such conditioning presents the opportunity of almost instantaneously trading absolute count rate against frequency response on the same muon spectrometer.

At the extreme, quasi-CW could be possible through “muon on request” techniques, offering intensities close to those at PSI


nmi3 workshop summaryThe proton driverIn order to be world leading a next generation muon source should therefore be driven by a 500kW proton driver and be capable of delivering three modes of operation –

1. A pure pulsed mode (ideally 25kHz) with an integrated count rate of at least x100 ISIS and with better frequency response than ISIS (30ns pulse width)

2. An electrostatically-tailored pulse mode (~5ns, 25KHz) with an order of magnitude higher count rate than ISIS but with significantly improved frequency response

3. A quasi-CW mode – which at least matches the experimental count rate at PSI

A dedicated proton driver unconstrained by parasitic uses of the proton beam will enable precise tailoring of beam/target assemblies, allowing smaller proton/muon beams, and more efficient pion/muon collection and will also facilitate the implementation of multiple muon production targets.


The proton driverCyclotron FFAG Synchrotron

Energy ~ 1 GeV

No Yes Yes

Current > 1 mA

Yes Yes No

Frequency CW 0.1 – 2 kHz 30 – 60 Hz

Pulse length Continuous (~ 1 ns) 10 ns – 1 µs 100ns to ~ 1 µs

Beam size ~mm2 No Yes No

Extraction efficiency

Good Good Good

Operation Easy Easy Not easy

Maintenance Hard Normal Normal

Static fields Yes Yes No

Size Moderate Compact Very large

Mult. beam extraction

No Yes Difficult

Construction cost

High Moderate Very high

Existing technology

Yes No! Yes


An FFAG driven muon source?

A FFAG appears to satisfy most of the requirements demanded of an optimised proton driver for a stand-alone muon facility.

Although repetition rates of 25kHz may not be achievable, even 2kHz offers considerable advantages

FFAG technology is also likely to keep costs within acceptable limits

The CONFORM project is currently exploring this possibility


Acknowledgements

Prof Roger Barlow Cockcroft Institute/Manchester University, UKDr Adriana Bungau University of Huddersfield, UKDr Kurt Clausen Paul Scherrer Institut, SwitzerlandDr Pierre Dalmas de Reotier CEA Grenoble, FranceDr Rob Edgecock, Rutherford Appleton Laboratory, UKDr Philip King ISIS Facility (RAL), UKDr James Lord ISIS Facility (RAL), UKProf Mike Poole ASTeC (STFC) Daresbury Laboratory, UKDr Francis Pratt ISIS Facility (RAL), UKDr Toni Shiroka sμs, Paul Scherrer Institut, SwitherlandDr Sue Smith ASTec, (STFC) Daresbury Laboratory, UK

towards a next generation sr facility bob cywinski school of applied sciences ffag’08,...

Documents

generation muon source

muon intensity

muon sites

hz pulsed muon source

continuous cw muon source

generation sourcequestion

spin glasses

ffag08muon productiontowards