R. Iwai (ETHZ) on behalf of muCool collaboration

muCool: Towards a high-brightness ultra-cold positive muon beam line at PSI

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Muon beams at PSI

p+

• High power proton beam colliding graphite target

protons

π+μ+

surface muons from pions stopped on the target surface

x

π+/-

μ+/-

cloud muonspion decay-in-flight

• Surface μ+ at 4.1 MeV most widely used (Δx ~cm)

�2

1.4 MW cyclotron Production target

• Standard beam cooling techniques not applicable due to limited τμ (2.2 μs)muCool scheme

Large room for improvement in phase space quality

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019 3

muCool project

0 mm

10 mm

20 mm

30 mm

• Reduce energy (4.1 MeV → <1 eV) and beam size (10 mm → <1mm)

• Cryogenic He gas stopping target with complex fields

• Device to compress the phase space of a standard muon beam by 10 orders of magnitude

• Efficiency ~10-3

• Conserve initial polarisation

3

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Motivation

4

• Muon g-2/EDM

Particle physics

Test anti-matter gravity

• High quality Mu beam from SFHe

1S-2S spectroscopy μ+

e-

Solid state physics (μSR)

• Study thin materials, small samples

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Q

�5

Working principle (old)Working Principle

4

D. Taqqu, PRL97, 194801 (2006)

IN

OUT

Transverse compression

Longitudinal compression

Y. Bao et al., PRL 112, 224801 (2014)He gas

Muon Cooling: Longitudinal Compression

Yu Bao,1 Aldo Antognini,2,* Wilhelm Bertl,1 Malte Hildebrandt,1 Kim Siang Khaw,2 Klaus Kirch,1,2 Angela Papa,1

Claude Petitjean,1 Florian M. Piegsa,2 Stefan Ritt,1 Kamil Sedlak,1 Alexey Stoykov,1 and David Taqqu21Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland

2Institute for Particle Physics, ETH Zurich, 8093 Zurich, Switzerland(Received 11 February 2014; published 4 June 2014)

A 10 MeV=c positive muon beam was stopped in helium gas of a few mbar in a magnetic field of 5 T.The muon “swarm” has been efficiently compressed from a length of 16 cm down to a few mm along themagnetic field axis (longitudinal compression) using electrostatic fields. The simulation reproduces the lowenergy interactions of slow muons in helium gas. Phase space compression occurs on the order ofmicroseconds, compatible with the muon lifetime of 2 μs. This paves the way for the preparation of a high-quality low-energy muon beam, with an increase in phase space density relative to a standard surface muonbeam of 107. The achievable phase space compression by using only the longitudinal stage presented hereis of the order of 104.

DOI: 10.1103/PhysRevLett.112.224801 PACS numbers: 29.27.-a, 14.60.Ef, 29.38.Db, 82.20.Tr

Standard muon (μþ) beams have a relatively high energyand poor phase space quality. A new scheme has beenproposed making use of stopped μþ in a He gas target [1].Through the stopping process high intrinsic phase spacecompression is achieved. The remaining challenge is toextract the muons fast enough into vacuum. This is done bycompressing the stopped muon swarm with electric fieldsand guiding it into a small extraction hole. In this Letter wereport the successful demonstration of muon swarm com-pression along the magnetic field lines.Related schemes have been used in the field of rare

isotope investigations [2,3]. High energy ion beams arestopped in He gas and compressed. Typical manipulationand extraction times are 5–200 ms. For muons, much fastertechniques are vital.The new concept for fast compression is based on a

position-dependent muon drift velocity ~vD in gas. In a longHe gas target placed in a longitudinal high magnetic field,the stopping muons are first transversely, then longitudi-nally compressed and finally extracted through a 1 mm2

side hole in the transverse direction (see Fig. 1). Theoperation takes place in 8 μs [1].This tertiary beam line is an add on to a secondary

surface muon beam line, reducing the phase space of theinput beam by 1010 with 10−3 efficiency.By using, e.g., the PSI μE4 surface beam with

4 × 108 μþ=s, an eV-energy beam can be obtained with1 mm2 transverse area and intensity of 5 × 105 μþ=s, 2orders of magnitude larger than presently achieved [4]. Thisdevelopment is also highly attractive in view of the HiMBproject [5] aiming at a surface muon beam of ∼1010 μþ=s.Space charge effects will limit the output intensity of thecompressed beam to about 1 × 107 μþ=s.It is important to note that depolarization during muon

swarm compression is only a few percent because of the

strong longitudinal magnetic field [6]. Thus, starting fromhighly polarized muons, such a beam can be used for nextgeneration μSR applications, muonium (Mu) spectroscopy,a muon g-2 experiment, searches for a muon electric dipolemoment, and Mu-Mu conversion. Because of space chargeeffects and muon losses it is not suited for muon colliderprojects.The drift velocity of charged particles in gas in the

presence of electric ~E and magnetic ~B fields is [7]

~vD ¼ μE1þ ω2τ2c

½Eþ ωτcE × Bþ ω2τ2cðE · BÞB&; (1)

where E and B are the unit vectors along ~E and ~B, ω ¼eB=m the cyclotron frequency withm the muon mass, μ themuon mobility in the gas, and τc the mean time between

µ+ Tertiary beamE < eVD < 1 mmTagged beam

D=10 mmContinuos beam

E=4 MeV

T = 290 K

compression

compression

T = 4 K

Transverse

Longitudinal

E

E

y

xz

µ+

B−field

Secondary beam

T = 12 K

FIG. 1 (color online). Schematic of the beam line proposed in[1]. μþ traveling in the −z direction are stopped in 5 mbar He gasat cryogenic temperatures. First, transverse (y direction) com-pression occurs within a density gradient in He gas (blue region).Then at room temperature the longitudinal (z direction) com-pression takes place (red region). In the yellow region a mixedtransverse-longitudinal compression precedes before extractioninto vacuum.

PRL 112, 224801 (2014) P HY S I CA L R EV I EW LE T T ER Sweek ending6 JUNE 2014

0031-9007=14=112(22)=224801(5) 224801-1 © 2014 American Physical Society

!⌧c

µ : muon mobility: cyclotron frequency: mean time between collisions

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Transverse compression‣ 5 mbar He gas

�6

‣ Temperature gradient at cryogenic temperature

‣ Crossed E- and B-field

Muon Cooling: Longitudinal Compression

Yu Bao,1 Aldo Antognini,2,* Wilhelm Bertl,1 Malte Hildebrandt,1 Kim Siang Khaw,2 Klaus Kirch,1,2 Angela Papa,1

Claude Petitjean,1 Florian M. Piegsa,2 Stefan Ritt,1 Kamil Sedlak,1 Alexey Stoykov,1 and David Taqqu21Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland

2Institute for Particle Physics, ETH Zurich, 8093 Zurich, Switzerland(Received 11 February 2014; published 4 June 2014)

A 10 MeV=c positive muon beam was stopped in helium gas of a few mbar in a magnetic field of 5 T.The muon “swarm” has been efficiently compressed from a length of 16 cm down to a few mm along themagnetic field axis (longitudinal compression) using electrostatic fields. The simulation reproduces the lowenergy interactions of slow muons in helium gas. Phase space compression occurs on the order ofmicroseconds, compatible with the muon lifetime of 2 μs. This paves the way for the preparation of a high-quality low-energy muon beam, with an increase in phase space density relative to a standard surface muonbeam of 107. The achievable phase space compression by using only the longitudinal stage presented hereis of the order of 104.

DOI: 10.1103/PhysRevLett.112.224801 PACS numbers: 29.27.-a, 14.60.Ef, 29.38.Db, 82.20.Tr

Standard muon (μþ) beams have a relatively high energyand poor phase space quality. A new scheme has beenproposed making use of stopped μþ in a He gas target [1].Through the stopping process high intrinsic phase spacecompression is achieved. The remaining challenge is toextract the muons fast enough into vacuum. This is done bycompressing the stopped muon swarm with electric fieldsand guiding it into a small extraction hole. In this Letter wereport the successful demonstration of muon swarm com-pression along the magnetic field lines.Related schemes have been used in the field of rare

isotope investigations [2,3]. High energy ion beams arestopped in He gas and compressed. Typical manipulationand extraction times are 5–200 ms. For muons, much fastertechniques are vital.The new concept for fast compression is based on a

position-dependent muon drift velocity ~vD in gas. In a longHe gas target placed in a longitudinal high magnetic field,the stopping muons are first transversely, then longitudi-nally compressed and finally extracted through a 1 mm2

side hole in the transverse direction (see Fig. 1). Theoperation takes place in 8 μs [1].This tertiary beam line is an add on to a secondary

surface muon beam line, reducing the phase space of theinput beam by 1010 with 10−3 efficiency.By using, e.g., the PSI μE4 surface beam with

4 × 108 μþ=s, an eV-energy beam can be obtained with1 mm2 transverse area and intensity of 5 × 105 μþ=s, 2orders of magnitude larger than presently achieved [4]. Thisdevelopment is also highly attractive in view of the HiMBproject [5] aiming at a surface muon beam of ∼1010 μþ=s.Space charge effects will limit the output intensity of thecompressed beam to about 1 × 107 μþ=s.It is important to note that depolarization during muon

swarm compression is only a few percent because of the

strong longitudinal magnetic field [6]. Thus, starting fromhighly polarized muons, such a beam can be used for nextgeneration μSR applications, muonium (Mu) spectroscopy,a muon g-2 experiment, searches for a muon electric dipolemoment, and Mu-Mu conversion. Because of space chargeeffects and muon losses it is not suited for muon colliderprojects.The drift velocity of charged particles in gas in the

presence of electric ~E and magnetic ~B fields is [7]

~vD ¼ μE1þ ω2τ2c

½Eþ ωτcE × Bþ ω2τ2cðE · BÞB&; (1)

where E and B are the unit vectors along ~E and ~B, ω ¼eB=m the cyclotron frequency withm the muon mass, μ themuon mobility in the gas, and τc the mean time between

µ+ Tertiary beamE < eVD < 1 mmTagged beam

D=10 mmContinuos beam

E=4 MeV

T = 290 K

compression

compression

T = 4 K

Transverse

Longitudinal

E

E

y

xz

µ+

B−field

Secondary beam

T = 12 K

FIG. 1 (color online). Schematic of the beam line proposed in[1]. μþ traveling in the −z direction are stopped in 5 mbar He gasat cryogenic temperatures. First, transverse (y direction) com-pression occurs within a density gradient in He gas (blue region).Then at room temperature the longitudinal (z direction) com-pression takes place (red region). In the yellow region a mixedtransverse-longitudinal compression precedes before extractioninto vacuum.

PRL 112, 224801 (2014) P HY S I CA L R EV I EW LE T T ER Sweek ending6 JUNE 2014

0031-9007=14=112(22)=224801(5) 224801-1 © 2014 American Physical Society

x

y

z

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019�711

Detector 1

Detector 2

Detector 1

Detector 2

• Demonstrated in 2015

I. Belosevic, Joint annual meeting of

Swiss and Austrian Physical Societies 2017

+ + data

— — simulation

μ+

Demonstration of transverse comp.‣ Demonstrated in 2015‣ Excellent agreement between measurement and simulation

I. Belosevic et al., Hyperfine Interact (2019) 240: 41.

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Longitudinal compression‣ 5 mbar He gas

�8

‣ Room temperature

‣ Parallel E- and B-field

Muon Cooling: Longitudinal Compression

Yu Bao,1 Aldo Antognini,2,* Wilhelm Bertl,1 Malte Hildebrandt,1 Kim Siang Khaw,2 Klaus Kirch,1,2 Angela Papa,1

Claude Petitjean,1 Florian M. Piegsa,2 Stefan Ritt,1 Kamil Sedlak,1 Alexey Stoykov,1 and David Taqqu21Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland

2Institute for Particle Physics, ETH Zurich, 8093 Zurich, Switzerland(Received 11 February 2014; published 4 June 2014)

A 10 MeV=c positive muon beam was stopped in helium gas of a few mbar in a magnetic field of 5 T.The muon “swarm” has been efficiently compressed from a length of 16 cm down to a few mm along themagnetic field axis (longitudinal compression) using electrostatic fields. The simulation reproduces the lowenergy interactions of slow muons in helium gas. Phase space compression occurs on the order ofmicroseconds, compatible with the muon lifetime of 2 μs. This paves the way for the preparation of a high-quality low-energy muon beam, with an increase in phase space density relative to a standard surface muonbeam of 107. The achievable phase space compression by using only the longitudinal stage presented hereis of the order of 104.

DOI: 10.1103/PhysRevLett.112.224801 PACS numbers: 29.27.-a, 14.60.Ef, 29.38.Db, 82.20.Tr

Standard muon (μþ) beams have a relatively high energyand poor phase space quality. A new scheme has beenproposed making use of stopped μþ in a He gas target [1].Through the stopping process high intrinsic phase spacecompression is achieved. The remaining challenge is toextract the muons fast enough into vacuum. This is done bycompressing the stopped muon swarm with electric fieldsand guiding it into a small extraction hole. In this Letter wereport the successful demonstration of muon swarm com-pression along the magnetic field lines.Related schemes have been used in the field of rare

isotope investigations [2,3]. High energy ion beams arestopped in He gas and compressed. Typical manipulationand extraction times are 5–200 ms. For muons, much fastertechniques are vital.The new concept for fast compression is based on a

position-dependent muon drift velocity ~vD in gas. In a longHe gas target placed in a longitudinal high magnetic field,the stopping muons are first transversely, then longitudi-nally compressed and finally extracted through a 1 mm2

side hole in the transverse direction (see Fig. 1). Theoperation takes place in 8 μs [1].This tertiary beam line is an add on to a secondary

surface muon beam line, reducing the phase space of theinput beam by 1010 with 10−3 efficiency.By using, e.g., the PSI μE4 surface beam with

4 × 108 μþ=s, an eV-energy beam can be obtained with1 mm2 transverse area and intensity of 5 × 105 μþ=s, 2orders of magnitude larger than presently achieved [4]. Thisdevelopment is also highly attractive in view of the HiMBproject [5] aiming at a surface muon beam of ∼1010 μþ=s.Space charge effects will limit the output intensity of thecompressed beam to about 1 × 107 μþ=s.It is important to note that depolarization during muon

swarm compression is only a few percent because of the

strong longitudinal magnetic field [6]. Thus, starting fromhighly polarized muons, such a beam can be used for nextgeneration μSR applications, muonium (Mu) spectroscopy,a muon g-2 experiment, searches for a muon electric dipolemoment, and Mu-Mu conversion. Because of space chargeeffects and muon losses it is not suited for muon colliderprojects.The drift velocity of charged particles in gas in the

presence of electric ~E and magnetic ~B fields is [7]

~vD ¼ μE1þ ω2τ2c

½Eþ ωτcE × Bþ ω2τ2cðE · BÞB&; (1)

where E and B are the unit vectors along ~E and ~B, ω ¼eB=m the cyclotron frequency withm the muon mass, μ themuon mobility in the gas, and τc the mean time between

µ+ Tertiary beamE < eVD < 1 mmTagged beam

D=10 mmContinuos beam

E=4 MeV

T = 290 K

compression

compression

T = 4 K

Transverse

Longitudinal

E

E

y

xz

µ+

B−field

Secondary beam

T = 12 K

FIG. 1 (color online). Schematic of the beam line proposed in[1]. μþ traveling in the −z direction are stopped in 5 mbar He gasat cryogenic temperatures. First, transverse (y direction) com-pression occurs within a density gradient in He gas (blue region).Then at room temperature the longitudinal (z direction) com-pression takes place (red region). In the yellow region a mixedtransverse-longitudinal compression precedes before extractioninto vacuum.

PRL 112, 224801 (2014) P HY S I CA L R EV I EW LE T T ER Sweek ending6 JUNE 2014

0031-9007=14=112(22)=224801(5) 224801-1 © 2014 American Physical Society

x

y

z

‣ Demonstrated in 2011/2014

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Mixed transverse & longitudinal compression

‣ New scheme with simpler setupB

ET

EL

y

xz

x [mm]40− 20− 0 20 40

y [m

m]

20−

10−

0

10

20h

Entries 1380652Mean x 3.905Mean y 2.016− RMS x 17.37RMS y 3.291

0

200

400

600

800

1000

1200

1400

hEntries 1380652Mean x 3.905Mean y 2.016− RMS x 17.37RMS y 3.291

step_y:step_x {step_x < 34}

x [mm]20− 0 20 40

z [m

m]

40−

20−

0

20

40h

Entries 1380652

Mean x 3.905

Mean y 0.7988−

RMS x 17.37

RMS y 9.545

0

200

400

600

800

1000

1200

1400

1600

1800

2000

hEntries 1380652

Mean x 3.905

Mean y 0.7988−

RMS x 17.37

RMS y 9.545

step_z:step_x {step_x < 34}

‣ Single stage at cryogenic temperature

�9

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Realised target • Kapton foil with equipotential lines

10

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019 11

Beam test setup

11

• First demonstration of mixed compression at the end of 2017

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Solenoid

Cryo-cooler

μ+

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019 13

Target installation

13

Heating pad He line

Collimator

Cold-finger

μ+

Detectors

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019 14

Measurement of mixed compressionTransverse direction

x

y

Trans 3Trans 2

Trans 1

Tiles 1, 2, 3

Tiles 4, 5, 6

z

x

Tiles 3, 6

Tiles 2, 5

Tiles 1, 4

Longitudinal direction

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Extract muons into vacuum

μ

‣ Through 1×1 mm2 orifice‣ He gas injection and efficient pumping to maintain target pressure

15

muCool: Next steps

He IN

He OUT

μ＋

He IN

He OUT

He OUT

μ＋

B

E

Increase E-field strength

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

ConclusionsmuCool project developing a high-brightness ultra-cold positive muon beam

‣ Transverse and longitudinal compressions separately demonstrated. Excellent agreement between data and simulation.

‣ Mixed compression scheme partially demonstrated

‣ Next: Improve mixed compression & develop muon extraction

Supported by SNF grant 200020_172639

16

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Back up

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019�18

Demonstration of longitudinal comp.‣ Demonstrated in 2011/2014‣ Excellent agreement between measurement and simulation

-500 V

+500 V 0 V

I. Belosevic et al., Eur. Phys. J. C (2019) 79: 430

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019�19

Target for surface muons‣ ~100 cm long needed

R. Iwai Zurich PhD student seminar - Villigen PSI, - 9, Oct, 2019

Improve maximum E-field

‣ Limited by discharging He gas

‣ Coverlay on electrodes, reducing “hot spot”, materials with lower εr

E-field strength

Kapton

Sapphire

Electrode

He

Pressure (mbar)0 5 10 15 20 25

Volta

ge (V

)

0

1000

2000

3000

4000

5000

Discharged Voltage

Graph

20