Laser Stripping for H - Injection

Laser Stripping for H- Injection

Wolfgang Bartmann

ATS Seminar, 23-Jan-2014

CERN Seminar, Laser Stripping 2

Outline• Introduction to multiturn injection• Motivation for H- injection• Principles of H- injection with foil

– Required hardware – Limits– Machines with different parameters

• Laser stripping as alternative– Principles– Laser requirements– Implications for lattice and optics– Challenges– Status

• References

23-Jan-2014


• For hadrons the beam density at injection can be limited either by space charge effects or by the injector capacity

• If we cannot increase charge density, we can sometimes fill the horizontal phase space to increase overall injected intensity.– Condition that the acceptance of receiving machine is larger than the

delivered beam emittance

23-Jan-2014

Multi-turn injection for hadrons



Septum magnet

• No kicker • Bump amplitude decreases and inject a new bunch at each turn• Phase-space “painting”

Closed orbit bumpers

Varying amplitude bump

23-Jan-2014



1

Turn 1

Septum

X

'X

Example: CERN PSB injection,

fractional tune Qh = 0.25

Beam rotates p/2 per turn in phase space

On each turn inject a new batch and reduce the bump amplitude

23-Jan-2014



1

2

Turn 2

X

'X

23-Jan-2014



3

2

1

Turn 3

X

'X

23-Jan-2014



4

1

2

3

Turn 4

X

'X

23-Jan-2014



3

2

4

51

Turn 5

X

'X

23-Jan-2014



6

5

4

3

21

Turn 6

X

'X

23-Jan-2014



7

6

5

4

3

2

1

Turn 7

X

'X

23-Jan-2014



7

6

5

4

3

21

8

Turn 8

X

'X

23-Jan-2014



7

6

5

4

3

2

1

8

9

Turn 9

X

'X

23-Jan-2014



7

6

5

4

3

21

8

9

10

Turn 10

X

'X

23-Jan-2014



7

6

5

4

3

2

1

8

9

10

11

Turn 11

X

'X

23-Jan-2014



7

6

5

4

3

21

8

9

10

11

Turn 12

X

'X

23-Jan-2014



7

6

5

4

3

2

1

8

9

10

11

Turn 13

X

'X

23-Jan-2014



7

6

5

4

3

21

8

9

10

11

Turn 14

X

'X

23-Jan-2014



7

6

5

4

3

2

1

8

9

10

11

Turn 15

In reality filamentation occurs to produce a quasi-uniform beam

X

'X

Phase space has been “painted”

23-Jan-2014


Charge exchange H- injection

• Multiturn injection is essential to accumulate high intensity

• Disadvantages inherent in using an injection septum– Width of several mm reduces aperture– Beam losses from circulating beam hitting septum– Limits number of injected turns to 10-20

• Charge-exchange injection provides elegant alternative– Possible to circumvent Liouville’s theorem, which says that emittance is

conserved

– Convert H- to p+ using a thin stripping foil, allowing injection into the same phase space area

23-Jan-2014

CERN Laser Stripping, WS Fermilab 21

H- injection foil stripping

140

mm

p+, H0 ,H-

B~1.6 T

p+, p+(H0n≥ 2)

H0 (n=1), H-

B ≤ 0.13 T40

Chicane bump

QF QF

D4D3D2D1

24 m

26-Sept-2013


H- injection painting

• Now we “overinject” onto the same phase space area already occupied by the circulating beam

23-Jan-2014


Foil stripping - HW• Chicane and painting bumpers (~4 each)

– Chicane: one of these septum like depending on geometry

• Stripping foil – Thin foil: Foil holder with exchange possibility – Thick Foil: convert unstripped H- and H0 to protons to guide them into dedicated waste

beam channel– Vacuum and radiation compatibility

• Waste beam handling– Internal dump (PSB)– Short line to external dump– Electron collector– Careful design of beam trajectories required due to different angles for H -,H0 in chicane

magnets– Machines with running H- injection suggest tracking studies of waste beams and a high

number of diagnostic possibilities

23-Jan-2014


Problems with foils

Foil damage• Present machines on the

high beam power frontier are approaching the limits of foils

23-Jan-2014

Sarah Cousineau, Laser stripping WS, Fermilab, 26-27 Sept-2013


Problems with foils• Beam loss and radiation

– Beam loss due to foil scattering (foil is highest loss point in SNS accelerator complex)

• Emittance growth due to foil scattering– Relevant for multi-purpose machines

• Laser stripping avoids a mechanical interaction with the beam

23-Jan-2014


Comparison of machine parameters

…where laser stripping is considered (except for ISIS) – likely not complete

Parameter Units ISIS SNS HP-PS FNAL ESS accumulator

Kinetic energy at inj [GeV] 0.8 1 4 8 2

Beam power at inj [MW] 0.16 1.2 - 3 0.19 0.34 5

Repetition rate [Hz] 50 60 1 10 14

Total beam intensity 2.5e13 1.25-3.1e14 2.5e14 2.7e13 2.8e14 (x 4)

# injected turns 150 - 200 1100 600 360 650

Macropulse length ms 0.2 1 2 4 0.73 (x 4)

Microbunch structure MHz 202.5 402.5 352 162.5 704.42

Motivation for laser stripping

Loss control,radiation

Loss control,radiation,

em growthLoss control,

radiation

23-Jan-2014


Include laser into injection setup

23-Jan-2014

140

mm

p+, H0 ,H-

B~1.6 T

p+, p+(H0n≥ 2)

H0 (n=1), H-

B ≤ 0.13 T

40

Chicane bump

QF QF

D4D3D2D1

Wiggler or laser for neutralisation

Laser for excitation Dedicated design of D3


Laser stripping concept: H- to H0

• Either Lorentz-stripping in a wiggler magnet…

• or Photo-dissociation

23-Jan-2014


Lorentz stripping

• H- ion moving in magnetic field Lorentz force tends to break it up

• Binding energy of extra electron 0.76 eV

• In ion rest-frame electric field E is the Lorentz-transform of the magnetic field B in the lab-frame

• The ion’s lifetime can be parametrized as

,

23-Jan-2014


H- neutralisation: Wiggler

• Magnet with ∫B·dl = 0 Lorentz stripping

• Vertical no extra horizontal angular spread which would increase the laser power

• HP-PS: at least 0.6 T to keep emittance growth small

• Integration could be an issue

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 210

-1

100

101

102

103

B [T]

Nor

mal

ised

em

itta

nce

grow

th [

pi.m

m.r

ad]

H-

p+

B < 0.1 TB > 0.6 T

B.dl = 0

H0 , p+

stripping wigglermerging chicane dipole

23-Jan-2014


H- neutralisation: Laser• No emittance growth• Easier integration than wiggler?• Numerical calculation of neutralisation degree

20 40 60 80 100 120 140 160 18010

1

102

103

Intersection angle [deg]

Las

er m

icro

bun

ch e

nerg

y [m

J]

Laser micro pulse energy required for 99% neutralisation:

60mJ per micropulse- with factor 1000 reduction

from recycling- fRF=352MHz and Tinj =2msgives 42 J per macropulse

23-Jan-2014

Demanding for the laser (and vacuum window)


H- neutralisation: LaserFeshbach resonance as alternative?

• Feshbach resonance appears at 113.49 nm (10.93 eV) in the H- photodissociation spectrum

• HP-PS:– With 1064 nm laser accessible with intersection angle of 37°

– To reach 99% neutralisation (crosssection~1.39e-15 cm2) a laser micropulse energy of 190 uJ is required

– However: Δλ/λ~5.2e-6 while beam Doppler spread ~2e-3

– Need another factor 5e2 for full beam neutralization, leads to a laser micropulse energy of 73 mJ

23-Jan-2014


H- neutralisation: Lasern=2 shape resonance as alternative?

• The resonance at 112.95 nm for the reaction

cross-section of 9.8e-17 cm2

• HP-PS– Requires 2.8 mJ micropulse energy but with much larger linewidth of 1.8e-4 resulting in

a total micropulse energy of 31 mJ

• The H0 is then already in an excited state and can be resonantly excited from n=2 to n=3

• Dual advantage of longer lifetime for spontaneous decay and shorter lifetime for stripping to p+ in magnetic field

23-Jan-2014


H0 --> p+ strippingDoppler shift of laser frequency

HP-PS:• 1064 nm laser• n=2 can be reached

with 47.5° between ion beam and laser

• n=3 with 8.39 °

0 0.5 1 1.5 2 2.5 30

2

4

6

8

10

12

14

Intersection angle [rad]

Pho

ton

ener

gy i

n at

omic

fra

me

[eV

]

4 GeV4.5 GeV3.5 GeV

n=4,5,...

n=3

n=2

23-Jan-2014

𝑓 𝑟𝑒𝑠𝑡=𝛾 (1+𝛽cos𝜃 ) 𝑓 𝑙𝑎𝑏


H0 --> p+ strippingdivergent laser beam

*00 HHh

laser

H0

0*0 HhH

spontaneousdecay B > 0.5 T

H0

p+

stripping dipole

Scheme developed and tested at SNS (Danilov et al.):• Resonant excitation of ground-state H0 in field

free region• Stripping of excited electron in magnetic field• Large spread of effective resonance

frequencies divergent beam

23-Jan-2014

H-

Divergent laser light


H0 --> p+ strippingdivergent laser beam

• Using this scheme for HP-PS:• Excitation to n=2 or n=3: 360 and 92 uJ laser micropulse energy• This leads to > 20 MW laser peak power!• Spontaneous decay reduces the efficiency:• 1.7% of n=3 decay in 25 cm drift between laser interaction and stripping

point

23-Jan-2014

Using a divergent beam to compensate the doppler broadening of the transition affects strongly the required laser power

How to reduce laser power (Danilov et al., Future prospects for laser stripping):- Locking between laser and beam temporal

structure- Dispersion tailoring- Photon recycling- Bunch length minimisation- Vertical beam size minimisation- Beam angular spread minimisation


Dispersion tailoring• For off momentum particles:

and

• Together with the effective laser frequency in the rest frame – - the dispersion angle can be chosen such that the doppler broadening due to dp/p is compensated

• HP-PS– Need a D’ of about 2.0 rad– LSS has zero dispersion– Target emittance should not be significantly affected by injection process– Assuming a target emittance of ~13 um– Injected transv emittance: 0.4 um, emittance growth less than 0.3 - 0.5 um– Assuming a maximum acceptable dispersion of 0.2 m– This leads to a maxiumum D’ of 0.05 rad

23-Jan-2014

Interesting option if D’ can be accomodated in optics design of injection region


Dedicated design of stripping magnetFringe field stripping – emittance growth

• Lifetime in magnetic field depends on quantum state, B-field and ion momentum

• Lifetimes of 4 GeV H0* in dependence of B and a simulated fringe field

• numerically integrated to get rms angle error and hence emittance growth

• n=3 gives Δε of 2 – 4 um for B = 1 T

• Careful fringe field design needed!

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 210

-2

10-1

100

101

102

103

Peak magnetic field [T]

Nor

mal

ised

em

itta

nce

grow

th [

pi.m

m.m

rad]

n=2

n=3

n=4

n=5

n=6

Normalised emittance growth for different H0* quantum states as a function of peak magnetic field

23-Jan-2014


Stark broadening• Stark effect: charged particle in an electric field transitions will be broadened

• Overcome Doppler broadening by placing interaction region in a magnetic field large Stark-broadening of transition

• Single frequency can excite the resonance for all atoms

• Stimulated emission suppressed

• HP-PS:

– To reach Doppler width of 2e-3 need 0.3 T for n=3 transition

– Lifetime of H0* only 1e-10 s

– Required laser micropulse energy slightly higher than previous method

– Excitation takes place over ~0.5 mwhich introduces a large angle between injected and circulating protons difficult integration

10-2

10-1

100

101

102

10-8

10-6

10-4

10-2

100

Magnetic field [T]

Rel

ativ

e li

new

idth

n=1n=2n=3n=4n=5

Stark-broadening of quantum levels vs B-field for 4 GeV H0

23-Jan-2014


Laser characteristics for HP-PS (H0 --> p+ stripping)

Parameter n=2 n=3

Wavelength nm 1064 1064

Laser/H- angle deg 47.50 8.39

Angular spread deg ±0.10 ±0.42

Micropulse energy uJ 360 92

Macropulse length ms 2 2

Macropulse energy J 253 65

Peak power (single pass) * 3 (margin) MW 21.6 5.5

Average power (single pass) kW 43.2 11.0

Average power (mode-locking) W 760 193

Average power (Q=100) W 432 110

Vertical laser beam height (1 σ rms) mm 1.5 1.5

Linac pulse, 1 σ rms separated by 2.84 ns ps 15 15

Laser repetition rate (max) Hz 1 1

Availability % 99 99

23-Jan-2014

Relevant parameters as input for laser feasibility discussion


Laser stripping - HW• Chicane and painting bumpers (~4 each)

– Chicane: one of these septum like depending on geometry and specially designed stripping magnet

• Stripping foil – Thin foil: Foil holder with exchange possibility – Thick Foil: convert unstripped H- and H0 to protons to guide them into dedicated waste beam

channel– Vacuum and radiation compatibility

• Waste beam handling– Internal dump (PSB)– Short line to external dump– Electron collector– Careful design of beam trajectories required due to different angles for H-,H0 in chicane magnets– Machines with running H- injection suggest tracking studies of waste beams and a high number of

diagnostic possibilities

• Laser– Wiggler magnet or laser for H- neutralisation– Laser for excitation + optical resonator– All the equipment to run the laser (optical table, fiber,…)– Vacuum window

23-Jan-2014


Implications for beam, lattice and optics

• General– A combined foil/laser stripping system for beam energies of several GeV

requires (eg. HP-PS and FNAL) about 20-25 m drift space– Matching of curvature of ring phase space and incoming turn implies:

• Foil– Minimum βi given by foil heating

– βi / βr : Smaller ratio helps to reduce foil hits but increases foil temperature

– αi = αr = 0 – Place foil in fringe field of chicane dipole such that H0*are stripped into the

machine acceptance

23-Jan-2014

• Laser– Laser peak power is proportional to vertical injected beam size if the beam-

laser interaction is horizontal• Minimize as much as possible vertical beam size in IR triplet structure in LSS for

FNAL and CERN studies– D’ at interaction point to eliminate Doppler broadening– Longitudinal painting:

• large momentum range increases tremendously the required laser power– Bunch length:

• increases laser average power– Energy jitter:

• increases frequencies to be swept• should be an order of magnitude lower than initial dp/p

– Trajectory jitter

Implications for beam, lattice and optics


Injection area optics

• Contradicting requirements for foil and laser two different optics settings

23-Jan-2014

Foil optics Laser optics


Combined chicane for foil and laser

23-Jan-2014

Foil 1

Foil 2

Waste beam

p+

B=1.6 TB ≤ 0.13 T

Stripping of first e-

H-

H0

Laser


Items which deserve attention• Laser

– Locking to beam temporal structure– Optical resonator– Transport of laser light, radiation hardness

• 300 J through a vacuum window• Integration of wiggler and laser with variable interaction angle• Magnet design of wiggler and stripping chicane magnet (fringe field)• Combination of foil and laser system, 3- and 4- magnet bumps with

dispersion closed

23-Jan-2014


Summary• High-intensity/brightness H- injection

• H- injection for high beam power– Challenges with foils: damage, losses, radiation, emittance growth

• Laser stripping avoids mechanical interaction with beam– H- neutralisation: Lorentz-stripping in wiggler magnet or Photo-dissociation with laser

(resonances)– H0 to p+: excite ions on resonance and use the therefore higher probability of Lorentz-stripping

in a chicane magnet

• Challenges: – Commercial lasers don’t fully match yet the requirements from accelerators but this

community is extremely fast evolving– Wiggler design/integration– Fringe field design of stripping chicane magnet– Vacuum windows withstanding laser power

23-Jan-2014


Status quo of laser stripping• At present

– R&D phase– Proof of principle at SNS demonstrated 90% stripping efficiency for ~7 ns (see

V. Danilov et al., PRST)• Midterm:

– 3-year experiment at SNS started with the aim of more than 90% stripping for 10 us (see talk S. Cousineau at FNAL workshop)

– Requires a photon recycling optical cavity (see talk M. Notcutt at FNAL workshop)

– Final experiment foreseen for Jan-2016• Longterm

– Laser stripping to be operational with similar or better performance than foil stripping … 2020-2025 ???

23-Jan-2014


References• V. Danilov et. al., Physical Review Special Topics – Accelerators and Beams 6, 053501• V. Danilov, Future prospects for laser stripping in high intensity machines• B. Goddard, Injection and Extraction, CAS slides• B. Goddard et al., Laser Stripping for the PS2 Charge-Exchange Injection System, PAC09• S. Cousineau, SNS Laser Stripping, FNAL workshop, see below• I. Yamane et al., POP experiment of Laser Stripping via a Broad Stark State Using BNL 200

MeV H- beam, ICFA-HB2004• D. Johnson, Conceptual Design Report of 8 GeV H- Transport and Injection for the Fermilab

Proton Driver• W. Bartmann et al., Laser stripping for CERN HP-PS, FNAL workshop, see below• W. Bartmann, B. Goddard, H-Injection into PS2 and Laser Stripping, SNS workshop, see below• H. Schönauer, ESS accumulator parameters• Laser stripping workshop 2013 at FNAL: https://

indico.fnal.gov/conferenceOtherViews.py?view=standard&confId=6855• Laser stripping workshop 2009 at SNS:

http://wiki.ornl.gov/events/lahbsa/default.aspx

23-Jan-2014

Thank you!

https://indico.fnal.gov/conferenceOtherViews.py?view=standard&confId=6855

https://indico.fnal.gov/conferenceOtherViews.py?view=standard&confId=6855




Laser Stripping for H - Injection

Documents

Transcript of Laser Stripping for H - Injection