Realizing an Ion Channel Laser at FACET-II

Mike Litos

Oct. 15, 2015

SLAC National Accelerator Laboratory

FACET-II Science Opportunities Workshop

2M. Litos - FACET-II Workshop - Oct. 15, 2015

What is an Ion Channel Laser and who cares?

• Ion Channel Laser (ICL) is coherent betatron radiation

produced by the undulating of a beam in an ion channel

• Put forth by Whittum in 1990* piggybacking on L/PWFA

• Compact alternative to a magnetic undulator FEL

• Advantages:

• linear focusing of ion column simultaneously transports

and undulates beam• can reach extremely high effective fields if desired• field strength and length tunable over wide range• like everything plasma, it’s compact!

*D. Whittum, A. Sessler, J. Dawson, Phys. Rev. Lett. 64 21 (1990)

3M. Litos - FACET-II Workshop - Oct. 15, 2015

Wigglers and Undulators

Synchrotron radiation emitted in cone with angle Dq ~ 1/g

Max. angle of deflection (i.e. angle at zero crossing) is y0 = 2p rmax / l0

Characterized by strength, K = g y0 = g 2p rmax / l0

Wiggler: K > 1 incoherent, broadband synchrotron spectrum

Undulator: K < 1 coherent, narrow bandwidth spectrum

2Dqrmaxl0

e-e-

4M. Litos - FACET-II Workshop - Oct. 15, 2015

The K Challenge

• Plasma wigglers are cool,

but not cool enough for

this talk

• Want an undulator:

K = g k0 rmax < 1

• If g = 1000, then

rmax k0 < 1/1000, where

k0 = kb = 2p / lb• In other words:

rmax ≲ lb / 1000

• …And sr << rmax

rmax

lb

e-

5M. Litos - FACET-II Workshop - Oct. 15, 2015

Lower Limit on Plasma Density

• lb = lp √2g ~50 lp n∝ p1/2

• Gain length:

Lg = lb (3 p r)-1/2

• Pierce parameter: r ~10-3

Lg ≈ 10 lb Lp > 10 lb

• for FEL, need Rayleigh length LR

~ sr R b / l > Lg

• radiation is guided in plasma, so

Lg > LR is OK

• for Lp < 1m lb < 10 cm

np > 3x1014 cm-3

radiated power vs. length

Lg

6M. Litos - FACET-II Workshop - Oct. 15, 2015

Upper Limit on Plasma Density

• Blowout radius RB ~ kp-1

• rmax >> sr, and RB > rmax

kp-1 >> sr

• If sr ~ 100 nm, kp-1 > 10 µm

• np < 3x1017 cm-3

• plasma density range with

g = 1000, sr = 100 nm, r = 10-3:

3x1014 cm-3 < np < 3x1017 cm-3

e-

RB

rmax

7M. Litos - FACET-II Workshop - Oct. 15, 2015

Emittance and Energy Spread

• l > p en / g l > en / 300

• for l = 1 nm, en < 300 nm-rad

• for SASE, need sd < r ~10-3:

• this is the hardest parameter <0.1% energy spread!

• Will need fancy injection technique to make tiny beam

8M. Litos - FACET-II Workshop - Oct. 15, 2015

Fancy Injection (Trojan Horse)

• Many injection techniques; will consider Trojan Horse

• Laser-ionized He electrons trapped in high-gradient wake

• Ultra-bright beams with tunable charge

• Can make short, low-charge, high peak current beams

• Can inject off-axis

• Let’s assume the following:

• Q = 10 pC

• en = 100 nm-rad

• g = 1000

• sd = 0.1%

• sr = 100 nm

• rmax = 10 µmB. Hidding

9M. Litos - FACET-II Workshop - Oct. 15, 2015

The Other Magic Ingredient: Wakeless Plasma Source

• It is possible to bare ion

column with no longitudinal

fields

• Requires RB > Rion

• Can pre-ionize narrow column

• Or use large beta-function

drive beam

• Leaves behind purely

focusing, non-accelerating ion

channel

• Important for

monochromatic light!

W. An

S. Corde

10M. Litos - FACET-II Workshop - Oct. 15, 2015

Putting it Together: Injection + Undulation

• Injection must occur in strong blowout wake

• Undulation should occur in wakeless ion channel

• There must be seamless matching from one to the next

• Two ideas: stepped plasma and/or tapered plasma

e-

e-

g

g

injection: 20 cm undulation: 1 m: ~20 Nb

both pre-formed in gas with laserne ~1017 cm-3

~50 µm

11M. Litos - FACET-II Workshop - Oct. 15, 2015

Challenges with Plasma Source

• Would require new custom optic (axilens-kinoform?)

• Might require two counter-propagating lasers

• Need to make transition sufficiently short (how short?)

• Most importantly: need to avoid dark current from down-

ramp injection at boundary between stages

e-

e-

g

g

injection: 20 cm undulation: 1 m: ~20 Nb

both pre-formed in gas with laserne ~1017 cm-3

~50 µm

12M. Litos - FACET-II Workshop - Oct. 15, 2015

Summary

• Ion Channel Laser is a compact and tunable alternative

to a magnetic undulator FEL

• Requirements for SASE in ion channel are stringent

• Need to use fancy injection scheme, like Trojan Horse

• Preferably use wakeless ion channel for undulator

• Need to stitch together injection and undulation stages

• Tolerances may really be in reach at FACET-II

• But clearly much, much more development needed