1. Realizing an Ion Channel Laser at FACET-II Mike Litos Oct. 15, 2015 SLAC National Accelerator Laboratory FACET-II Science Opportunities Workshop

2. 2 What is an Ion Channel Laser and who cares? M. Litos - FACET-II Workshop - Oct. 15, 2015 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)

3. 3 Wigglers and Undulators M. Litos - FACET-II Workshop - Oct. 15, 2015 Synchrotron radiation emitted in cone with angle  ~ 1/  Max. angle of deflection (i.e. angle at zero crossing) is  0 = 2  r max / 0 Characterized by strength, K =  0 =  2  r max / 0 Wiggler: K > 1  incoherent, broadband synchrotron spectrum Undulator: K < 1  coherent, narrow bandwidth spectrum 2  r max 0 e-e- e-e-

4. 4 The K Challenge M. Litos - FACET-II Workshop - Oct. 15, 2015 Plasma wigglers are cool, but not cool enough for this talk Want an undulator: K =  k 0 r max  If  = 1000, then r max k 0 < 1/1000, where k 0 = k  = 2  /  In other words: r max ≲   …And  r << r max r max  e-e-

5. 5 Lower Limit on Plasma Density M. Litos - FACET-II Workshop - Oct. 15, 2015  = p √2   ~50 p ∝ n p 1/2 Gain length: L g =  (3  ) -1/2 Pierce parameter:  ~10 -3  L g ≈ 10   L p > 10  for FEL, need Rayleigh length L R ~  r R  / > L g radiation is guided in plasma, so L g > L R is OK for L p 3x10 14 cm -3 radiated power vs. length Lg

6. 6 Upper Limit on Plasma Density Blowout radius R B ~ k p -1 r max >>  r, and R B > r max  k p -1 >>  r If  r ~ 100 nm, k p -1 > 10 µm  n p < 3x10 17 cm -3 plasma density range with  = 1000,  r = 100 nm,  = 10 -3 : 3x10 14 cm -3 < n p < 3x10 17 cm -3 M. Litos - FACET-II Workshop - Oct. 15, 2015 e-e- RBRB r max

7. 7 Emittance and Energy Spread M. Litos - FACET-II Workshop - Oct. 15, 2015 >  n /    >  n / 300 for = 1 nm,  n < 300 nm-rad for SASE, need   <  ~10 -3 : this is the hardest parameter  <0.1% energy spread! Will need fancy injection technique to make tiny beam

8. 8 Fancy Injection (Trojan Horse) M. Litos - FACET-II Workshop - Oct. 15, 2015 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  n = 100 nm-rad  = 1000   = 0.1%  r = 100 nm r max = 10 µm B. Hidding

9. 9 The Other Magic Ingredient: Wakeless Plasma Source M. Litos - FACET-II Workshop - Oct. 15, 2015 It is possible to bare ion column with no longitudinal fields Requires R B > R ion 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

10. 10 Putting it Together: Injection + Undulation M. Litos - FACET-II Workshop - Oct. 15, 2015 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- e-e-   injection: 20 cm undulation: 1 m: ~20 N  both pre-formed in gas with lasern e ~10 17 cm -3 ~50 µm

11. 11 Challenges with Plasma Source M. Litos - FACET-II Workshop - Oct. 15, 2015 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- e-e-   injection: 20 cm undulation: 1 m: ~20 N  both pre-formed in gas with lasern e ~10 17 cm -3 ~50 µm

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