Laser Accelerators: The Technology of the Future (They Always Have Been and They Always Will Be ?)

1. Laser Accelerators: The Technologyof the Future(They Always Have Been and They Always Will Be ?) • Cockcroft Institute Laser Lectures • April 2008 Graeme HirstSTFC Central Laser Facility

2. Lecture 6 Plan • HHG (from lecture 4) • Context • Plasmas and acceleration • Early results • “Dream beams” • Prospects • Summary

3. An electron’s return energy (the t=0trajectory slope) is set by the time atwhich it tunnelled out. High energiesappear only in a narrow time window High energy electrons generateattosecond X-rays. Once again atrain of short pulses in time mapsto a comb in frequency High Harmonic Generation HHG can be understoodusing the 3-step model:tunnel ionisation, classicalacceleration and recollision

4. CutoffIp+3Up Plateau w  High Harmonics - Spectrum The spectrum consists of oddharmonics whose energy fallsrapidly to a broad plateauwith a sharp cutoff Each plateau harmonic canhave~10-6 of the drive energy Conversion efficiency is affected by target ionisation, absorptionand phase matching. The spectrum can be tailored by control ofthe driving E-field and quasi phase matching in a target capillary E-field control can also fine tune the harmonic wavelengths tofill spectral gaps above ~40eV. Reasonably efficientpulse-length preserving monochromators based onsagittal grating designs are nowbecoming available

5. High Harmonics - Prospects • AVERAGE POWER: 10-6 conversion of a 10mJ laser pulse should already give 109 hn/pulse at 50eV which exceeds the pulse performance of undulators by a factor of ~104. Kilowatt class lasers should deliver >1014 hn/s which is 1-2 orders of magnitude below the average power from undulators. • PHOTON ENERGY: Raising the cutoff energy will involve raising the electron energy. Options include using ions (higher Ip) or longer drive wavelengths (higher Up, but with more time for core wavepacket expansion hence lower recollision probability) • SURFACE HARMONICS: Laser scattering from a rapidly oscillating dense plasma delivers keV harmonics with higher conversion efficiency than HHG in gases but requires a “national scale” drive laser

6. Nature Phys,2 (10) 696 Phil Trans R Soc A,364 (1840) Nature, 431 (7008) Laser Acceleration - References Phys Rev Letts, 43 (4) 267

7. Motivation ~13km ~13km ~4km ILC (1TeV) cost $6.6bn, 22km of SCRF Conventional acceleratorsare large and expensive(and this is not only trueof world class particlephysics machines) FLASH (1GeV)cost €190M(not “greenfield”), and has>50m of SCRF The accelerating field is limited to~100 MV/m (often less)

8. E B P l Laser Generated Fields The peak field in an EM wave is: But the field is transverse and oscillatingat >1014 Hz so electrons acquire only keVenergies (hence HHG cutoff) The field can be converted to a quasi-static (for co-propagatingelectrons) longitudinal one using plasma waves. Breakdown is not an issue asplasmas are already ionised.

9. Basic plasma theory predicts electron density oscillations with which is ~40 times lower than optical frequencies if theelectron density, ne, is 1018 cm-3 However laser accelerators operate in an extreme regimewhere basic theory can break down. Complications include: • Electrons becoming relativistic • Plasma and optical behaviour becoming nonlinear • Fields approaching the wave-breaking limit Analytic modelling now becomes difficult and large scalecomputational approaches (e.g. PIC)become important Plasma Waves

10. WAKE FIELD (LWFA) A single short pulse drives electrons forwardsand sideways leaving a depleted volume in itsimmediate wake. In the so-called “bubbleregime” electron re-injection can be restrictedto a narrow phase window leading to“monoenergetic” output. Laser-Plasma Coupling Several mechanisms for coupling laser energy into plasmawaves have been tried. They include: BEAT WAVE (PBWA) Two long pulses from lasers whose frequenciesdiffer by wp co-propagate through the plasma.The electrons respond to the field envelope.

11. Acceleration Limits DEPHASING: Highly relativistic electrons travel faster than theplasma wave and eventually leave the high field region The velocity difference can be reduced by lowering ne, giving alonger dephasing length but also lowering Ewb. It turns out: An energy gain of ~1GeV needs a dephasing length of >30mm DEFOCUSING: A lab-scale 1019 W/cm2 laser has a Rayleighrange of a few mm. So ~1GeV needs beam confinement This can be achieved in free plasma by filamentation or by“pre-drilling” with another laser Or a capillary can be used

12. 1011 1010 109 Number of electrons (/MeV/sr) 108 107 106 0 50 100 150 200 250 Electron energy (MeV) Early Results Laser pulses longer than the plasma period can be temporallysubdivided by self modulation. The sub-pulses drive wakeswhich accelerate electrons to high energies in a few mm. Butall oscillation phases are populated so the spectrum is broad. The highest energyelectrons are rapidlyaccelerated in a smallpart of the wake, sotheir emittance is low. However their numberis also low and varieswidely from shot to shot.

13. Energies were still limited by laserdefocusing. Parameter windowswere tight enough for shot-to-shotreproducibility still to be poor. Butthe principles were proven. ne = 6×1018 cm-3 (upper), 2×1019 cm-3 (lower) Low DE/E – “Dream Beam” Early in this decade ~100MeV beams with few percent DE/Ewere produced in three laboratories. Bunch charges could betens or hundreds of pC and beam divergences just a few mrad. The secret was precise tuning of laser andplasma parameters to generate a strong wake,to self-inject over a very narrow phase rangeand to extract the beam before degradation.

14. E=1.0±0.06 GeV, DE=2.5% rmsDQ=1.6mrad, Q=30pC 1 GeV Guiding of the drive laser beam using another laser had beenreported in one of the “dream beam” papers A pulsed electric capillary discharge nowcreated a radial density gradient in thetarget gas. The resulting light guideconfined the 40TW laser beam for 33mm

15. A pulsed electric capillary discharge nowcreated a radial density gradient in thetarget gas. The resulting light guideconfined the 40TW laser beam for 33mm 1 GeV Guiding of the drive laser beam using another laser had beenreported in one of the “dream beam” papers With 12TW drive and a narrowercapillary, 50pC beams at0.46±0.05GeV with DE=6% wereproduced on every shot where thedischarge-to-laser delay was correct A 0.9J laser pulse had produced a 25mJelectron bunch i.e. 3% energyconversion efficiency

16. Prospects STABILITY: Controlling self-injection is critical. It is very sensitiveto experimental conditions, but “reliably” so. Current jitters areE ~5-10% and Q >10%. DE/E is 2-5%. In addition to betterexperimental control, new approaches are being considered. “EXTERNAL” INJECTION: May be another route for controllingthe beam. Electrons may be prepared using a second laser forPBWA. Alignment, synchronisation, emittance requirementsare demanding. But they may need to be solved in any case for: STAGING: Controlling dephasing by further reducing ne willeventually become unmanageable (if only because of drivelaser depletion). As with conventional accelerators the solutionwill be to use a larger number of discrete moduleswith the beam re-phased between them

17. Few hundredMW turbines Few hundredwatt alternator Average Current - An Issue For some applications it hard to imagine plasma acceleratorsever generating sufficient average power The 4GLS Energy Recovery Linac was specified at0.6GeV/100mA (60MW power). The ILC beams are 45MW With 6% conversion a 1kW drive laser might deliver0.6GeV/100nA (60W power)

18. Prediction • Courtesy of Simon Hooker: • “It seems likely that in the next few years we will seevery compact laser-driven plasma accelerators with • Controlled electron injection • Energies up to a few GeV • Energy spread <1% • Pulse duration ~10fs • Bunch charge 10-100pC • Pulse repetition rate 10Hz”

19. Note goodfits betweenmeasuredspectra andpredictionsbased on theassumptionof undulatorradiation LWFA Undulator Radiation Hot off the press(Nature Phys, 4 (2) 130)

20. Summary • Over 25 years laser plasma accelerators became capable of generating ~100MeV electrons with reasonable emittance. However their relative number was small and their parameters hard to reproduce • In the last 5 years the advent of high power, short pulse lasers has delivered monoenergetic beams with low emittance and much higher bunch charge. As laser and plasma control improves, so shot-to-shot bunch variations decrease • In the last few months the first use of plasma accelerated electrons to produce undulatorradiation has been reported

21. Thank you !