ELI-NP laser IP cavity 1)Requests 2)Recirculating cavity 1) biblio 2)limits 3)Fabry-Perot cavity solution a)Technical Contraints  limits b)Possible solution : 2 frequencies 1)Requests 2)Recirculating cavity 1) biblio 2)limits 3)Fabry-Perot cavity solution a)Technical Contraints  limits b)Possible solution : 2 frequencies 1

2... 5ns (200MHz)... 5ns 1ms (100Hz) Assume the following Laser request at the Compton IP ~1µm  max ~10ps P peak =10 11 W =10kW ~1µm  max ~10ps P peak =10 11 W =10kW

Recirculating cavity M. Y. Shverdin et al. High Power Picosecond Laser Pulse Recirculation Opt Lett35(2010)2224 In the 2010 paper: incident 1µm beam and 177mJ after freq doubling. Pulse energy measurement turn after turn : In the 2010 paper: incident 1µm beam and 177mJ after freq doubling. Pulse energy measurement turn after turn : 6% loss per cavity round trip 3 LLNL (& now BNL/AFT) method 50th pulse

Recirculating cavity experimental results  Estimated integrated pulse energy : 3J in total : Need 33 times more  : need 10 times more ?  Nothing after 50 pulses : need less than 50 pulses  No information on the laser beam profil quality  Estimated integrated pulse energy : 3J in total : Need 33 times more  : need 10 times more ?  Nothing after 50 pulses : need less than 50 pulses  No information on the laser beam profil quality 4

5 Technique limitations Numerical estimate in : Non linear effects Induced in the freq doubler The highest the efficiency the worse the beam profile

6 ~50% Compton losses after 20 passes  spectrum modified after 20 round trips

A new démonstration experiment à BNL/ATF (2011 ) R&D experiment 2-3J total(idem paper Hz Upgrade foreseen at Hz But still an R&D R&D experiment 2-3J total(idem paper Hz Upgrade foreseen at Hz But still an R&D 7

8 Bibliography Summary  Advantage : easy to enter the cavity  Drawbacks/issues  Non-linear effects  Nb of passes limited (~20)  Beam profil not shown & beam ellipticity not adressed  mirror damage issue not adressed (40cmx40cm cristal for 1J…)  Still frar from being a mature techno  visit/contact BNL Bibliography Summary  Advantage : easy to enter the cavity  Drawbacks/issues  Non-linear effects  Nb of passes limited (~20)  Beam profil not shown & beam ellipticity not adressed  mirror damage issue not adressed (40cmx40cm cristal for 1J…)  Still frar from being a mature techno  visit/contact BNL

9 Puzzlingly LLNL proposes another Techniques for an ILC gg collider laser source (laser request not so far from ELI-NP !) Puzzlingly LLNL proposes another Techniques for an ILC gg collider laser source (laser request not so far from ELI-NP !)

Oscillator 200MHz Oscillator 200MHz Ampli  t~500ns 5mJ/pulses 100 pulses 50W Ampli  t~500ns 5mJ/pulses 100 pulses 50W « empty » optical resonator F~600 1J/pulses for 100 pulses : 10kW 1 circulating pulse of 1J « empty » optical resonator F~600 1J/pulses for 100 pulses : 10kW 1 circulating pulse of 1J Scheme Issues 1.Laser amplifier cost >3M€ 1.Faisable (1rst discussion with Amplitude System, SupOptics laser groupe) 2.Effects of a 1J pulse inside an optical resonator 1.Mirror Fluence damage threshold constraint 1.A priori also a problem for the recirculating cavity 2.Cavity feedback 1.Thermal load in the mirors 2.Radiation pressure Issues 1.Laser amplifier cost >3M€ 1.Faisable (1rst discussion with Amplitude System, SupOptics laser groupe) 2.Effects of a 1J pulse inside an optical resonator 1.Mirror Fluence damage threshold constraint 1.A priori also a problem for the recirculating cavity 2.Cavity feedback 1.Thermal load in the mirors 2.Radiation pressure 10 Fabry-Perot technique

Figure 1: Dielectric bulk material, exposed to 0.9 ps laser pulses at l=532 nm, two spots in the middle of the picture show damage onset. The weaker of those spots was irradiated with a laser fluence just above the damage threshold. Figure 1: Dielectric bulk material, exposed to 0.9 ps laser pulses at l=532 nm, two spots in the middle of the picture show damage onset. The weaker of those spots was irradiated with a laser fluence just above the damage threshold. Results, 0.9 ps pulses The same measurements as for 5 ns pulses were carried out in order to compare damage thresholds for different time domains. Single shot results were Fth = 2,37 J/cm2 for bulk material (fused silica), Fth = 0,35 J/cm2 for the untreated HR mirror stack and Fth = 0,25 J/cm2 for the dielectric grating. Multishot measurements (n=100) gave Fth = 0,20 J/cm2 for the mirror and Fth = 0,26 J/cm2 for the grating, respectively. Results, 0.9 ps pulses The same measurements as for 5 ns pulses were carried out in order to compare damage thresholds for different time domains. Single shot results were Fth = 2,37 J/cm2 for bulk material (fused silica), Fth = 0,35 J/cm2 for the untreated HR mirror stack and Fth = 0,25 J/cm2 for the dielectric grating. Multishot measurements (n=100) gave Fth = 0,20 J/cm2 for the mirror and Fth = 0,26 J/cm2 for the grating, respectively. 11 For =532nm Damage threshold measurements of gold and dielectric coated optical components at 50 fs – 5 ns R. Bödefeld, W. Theobald, J. Schreiber, H. Gessner, E. Welsch, T. Feurer, R. Sauerbrey Damage threshold measurements of gold and dielectric coated optical components at 50 fs – 5 ns R. Bödefeld, W. Theobald, J. Schreiber, H. Gessner, E. Welsch, T. Feurer, R. Sauerbrey

Depends on the spot size Almost a facteur 10 for this exemple Depends on the spot size Almost a facteur 10 for this exemple Depends on, for 0.5µm : 2 times worse than 1µm ! 12 Depends on nb of pulses Damage threshold à 200MHz ??? Depends on nb of pulses Damage threshold à 200MHz ???

13 For 10ps : damage fluence ~3 times % 1ps  Threshold Fmax~0.6J/cm 2 for  Assume F max =0.1J/cm 2 For 10ps : damage fluence ~3 times % 1ps  Threshold Fmax~0.6J/cm 2 for  Assume F max =0.1J/cm 2 spotsize of the beam on the cavity mirror  For E max =1J, spotsize=10cm 2 ! spotsize of the beam on the cavity mirror  For E max =1J, spotsize=10cm 2 !

d1 d3  1 er waist  2 ème waist d4 d2  ≠ 0 : 2 plans à considérer Plan tangentiel(=plan cavité) f=Rcos  /2 Plan sagittal (plan cavité) f=R/(2cos  ) Plus    h  grand plus le mode propre de la cavité est elliptique Condition de stabilité : INTRODUCTION Bow-tie cavity : basic paraxial expressions R f=R/2 Normal incidence  = 0 : h  L=d2+d3+d4 L tot =d1+L électrons electrons

Example : ThomX L tot = m, d1=2m  = atan(h/d1)/2=0.6°  cross = atan(2h’/d1) = 1.7° Waists in µm Radii on mirrors in mm

Elliptical mode

d1d1 d3  d4 d2 h  1 er et 2 ième waist au même endroit électrons Drawback : beam pipe cut Autre géométrie, plus astucieuse : (M. Lacroix pour ThomX) Autre géométrie, plus astucieuse : (M. Lacroix pour ThomX) X rays We can make use of the ellipticity : The highest h the highest the ellipticipty  h must be as small as possible  The X-angle can be minimized with concave mirror with rectangular edges A minima : Diamètre du miroir = 6  M,min ~ 6*[1,3]mm R mirror = [3,9]mm Soit h min = 2[3,9]mm = [6,18]mm X-angle ~(R mirror +R beam pipe )/(d1/2) ~[9,15]/250~[2°,3.5°] We can make use of the ellipticity : The highest h the highest the ellipticipty  h must be as small as possible  The X-angle can be minimized with concave mirror with rectangular edges A minima : Diamètre du miroir = 6  M,min ~ 6*[1,3]mm R mirror = [3,9]mm Soit h min = 2[3,9]mm = [6,18]mm X-angle ~(R mirror +R beam pipe )/(d1/2) ~[9,15]/250~[2°,3.5°]

d 1 =  +R/cos(  )

L tot =c/f rep  1.5m pour f rep =200MHz d1d1 d3  d4 d2  L=d 2 +d 3 +d 4 L tot =d 1 +L d 1 =  +R/cos(  ) L=d 2 +d 3 +d 4 L tot =d 1 +L d 1 =  +R/cos(  ) For =532nm

Non-paraxial region

For =532nm ~300mJ/pulse for =1µm 100MHz ~300mJ/pulse for =1µm 100MHz Pushing the parameters

22 Feddback issues Cavity finesse F   /(1-r1*r2*r3*r4)  ‘phase matching’ r1=r2*r3*r4  F   /(1-r1^2)   /T1  power/energy enhencement factor  F/   Cavity resonance linewidth  FWHM  L=2  F  if the cavity length is shifted by  L=  F half of the power is lost Pound-Dever-Hall feedback methode  Linear error signal if cavity length variation < /F  Laser resonance frequency  nc/L,  Feedback control accuracy : At LAL we have already achieved But here we see 2 difficulties related to the high pic power

23 1rst problem : To fill all the pulses within a time interval < feedback bandwidth (1MHz AT MOST !)  The pulse stacking must not change the cavity length by more than ~ /F 2nd problem If the cavity has been correctly filled : no perturbation on the cavity length > ~ /F must be induced by the circulation of the very high energy pulse 1rst problem : To fill all the pulses within a time interval < feedback bandwidth (1MHz AT MOST !)  The pulse stacking must not change the cavity length by more than ~ /F 2nd problem If the cavity has been correctly filled : no perturbation on the cavity length > ~ /F must be induced by the circulation of the very high energy pulse What can change the cavity length within  t>1/MHz ? The radiation pressure : for a pulse of 10ps & 1J stength=2P/c~700 N  equivalent to a weight of 70kg falling on the mirrors each 5ns ! Stress wave propagates ~ at the sound speed (~6km/s in glass) The thermo-elastic coupling Absorption of the mirror coating layers ~1ppm But very fast mechanism can occur with 10ps pulses… E.g. What can change the cavity length within  t>1/MHz ? The radiation pressure : for a pulse of 10ps & 1J stength=2P/c~700 N  equivalent to a weight of 70kg falling on the mirrors each 5ns ! Stress wave propagates ~ at the sound speed (~6km/s in glass) The thermo-elastic coupling Absorption of the mirror coating layers ~1ppm But very fast mechanism can occur with 10ps pulses… E.g.

24  One can ‘solve’ the 2 nd problem using two wavelengths with high/low finesse  CALVA (LAL R&D)/VIRGO upgrade, see next slide  To look at the 1rst problem a possible experiment at the LASERX facilitiy could help (Ti:sapph 30fs,…100ns)  (R. Chiche & LaserX ‘young’ group, K. Cassou, Guillebaud, S. Kazamias)  One can ‘solve’ the 2 nd problem using two wavelengths with high/low finesse  CALVA (LAL R&D)/VIRGO upgrade, see next slide  To look at the 1rst problem a possible experiment at the LASERX facilitiy could help (Ti:sapph 30fs,…100ns)  (R. Chiche & LaserX ‘young’ group, K. Cassou, Guillebaud, S. Kazamias) O L~10cm R R ND:YAG cw <1W laser ND:YAG cw <1W laser ~ ~ modulation X X 1GHz synthetiser demodulation pdiode Error Signal  cavity length variation induced by high pulse power bandwidth ~1GHz Dynamic range : /F~20nm if F=50 bandwidth ~1GHz Dynamic range : /F~20nm if F=50 High energy pulse

Yb Oscillateur ~200MHz >8nm bandwidth Stabilisable 20mW. Inside : Steper motor Pzt EOM Double wedge pump modulation Yb Oscillateur ~200MHz >8nm bandwidth Stabilisable 20mW. Inside : Steper motor Pzt EOM Double wedge pump modulation Freq doubler preamplifier 4-mirror cavity Optical switch+ Ampli :  T~500ns-1µs 5mJ/pulses 100 pulses 50W Optical switch+ Ampli :  T~500ns-1µs 5mJ/pulses 100 pulses 50W Servo feedback Servo feedback grating Cavity round trip length L=c/200MHz=1.5m Fibre connectorised ? Error signals Reflected signals transmited signals 2 frequencies solution 2 nd harmonic Freq doubler

26 Error signal Linearity range Corresponds to  L= /F Linearity range Corresponds to  L= /F Using the frequency doubling : F  ~1000 for  precise feedback BUT  L<0.5nm F  ~50 for   L<20nm  can recover the locking after the macro pulse pass (1-2µs) Using the frequency doubling : F  ~1000 for  precise feedback BUT  L<0.5nm F  ~50 for   L<20nm  can recover the locking after the macro pulse pass (1-2µs)

27 Recent improvement from VIRGO Linearity range increased by a factor ~10  L~ /(10F) ‘just’ by dividing the error signal by the transmited signal Linearity range increased by a factor ~10  L~ /(10F) ‘just’ by dividing the error signal by the transmited signal Using the frequency doubling : F    ~1000 for  precise feedback BUT  L<5nm F  ~50 for   L<200nm  recover the locking after the macro pulse pass (1-2µs) starts to be feasible with the doubled frequency  find the optimum for F  F  Using the frequency doubling : F    ~1000 for  precise feedback BUT  L<5nm F  ~50 for   L<200nm  recover the locking after the macro pulse pass (1-2µs) starts to be feasible with the doubled frequency  find the optimum for F  F 

Yb Oscillateur ~200MHz >8nm bandwidth Stabilisable 20mW. Inside : Steper motor Pzt EOM Double wedge pump modulation Yb Oscillateur ~200MHz >8nm bandwidth Stabilisable 20mW. Inside : Steper motor Pzt EOM Double wedge pump modulation Frequency doubler preamplifier 4-mirror cavity Optical switch+ Ampli :  T~500ns-1µs 5mJ/pulses 100 pulses 50W Optical switch+ Ampli :  T~500ns-1µs 5mJ/pulses 100 pulses 50W Servo feedback Servo feedback grating Cavity round trip length L=c/200MHz=1.5m Fibre connectorised ? Error signals Reflected signals transmited signals 2 frequencies solution 1 rst harmonic ~1µm

29 summary  Damage threshold limits the max pulse energy in a ‘ring cavity’  We have some experience in high finesse cavity locking  A ‘burst’ regime should work but one must estimate the effects of a ‘huge’ circulating pulse energy

30 LMA designed mirrors with F=50 at /2

31 Il faut faire tourner du code aux elements fini !  calculer l’evolution temporelle des deformations  induite par la pression de radiation  induites par la diffusion de la chaleur Et penser à une manipe auprès d’une source de laser intense … Il faut faire tourner du code aux elements fini !  calculer l’evolution temporelle des deformations  induite par la pression de radiation  induites par la diffusion de la chaleur Et penser à une manipe auprès d’une source de laser intense …

On pose h=15mm, d 1 =R/cos , R=0.5m, On tilt tous les miroirs de  x,y =(-1,0,1)µrad  3 8 =6561 combinaisons de désalignements – Pour chaque combinaison on applique le principe de Fermat pour trouver l’axe optique (on itère trois fois  précision Matlab) – On calcul le déplacement de l’axe optique sur tous les miroirs par rapport aux centres (alignement parfait) – Tolérance = le plus grand déplacement parmi les 3 8 combinaisons Résultats : – 2µm sur les miroirs sphériques – 1µm sur les miroirs plans – 2µm au point de croisement laser électron 0.5’ ~13nm 1µrad 2 ème ‘bonne propriété’ d’une cavité 4 miroirs : tolérances mécaniques

On translate tous les miroirs de  x,y,z =(-1,1)µm  2 12 =4096 combinaisons on obtient –3µm sur les miroirs sphériques –5µm sur les miroirs plans –1µm au point de croisement laser électron Calculation of the cavity eigenmodes  Linear polarisation is preserved  for 1µm, 1µrad mirror motions  And 1mm, 1mrad missalignments Calculation of the cavity eigenmodes  Linear polarisation is preserved  for 1µm, 1µrad mirror motions  And 1mm, 1mrad missalignments