Ultrafast Laser-Driven Plasma for Space Propulsion

Transcription

1 Ultrafast Laser-Driven Plasma for Space Propulsion Terry Kammash, K. Flippo, T. Lin, A. Maksimchuk, M. Rever, S. Banerjee, D. Umstadter FOCUS Center / Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, MI , USA Y. Sentoku General Atomics, San Diego, CA V. Yu. Bychenkov P. N. Lebedev Physics Institute, Russian Academy of Science, Moscow, Russia Lasers supported by the National Science Foundation FOCUS Center and the U of M Center for Ultrafast Optical Science, and funding from NASA Institute For Advanced Concepts

2 Accelerator Setup CR-39 Detector Target Normal Forward Direction Proton Beam Laser Forward Direction CUOS T 3 Laser Parameters: Ti:Sapphire / Nd:Glass mm (ω o ), 527nm (2 ω o ) up to ~12 TW 5 J 400 fs Contrast: 10-5 ω o, 10-7 ω o 2x x10 19 W/cm 2 Incident Laser Spot FWHM = 4.3 um

3 Front Surface Deuteron Acceleration Activation of 10 B to 11 C is achieved only by illuminating deuterons on the front surface. No activation when deuterons were on the back surface, or without deuterons (i.e. no production of 11 C detected from 11 B (p,n) 11 C reaction). Deuterons have about ½ the E max of the measured protons Counts/2 min I las =6x10 18 W/cm 2 Detection efficiency 15% Decay for 11 C Laser CD Mylar film Deuterons Boron sample 10 B 11 C n Time after shot (min) 10 B(d,n) 11 C reaction K. Nemoto, S. Banerjee, K. Flippo, A. Maksimchuk, D. Umstadter App. Phys. Lett, 78, 595 (2001)

4 Radioisotope Activation with Protons collimator & shield Laser Counts target protons Singles Spectrum 11 B (p,n) 11 C NaI PMT Sample NaI PMT MeV Sort window Channel to MCA to MCA Count (0.511 MeV) Count (0.511 MeV) Laser Induced B (p, n) C Time (sec) Laser Induced 63 Cu (p,n) 63Zn t ~ 38 min Time (sec) t = 20 min

5 Material Effect on Proton Production Conductor Aluminum ρ~2.7 g/cm 3 σ= Ω -1 m -1 Z=13 Insulator Mylar (polyethylene terephthalate C 10 H 8 O 4 ) ρ~1.2 g/cm 3 σ=10-12 Ω -1 m -1 Z=4.3 p + e - p + E p + e - E e - p + laser B target p + p + laser B target

6 Beam Profile Dependence on Initial Target Composition RCF (a,c,e,g) / CR-39 (b,d,f,h) detector stack images for 13µm Mylar, 10 µm silicon, 12.5 µm aluminum, and 12.5 µm copper targets. All pairs are single shot except (c) and (d) which were two separate shots. RCF records protons between ~0.2 and ~2 MeV, CR-39 records protons between ~2.5- ~4 MeV. Except (d) which recorded between ~1.2 MeV and 3 MeV

7 (a) 4 µm, (b) 12.5 µm, (c) 25 µm, (d) 50 µm, and (e) 75 µm Beam Profile Dependence on Target Thickness (a) 6 µm, (b) 13 µm, (c) 25 µm, (d) 50 µm, and (e) 100 µm Call out: White arrows point to beam filamentation, most likely a manifestation of the Weibel, instability.

8 Comparison with Simulation Images: Contrast enhanced RCF images of proton beam profiles after a drift of 5 cm from target exit from experiments with 13 microns of Mylar (a) top left, and 12.5 microns of aluminum (b) bottom left. Compare an electron beam profile from a simulation (c) by L. Gremillet, et al. [Phys. Plasmas 9, 941(2002)], showing the transverse electron profile jb/enc at 20 microns inside a silica target for a propagating monoenergetic electron beam of energy 500 kev, after 405 fs of propagation, which is also the beam duration. Image reproduced with permission. Observed profiles Silica e-beam simulation

9 Magnetic Field from Simulation vs. Proton Beam Profile And as shown by M. Honda, J. Meyer-ter-Vehn and A. Pukov, PRL (2000) the ions can follow the electron filaments in as little as 60 fs. E field configuration plot from the simulation at 405 fs. Notice the similarities in the simulation slices to proton beam images in row (I) of the previous slide. e-beam induced B field evolution is very similar to that of the proton beam profile seen from Mylar previously.

10 Electron Distribution From Al Target X-ray Film Line Out Target Holder Shadow X-ray Film 0 Target Top View Holder Protons laser X-ray Film

11 Protons From Front Surface Maximum Proton Energy [MeV] E imax ~ 13 µm Target Thickness [microns]

12 Simulation of proton beam Sentoku s[1] recent 1-D PIC simulations predict a 5 MeV beam from the front surface for a 400fs laser pulse, with about 13 MeV from the rear. This agrees well with the observed 4 MeV trend, and a maximum of about 12 MeV. [1] Y. Sentoku Phys. Plasmas (2003)

13 Deuteron Acceleration Preliminary Results Deuteron coating d+ Where p+ No deuteron coating p+ do highest energy deuterons come from? The BACK of 12.5um Al The FRONT of 6 um Mylar The FRONT of 13 um Mylar The FRONT of 12.5 um Al The BACK of 13 um Mylar

14 Proton Energy Scaling with Pulse Duration and Intensity 14.5 MeV Current T-cubed System > 30 MeV Future HERCULES System From Y. Sentoku, T. E. Cowan, A. Kemp, and H. Ruhl Physics of Plasmas 10, 2009 (2003)

15 Peak Proton Energy vs. Spot Size 6000 f/3.3 off-axis parabola 5000 f/1.5 off-axis parabola Peak Proton Energy [kev] Power Scaling Fit For intensities of ~ 2.5 x W/cm 2 E = E = d x d For intensities of ~ 1.4 x W/cm Spot size diameter [microns]

16 Spot Size Comparison 120 Total Intensity vs. Diameter for f/1.5 Paraboloid 4.3 FWHM Spot Size 100 Total Energy [%] % in FWHM Intensity [a.u.] Profile of 4.3µm FWHM Spot Radial Position [µm] Spot Size Diameter [um]

17 Spot Size Comparison 120 Total Intensity vs. Diameter for f/3.3 Parabaloid FWHM Focal Spot of 6.4 Microns Total Energy [%] % in FWHM Intensity [a.u.] Profile of 6.4 µm FWHM Spot Radial Position [µm] Spot Size Diameter [um]

18 Material Effects on Plume Profile Proton Beam Images Using a CCD Target Plane Dark Side Illuminated Side Laser Propagation Direction Proton Beam is Emitted Normal to Target 25 um Al Target 25 um Mylar Target 25 um Mylar Target with 2.4 Torr He 4 Al Target, 4MeV beam No backfilled gas, 200 mtorr ambient 4 um Al Target with 2 Torr H 2

19 Plume Evolution in 1 Torr H 2 Ambient Backfill 12.5 mm Al 25 mm Mylar +1 ms +4 ms +1 ms +4 ms ms 65.5us ~32000 m/s ~31000 m/s cm cm 2.222cm cm 1 cm

20 Target Geometry Radius of curvature ~ 0.2 mm >1.4 MeV, 55º 1.5x10 19 W/cm 2 >2 MeV, 38º 1.2x10 19 W/cm 2 Laser Curved Target Geometry 25 µm Al Target Radius of curvature ~ 0.5 mm Target Holder Protons > 1.4 MeV, 44º 1.6x10 19 W/cm 2 > 3 MeV, 28º 1.2x10 19 W/cm 2

21 Target Geometry Wire orientation: ~100 Micron Half Wire Cross-sections Focus on flat surface Protons CR-39 Focus on round surface Protons Protons Laser Laser Flat Wire position Round Beam Images: Focusing on flat surface (840) creates an ion beam, while focusing on the round side produces a cylindrical-like spray laser laser

22 Target Surface Geometry Use a material which will trap the laser light, to enhance the generation of hot electrons. Electron Microscopy of LaserBlack Murnane et al. APL 62 (1993) used gratings and clusters, Kulcsar et al. PRL 84 (2000) used metallic velvet. Both showed enhanced X-ray yield from enhance electron heating from efficient coupling. 100 µm 2 µm Laser Spot Size ~ 6 microns LaserBlack is > 96% absorptive at 1 mm. Results: 30 mm Laserblack target ~ 8.2 MeV Enhancement in the number of maximum energy protons Beam profile does not suffer, regardless of which surface has been coated, i.e. no imprinting even from rear-side >1.3MeV 31º div.

23 Proton Radiography Thin Film Target T-cube Laser Proton Beam Mesh Radiochromic Film Approximate Region Sampled by Beam 1 mm Area of Image at Right The possibility exists to use the laser produced proton beam for very small scale imaging or even lithography. The image on the left is a 5x magnified proton radiograph captured on RCF of a mesh with 10 micron wires and 30 micron grid spacing. 1 mm 51.8 lines high

24 Future Laser Development Oscillator Cleaner (10 6 contrast) Regenerative Amplifier 4-pass Amplifier 2-pass Amplifier High-Power Amplifier Energy Pulse width Repetition Rate 1 nj fs 80 Mhz 1 mj 15 fs 10 Hz 100 mj 350 ps 10 Hz J 7-10 J 350 ps Current Hercules 350 ps 10 Hz 0.1 Hz 50 J 350ps 0.1 Hz Compressed Output N/A N/A N/A fs fs fs

25 Proton Acceleration Summary Simulation and experiment support proton acceleration at the laser-irradiated side of the target of a 4 MeV beam, on the back of the underdense plasma under these conditions. And a 12 MeV beam from the rear-surface of Al due to recirculation sheath enhancement. Beam spectrum has bands of energies due to ion fronts. Beam profile smoothes out as initial target conductivity increases. Filamentation and structures similar to the electron simulation by Gremillet et. al have been observed. Demonstrated beam profile modification with modest geometry, and enhancement of number at the maximum energy achieved by initial target geometry and surface conditions CR-39 response is highly non-linear when scanned optically. By using a highly absorptive material we have increased the number of maximum energy protons without sacrificing beam quality. No imprint of LB on beam profile, unlike Roth et. al New 30 fs laser has produced W/cm 2 on target in a 1 micron spot, expect high efficiency acceleration

26 Ion Acceleration Physics Relativistic Electron Cloud (Beam) Model One- Dimensional Poisson s Equation E=-4 πen b Where: e=electron charge n b =beam electron density d R Can readily show: E z =2πen b h Where: h=thickness of electron cloud R=radius of electron cloud d=diameter of electron cloud E z

27 Physics Continued Energy conservation for electrons in cloud PE=KE PE πe 2 n b h 2 KE=( γ b -1)m o c 2 where γ b =Relativistic Parameter Hence: h= (γ b -1)m o c 2 /πe 2 n b = = (γ b -1)/πr e n b Where: r e =classical electron radius r e =e 2 /m o c 2 = Substituting into exp. for E z we get E z =2c πm o (γ b -1)n b

28 Example We begin with γ b =10 n b =10 19 cm -3 h=10µm E z =913GV/m Over a distance of h=10 µm, the electron acquires an energy of E b =9 MeV

29 Continued The Ion Energy E i =ZE b =ZeE z h E i =9MeV (Z=1) Mean Ion Velocity V i is given by ½m i V i2 =ZeE z h And the ion acceleration time t i is t i =h/v i or t i = m i /Ze 2 n b

30 Two Asymptotic Regimes for Ion Acceleration 1. Isothermal expansion relevant to long pulse lengths i.e. τ>t i (t =1ps) i Ions acquire exponential distribution in velocity dn i /dv ~ exp-( v/c S ) Where C S = ZT e /m i = ion sound speed

31 Two Asymptotic Regimes for Ion Acceleration 2. Adiabatic regime corresponding to shorter, sub picosecond pulses i.e. τ<<t i Here ion distribution is steeper and the form dn i /dv ~ exp-( v 2 /2C S2 ) For the adiabatic expansion electron cooling takes place according T e =T e0 (t i /t) 2

32 Ion Velocities Maxium Ion Velocities: Isothermal v max =2C S ln(d/h) Adiabatic v max =2 2C S ln(d/h) Note in both instances: Ion Acceleration is more efficient when (d/h)>>1 i.e. for larger focal spots

33 Relationship Between Ion Energy, Laser and Target Parameters Consider power balance between laser and ejected electrons: [n b (γ b -1)m o c 2 ]c=ηi Where η=efficiency of energy transfer Rewrites as ε e =ηi/n b c Also electron must exceed Coulomb Energy to penetrate the target i.e. n b = ε e /(πe 2 hr)

34 Relationship Between Ion Energy, Laser and Target Parameters Combining we get: ε e = πe 2 IRh η/c Since h λ = laser wave length, then ε e = πe 2 IRh η/c And ε i =Z ε e If we express intensity I in units of W/cm 2 and R and λ in microns then ε i =Z ε e ηirλ MeV

35 An Example I=10 21 W/cm 2 η=0.10 R=2.5 µm Then ε i =14 MeV

36 Thrust F=N i M i ωv i M i = ion mass (proton) = kg ω = representation rate 1kHz V i = ion velocity (14 MeV) = m/s

37 Plasma Expansion in Vacuum Ion acceleration time t i =h/v i = sec Pulse length (projected) τ= Then τ>t i Expansion is Isothermal v i max = 2 C S ln(d/h) C S = ZT e /m i = m/sec v i max = 10 8 m/sec V i initial m/sec

38 Specific Impulse Note improvement in energy transfer efficiency for increasing (d/h), namely for larger aspect ratios d/h ln(d/h) V imax (m/s) Max I sp (s)

39 Accomplishments Thus Far 1. Generate a Relativistically Consistent Mathematical Expression for the energy of the ejected ion as a function of laser and target parameters, i.e. E i =z ηirλ where z = ion charge η = energy conversion efficiency R = radius of focal spot λ = laser wave length

40 Accomplishments Thus Far 2. Experimentally validated E i ~ I Ei~ λ 3. Indirectly established relationships relating E i to R and dependence on η. More work is needed in this area! Just purchased 5 parabolic mirrors to investigate thoroughly dependence of E i and total number of ejected particles on R.

41 Accomplishments Thus Far 4. Experimentally established dependence of E i on target thickness t, optimized t 10λ 5. Experimentally established conditions for filamentation instability P =5P c =5[17(ω o /ω p ) 2 GW] 4Tc/ω p a 0 2R c = speed of light a 0 = λ [µm] I 1/2 [W/cm 2 ] ω p =plasma frequency R= radius of focal spot

42 Accomplishments Thus Far 4. Experimentally established energy of ions ejected from front and rear surfaces of target which appear to agree well with simulations 5. Established dependence of proton beam profiles on materials, surface conditions and geometry 6. Carried out designs of space Nuclear Reactor for use in LAPPS. Likely candidates are gas-cooled Cermet reactors using Uranium, Plutonium or Americium as fuel.