Multiple-beam fast ignition with KrF laser

1. Multiple-beam fast ignition with KrF laser István B Földes1, Sándor Szatmári2 1KFKI-Research Institute for Particle and Nuclear Physics, Department of Plasma Physics Association EURATOM, H-1525 Budapest, P.O.B. 49. Hungary 2University of Szeged, Department of Experimental Physics2 H-6720 Szeged, Dóm tér 9. Hungary (IAEA F1.30.11 CRP, Contract HUN 13759, Hungarian OTKA Foundation K-60531)

2. Content • Laser fusion with KrF lasers. • 2. Requirements for fast ignition with KrF lasers: How to reach • the required intensities with KrF amplifiers? • 3. The multiple-beam fast ignition: Multiple beams of 1ps • duration as alternatives for 20ps FI.

3. Advantages of KrF lasersas a fusion driver Advantages of short (248 nm) wavelength: • - Less disturbing nonlinearities due to the I2 scaling. • - Higher critical density, deeper penetration into the plasma. • - No frequency conversion needed for short wavelength in case of KrF lasers: The UV wavelength may reduce the electric energy requirements in case of a fusion test facility. • - A gas discharge laser can provide better beam quality, higher symmetry in target illumination. Direct drive compression is possible!

4. KrF lasers: real alternatives Properties of KrF lasers: - fast relaxation time (6 ns) - long pumping time (e-beam: 100 ns, discharge: 15 ns) - to use efficiently the full energy of the discharge angular beam multiplexing is necessary - high reprate possible - e-beam amplifier lifetime considerably increased - Electra program: 400 J, 5 Hz, 100000 shots obtained development of preionizer (hibachi) - IFE: 3108 shots (2 years) needed with 7% efficiency (Sethian)

5. NRL design of a KrF DD laser fusion test facility • 500 kJ system (2006) • Number of moduls: 32 • Modul aperture: 1m2 • Amplified energy of a modul: 30 kJ • Number of angularly multiplexed • beams: 40 • Pump duration: 225 ns • High repetition rate

6. Problems with present fast ignitor schemes Au cones for the direct coupling of radiation proved to be successful but it is hard to scale them for a reactor size. The target is very complicated, and it is difficult to inject them with 10 Hz rate keeping the correct alignment. Also the evaporating material will cover the chamber walls and even the optics. Without a cone however infrared lasers do not penetrate deep enough, the velocity of the accelerated electrons will be too high at the required intensities, therefore they do not stop inside the target. Short wavelength may be advantageous for fast ignition, too!! Hydrodynamical simulations (R. Betti): The optimal implosion for fast ignition has low velocity, low adiabat implosion with a large total mass. Optimal case: <>300-500g/cc,  homogeneous. Fusion gain may increase with a factor of 2, i.e. for a 1MJ laser 50 to 130 for 0.5 MJ laser it will be 100 (50-200 kJ PW laser)

7. Fast ignition and wavelength The energy of fast electrons can be scaled with the ponderomotive force, and the penetration depth is energy-dependent: If E>>1MeV, the electron energy will be significantly larger than the optimum for fast ignition, the efficiency will be low. Shorter laser wavelength reduces the average energy of electrons, the stopping range and thus the minimum ignition energy. According to the scaling laws, for the 248nm KrF wavelength 1.8 1020W/cm2 intensity is needed for 1 MeV electrons.

8. The KrF fast ignitor The idea of Zvorykin: Using the same KrF amplifier for the driver laser of ns duration and for the short pulse PW ignitor laser. KrF amplifiers: When an amplified pulse depletes the pump energy 2ns is needed for recovering population inversion. Thus the duration of electron-beam pumping (>100ns) allows its multiple using. This is the basis of multiplexed amplification. Another property: for short laser pulses there exists a maximum output energy of 6mJ/cm2 (saturated regime). Independent of pulse duration (<<2ns). Zvorykin-scheme: For 2MJ KrF system 100 kJ can be obtained by 25-pass amplification (10-20ps), which takes only 50ns from the 250ns of pumping time. The beams are then angularly demultiplexed and focussed onto the target. Due to the saturation properties of the KrF amplifiers 6mJ/cm2 can be obtained either with 20ps or with shorter (~1ps) pulses. Transform limited pulse: 100fs.

9. Fast ignition at the high repetition rate test facility The full aperture here is only 20m2 only 1.2kJ energy/pass. 40 pass: 48 kJ, 80 ns duration 80 pass: 96 kJ, 160 ns duration. A significant part of the total pumping time  either longer pump or reduced compressor pulse energy Flexible system !? Table shows focal diameters needed for 1MeV electron-energy in case of two pulse durations:

10. Properties of KrF lasers, 20ps vs 1ps pulses • 20ps pulses: The 20m focal radii needed for 1 MeV electron-energy may be obtained. Required accuracy: A 48kJ system corresponds to 80ns, which is 10-4 on 24 m, because the accuracy of demultiplexing must be some ps, which will be available. Problem: Bandwidth of the KrF laser: transform-limited pulse: 100fs, therefore the bandwidth for 20ps is small. Amplifier efficiency maybe low, coherence effects must be suppressed, beam smoothing techniques are needed. • Short pulse amplifiers work in saturation regime. • The output laser energy is independent on pulse duration from 100fs to several 10 ps. • It is possible to obtain the 48 kJ energy in a ~1ps pulse. • The power is high, it is enough to combine the beams into a spot of ~90m radius. But: Angular demultiplexing does not work well, the pulse duration corresponds to 300m. The wavefronts must be matched as well. Interferometric multiplexing is needed, e.g. with the method of polarization multiplexing.

11. Interferometric multiplexing The 2 beams have the same path in different directions, therefore the wavefronts are matched automatically. The problem is that it is applicable only for a few beams. For 2 beams 1.7 times energy multiplication obtained. Other problem: It is very difficult to combine the high number of beams with an accuracy of 300m on the target.

12. The alternative: multiple beam fast ignition Laser energy at the output of a 1m2 output of an amplifier after interferometric multiplexing of 2 beams of 1ps pulse duration: 120J. Focusing: r=8m  I=21020 W/cm2 .This is sufficient for 1MeV electrons. 400 separately focused beams fulfill both the energy- and intensity requirements!

13. Requirements for multiple beams fast ignition 400 beams focussed separately fulfills both the energy- and the intensity-requirements. - A diffraction-limited beam 8m spot size can be obtained by f/32 focusing, which covers ~10% of the total solid angle. - It is sufficient to focus the separate beams onto the target with 10ps accuracy. By changing the delay between the beams even the pulse-form can be varied. - The beams can be focused onto different parts of the target if the „corona implosion model” of fast ignition should work. - In the case of separate spots R for the ignition may be < 0.1-0.3 but for the high density isochoric case it may be reached even in this case. - Alternatively they can be focused to 1-2 spots with a larger total radius, then each beam accelerates electrons separately and they – after diverging – will ignite together. - The critical density is still 3 orders of magnitude lower than that of the fuel. 3-dimensional simulations for self-focusing, electron- and burn-propagation is needed.

14. Conclusion • A new fast ignitor scheme using multiple KrF laser beams from the driver amplifers is considered. • 2. It is shown that for KrF lasers the short, ~1ps pulses have several advantages as compared with the longer pulses. • 3. Experimental tests are needed for validating the interferometric multiplexing for large sized beams and for finding the paths for multiple amplification. • 4. The feasability of the proposal and the optimal geometry – including the number of focal spots – should be determined by 3D simulations. • Földes, Szatmári; Laser and Particle Beams, accepted.