grand goal pursued with modest tools: insight by experiments
Laser fusion relies on large facilities such as NIF
and MEGAJOULE. However, insight into the physical processes crucial for making laser fusion work reliably can be gained from carefully devised experiments requiring high laser pulse intensities rather than energies. These intensities can be provided by LAP’s state-of-the-art lasers thanks to their ultrashort pulse durations. LAP is currently pioneering the development of a source (Petawatt Field Synthesizer, PFS)
producing few-cycle, few-femtosecond laser pulses with several joules of energy, resulting in intensities in excess of 10^{22}\:{\rm W}/{\rm cm}^2. In relativistic interactions with electrons, these pulses will generate intense attosecond flashes of X-ray light and bursts of particles.
and MEGAJOULE. However, insight into the physical processes crucial for making laser fusion work reliably can be gained from carefully devised experiments requiring high laser pulse intensities rather than energies. These intensities can be provided by LAP’s state-of-the-art lasers thanks to their ultrashort pulse durations. LAP is currently pioneering the development of a source (Petawatt Field Synthesizer, PFS) producing few-cycle, few-femtosecond laser pulses with several joules of energy, resulting in intensities in excess of 10^{22}\:{\rm W}/{\rm cm}^2. In relativistic interactions with electrons, these pulses will generate intense attosecond flashes of X-ray light and bursts of particles.What is the relevance of this for inertial fusion, characterized by nanosecond implosion physics and nuclear burn on picosecond time-scales? We believe that the techniques now developed at LAP will play an important role in future inertial fusion research after ignition and burn has first been demonstrated on NIF and MEGAJOULE. We expect that the focus will then turn to advanced techniques such as fast ignition to reduce driver requirements and to simplified target design. The unique petawatt-scale, few-cycle source, PFS, and attosecond X-ray sources driven by it, which will become available at LAP in the foreseeable future, will afford unprecedented experimental capabilities of visualizing the transport dynamics discussed above in real time. The PFS will also allow to explore new approaches to fast ignition such as the use of circularly-polarized beams focused beyond 10^{22}\:{\rm W}/{\rm cm}^2. Recent theoretical work predicts stable hole boring accompanied by efficient acceleration of ions under these conditions. The ion beam could then heat the hot spot and possibly obviate the need for complicated cone targets.
Fig. 1. Fast ignition relies on the transport of ignitor energy to the core of the compressed fuel. The imploded fuel is so dense that the laser light can’t penetrate it. Rather, the energy has to be carried by relativistic electron currents generated by the igniting laser pulse on the surface of the fuel. Ignition requires currents of 100 million amperes and more. Such currents cannot be transported in vacuum due to self-generated magnetic fields, which stop the beam. In plasmas, these undesirable magnetic fields are suppressed by return currents. However, this comes at the expense of instabilities, which causes the igniting electron beam to break up into filaments. Hence energy transport by the electrons is a highly complex process, which needs to be well understood and controlled before fast ignition of fusion targets can be successful. Lap’s PFS and the intense attosecond soft-x-ray pulses produced with it will offer unique tools for investigating this key process for ignition. The PFS pulses – upon being focused on a solid target – will drive electrons into the solid target and attosecond x-ray pulses will be used to image the temporal evolution of the electron beam and its filamentation in real time.