First-of-a-kind engineered systems often burst with complexity – a major cause of project budget and time overruns. Our particular concern is the structural complexity of nuclear fusion devices, which is determined by the amount and entanglement of components. We seek to understand how this complexity rises during the development phase and how to manage it. This paper formulates a theory around the interplay between a problem-solving design process and an evolving design model. The design process introduces new elements that solve problems but also increase the quantifiable complexity of the model. Those elements may lead to new problems, extending the design process. We capture these causal effects in a hierarchy of problems and introduce two metrics of the impact of design decisions on complexity. By combining and incorporating the Function-Behavior-Structure (FBS) paradigm, we create a new problem-solving method. This method frames formulation, synthesis and analysis activities as transitions from problems to solutions. We demonstrate our method for a nuclear fusion measurement system. Exploring different design trajectories leads to alternative design models with varying degrees of complexity. Furthermore, we visualize the time-evolution of complexity during the design process. Analysis of individual design decisions emphasizes the high impact of early design decisions on the final system complexity.

It is argued that the natural and human vicissitudes of the Northern Hemisphere—or at least western European history between 1315 and 1648—provide a preview of the sort of consequences for humanity and its demography that will result from the serious if not catastrophic climate change that is now anticipated by the Intergovernmental Panel on Climate Change (IPCC). Game theory suggests that at least some nation-state players in the strategic problem that climate change raises will not choose Nash equilibria that mitigate the problem. The only feasible solution will be the discovery or invention of some non-greenhouse-gas-emitting energy source so cheap that its owner will be indifferent to free-riding by all other users of energy. Recent efforts to develop fusion reactors do not provide much hope for this eventuality.

A fundamental problem for the realization of laser fusion through the implosion of a spherical target is Kidder's E−1/6 law, where E is the energy needed for ignition, proportional to the 6th power of the ratio R/R0, where R0 and R are the initial and final implosion radii, respectively. This law implies that the ignition energy is very sensitive to the ratio R0/R, or vice versa, the ratio R0/R is very insensitive to the energy input, with R0/R limited by the Rayleigh–Taylor instability. According to still classified data of the Centurion–Halite experiment at the Nevada Test Site, ignition would require an energy of ${\rm E}\simeq 50\,{\rm MJ}$, 25 times larger than the 2 MJ laser at the National Ignition Facility (NIF) reported in the New York Times. This means that even a tenfold increase from 2 to 20 MJ would only decrease the R/R0 ratio by an insignificant factor of 10−1/6 ≃ 0.7. To overcome this problem, it is proposed that the spherical target is replaced with a hollowed-out, rapidly rotating, cm-size ferromagnetic target, accelerated by a rotating traveling magnetic wave to a rotational velocity of ~1 km/s, at the limit of its tensile strength. In a rotating reference system, the general theory of relativity predicts the occurrence of negative gravitational field masses in the center of rotation, with their source located in the Coriolis force field. The density of this negative gravitational field mass can be larger than the magnitude of the positive mass density of a neutron star. The repulsive gravitational force causes the centrifugal force. For a magnetized plasma placed in the rapidly spinning, hollowed-out target chamber, this repulsive force can be balanced by the magnetic force generated by thermomagnetic currents of the Nernst effect. Such a configuration does not suffer from the Rayleigh–Taylor instability, but becomes a small magnetohydrodynamic generator, amplifying the magnetic field to values about equal to those of the Nernst effect, axially confining the plasma. By placing the spinning target in the center of a lithium vortex, the fusion neutrons absorbed in the vortex can breed tritium, and at the same time remove heat from the target chamber to sustain the Nernst effect. A hot spot is thereby produced in the target chamber, which launches a thermonuclear burn wave into a cylindrical deuterium–tritium configuration. With the stability of a rapidly rotating target greatly increased, and the range of 10 MeV electrons in the wall of the cm-size ferromagnetic target, an intense 10 MeV relativistic electron beam drawn from a 10 MJ Marx generator should be sufficient to implode the target for thermonuclear ignition.

Karlsruhe Institute of Technology (KIT) is doing research and development in the field of megawatt-class radio frequency (RF) sources (gyrotrons) for the Electron Cyclotron Resonance Heating (ECRH) systems of the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Fusion Power Plant that will follow ITER. In the focus is the development and verification of the European coaxial-cavity gyrotron technology which shall lead to gyrotrons operating at an RF output power significantly larger than 1 MW CW and at an operating frequency above 200 GHz. A major step into that direction is the final verification of the European 170 GHz 2 MW coaxial-cavity pre-prototype at longer pulses up to 1 s. It bases on the upgrade of an already existing highly modular short-pulse (ms-range) pre-prototype. That pre-prototype has shown a world record output power of 2.2 MW already. This paper summarizes briefly the already achieved experimental results using the short-pulse pre-prototype and discusses in detail the design and manufacturing process of the upgrade of the pre-prototype toward longer pulses up to 1 s.

Self-emission x-ray shadowgraphy provides a method to measure the ablation-front trajectory and low-mode nonuniformity of a target imploded by directly illuminating a fusion capsule with laser beams. The technique uses time-resolved images of soft x-rays (${>}1$ keV) emitted from the coronal plasma of the target imaged onto an x-ray framing camera to determine the position of the ablation front. Methods used to accurately measure the ablation-front radius (${\it\delta}R=\pm 1.15~{\rm\mu}\text{m}$), image-to-image timing (${\it\delta}({\rm\Delta}t)=\pm 2.5$ ps) and absolute timing (${\it\delta}t=\pm 10$ ps) are presented. Angular averaging of the images provides an average radius measurement of ${\it\delta}(R_{\text{av}})=\pm 0.15~{\rm\mu}\text{m}$ and an error in velocity of ${\it\delta}V/V=\pm 3\%$. This technique was applied on the Omega Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] and the National Ignition Facility [Campbell and Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)].

In this paper we consider laser intensities greater than $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}10^{16}\ \mathrm{W\ cm}^{-2}$ where the ablation pressure is negligible in comparison with the radiation pressure. The radiation pressure is caused by the ponderomotive force acting mainly on the electrons that are separated from the ions to create a double layer (DL). This DL is accelerated into the target, like a piston that pushes the matter in such a way that a shock wave is created. Here we discuss two novel ideas. Firstly, the transition domain between the relativistic and non-relativistic laser-induced shock waves. Our solution is based on relativistic hydrodynamics also for the above transition domain. The relativistic shock wave parameters, such as compression, pressure, shock wave and particle flow velocities, sound velocity and rarefaction wave velocity in the compressed target, and temperature are calculated. Secondly, we would like to use this transition domain for shock-wave-induced ultrafast ignition of a pre-compressed target. The laser parameters for these purposes are calculated and the main advantages of this scheme are described. If this scheme is successful a new source of energy in large quantities may become feasible.

The inertial confinement fusion (ICF) philosophy is based on high compression. The reasoning is that (a) it is cheaper (energetically) to compress than to heat and (b) nuclear reactions are proportional to density square, therefore the more you compress the better you are in ICF. Of course the only limitations of compression are the hydrodynamic instabilities (like Rayleigh-Taylor, etc). Many of the references in the literature require extremely high compression and in particular the pB11 needs extremely huge compressions. In this paper it is shown that there is an optimum of compression, namely gain G is maximum for a definite compression. The value of this density (for a given fuel mass and particular ICF scheme) depends on the equation of state (EOS). We calculate this value for fast ignition (FI) schemes and compare it with the central spark ignition (CSI) model. The gain calculations are based on the ideal gas for the ions and the Fermi-Dirac EOS for the electrons with an effective alpha, as usually suggested from simulations. The “optimum compression” idea is easily understood from the following argument: From EOS data one needs an infinite energy to compress to an infinite density. Since the energy output is finite it is clear that G is zero for infinite compression. On the other hand for normal density with small fuel mass (~ few mg) the gain is also zero. Therefore a maximum should exist somewhere. For the deuterium-tritium fuel with a mass of few mg one gets an optimum at few hundred g/cc. If you compress more then the gain is going down. So there is a desired maximum compression fixed by EOS. Last but not least, bremsstrahlung losses in degenerate plasma are discussed and the clean fusion (i.e., without neutrons) of proton + B11 → 4α is analyzed.

ITER will be the first fusion reactor producing much more energy than that necessary to establish and heat the plasma. This nuclear energy is mainly exported from the plasma by neutrons and deposited in the plasma facing components, generating nuclear activation. One of the elements used in the fusion reaction is tritium, a radioactive isotope of hydrogen. Although some feedback may be used from in service fusion machines, such as JET in England, the ITER radiological protection issues need to be assessed as early as possible. The protection against radiological hazards is, as usual shielding, waiting for radioactive decay, limitation of airborne contaminants, reduction of exposure time, etc. This has been done in the present design and has followed a first step of ALARA studies. Radiation fields have been calculated and work effort estimated leading to the evaluation of a collective yearly dose of 356 p mSv, below the project dose target which is 500 p mSv. A second step of occupational radiological exposure studies should allow refining further the dose estimate as well as the ALARA optimization.

The properties of plasma (proton) block driven by the laser-induced skin-layer ponderomotive acceleration (S-LPA) mechanism are discussed. It is shown that the proton density of the plasma block is about a thousand times higher than that of the proton beam produced by the target normal sheath acceleration (TNSA) mechanism. Such a high-density plasma (proton) block can be considered as a fast ignitor of fusion targets. The estimates show that using the S-LPA driven plasma block, the ignition threshold for precompressed DT fuel can be reached at the ps laser energy ≤ 100 kJ.