## 1 Stellarator Configurations: Optimisation and the Scaling of the Confinement Time  
The Stellarator is a toroidal magnetic confinement device for the achievement of thermonuclear fusion, with potentially sufficient efficiency to become a realistic candidate for the final commercial reactor [1]. The main, common specificity of the various Stellarator configurations is that they use external magnets to generate nearly all the confining fields. The fact that Stellarators do not require any plasma current to form and sustain the configuration presents some very substantial advantages [2]. First, they are not affected by those plasma terminating instabilities called disruptions, which frequently occur in Tokamaks and are a potential showstopper on the route to the reactor. Secondly, relying on externally generated fields, the Stellarator is inherently a steady state device, contrary to the Tokamak, which would probably need challenging and expensive external current drive to operate continuously and not in pulse mode [3]. Thirdly, Stellarator plasmas are much more externally controlled by design, and therefore, being less self-organised, their extrapolation into the future generation of machines should be less uncertain. On the other hand, in the first generations of Stellarator devices, the energy confinement was severely affected by neoclassical losses, related to the asymmetries inherent in the 3D topology of the fields [4]. The strategy of designing the coils without too much regard for the plasma behaviour proved to be inadequate. Modern Stellarator machines are now conceived with a completely different approach, in which a plasma equilibrium with good neoclassical confinement properties is analysed first, and then, the external coils are configured to approximate the required magnetic fields as much as possible. The most recent devices, such as W7-X, have been built not only taking into account neoclassical transport but are also optimized relative to ideal MHD stability. They are basically stable to all ideal MHD perturbations, as described in [5,6]. These new configurations have certainly benefitted greatly from the substantial progress in theoretical understanding of neoclassical transport [7]. On the other hand, the most consequential aspects of energy transport remain too difficult to properly understand with only numerical simulations. Therefore, in the last years, the community has collected an international database explicitly built to investigate the confinement properties of the Stellarator configuration. This International Stellarator Confinement DB contains 0D quantities from all the major devices in the world. (0D quantities are global values, characterising the whole plasma). It has therefore been the basis for the identification of the most widely accepted scaling law for the Stellarator confinement time \(\tau_{E}\), the so called ISS04. This scaling is in power law monomial form and, in order to fit the experimental data acceptably, it employs a dimensionless renormalisation factor, called \(f_{ren}\), for each device and for each sufficiently different optimisation level. In the traditional interpretation, \(f_{ren}\) is considered to reflect the different aspects of the optimisation, which cannot be accounted for by 0D quantities. The assumptions adopted to derive and interpret the ISS04 scaling have been critically tested with a new approach to data fitting, called Symbolic Regression (SR) via Genetic Programming (GP) and briefly overviewed in Section 2. The main characteristics of the available database (DB) are summarised in Section 3. The deployment of the proposed methodology to analyse the DB, without any recourse to renormalisation factors, shows that power laws are too rigid and that the main Stellarator magnetic configurations can be better interpreted with different models (Section 4) [8]. The flexibility of the non-power law scalings, identified with SR via GP, are shown to properly reproduce also the signature left on the 0D quantities by the optimisation (Section 5).