A ‘Stepladder’ Approach to a Steady State Tokamak Fusion Power Plant

In the EU strategy to an FPP, DEMO is the single step between ITER and an FPP. It is not obvious how to arrive at a DEMO design point in this strategy. We propose to avoid large scenario development steps in an ITER-DEMO-FPP step-ladder, since no other machines can qualify the scenarios. Thus, DEMO becomes a technology demonstrator, not a plasma physics experiment. We characterize the plasma scenario in terms of the quantities $\beta_N$, q, H and $f_{GW}$. To ensure adequate divertor performance, constant $n_e$ is chosen. Different from previous approaches, $\rho^*$ and $\nu^*$ will vary throughout the stepladder based on physics arguments that below minimum values, their variation is no longer important. This leaves open the choice of machine parameters A, R and B. Fixing A to the ITER value, constant $f_{GW}$ and absolute $n_e$ lead to B/R = const. At constant q, $\beta_N$ and A, B and R increase proportional to $P_{fus}^{1/7}$ in the stepladder. The power needed to drive the current in steady state varies similarly, so from DEMO to an FPP a significant decrease in recirculating power fraction occurs. A viable divertor solution and access to H-mode are considered explicitly. An example for such a stepladder is discussed, based on recent ASDEX Upgrade results in steady state.