To sustain long-lasting plasmas in a magnetic fusion device, it is crucial to control the plasma for durations significantly longer than the energy and particle confinement times—approximately 100 milliseconds in the WEST tokamak and 2 seconds in ITER. Long-pulse operation in tokamaks and stellarators is addressing control of stable plasma for duration well above the plasma confinement time and approaching plasma-wall integration timescales where physics processes evolve on very long timescales, such as hydrogen saturation of the walls or surface erosion. Achieving simultaneous improvements in duration and fusion performance demands an integrated approach that addresses both the physical and technological aspects. This integrated vision is currently being implemented through an international effort that encompasses:

• (1) an experimental and simulation program on tokamaks and stellarators;
• (2) a technological R&D program on dedicated facilities (for example, actively cooled plasma facing components, superconducting magnets) ;
• (3) control methods to maintain a fusion burning plasma within a safe and stable operating domain, "transferable" to ITER and beyond.

To achieve this goal, the International Energy Agency (IEA) and the International Atomic Energy Agency (IAEA) established  a network of experts in 2020 called CICLOP, which stands for "Coordination on International Challenges on Long duration OPeration". The wink to Greek mythology might not be accidental, as the Cyclopes were reputed to be supernaturally skilled  craftsmen who pushed back the limits of know-how! [cf. ITER news https://www.iter.org/newsline/-/3823].  The group is chaired by an expert from CEA (France), with two co-chairs one from the Max-Planck-Institut für Plasmaphysik (Germany) and one from the National Institute for Fusion Science (Japan).

The objectives of the CICLOP group are to promote and coordinate experiments and to collect and share best practices on the operation of long-duration plasmas. In 2022, this activity was reported as part of the IAEA Technical Meeting devoted to Long-Pulse Operation of Fusion Devices  [November 14-16, 2022, IAEA headquarters, Vienna, Austria, https://conferences.iaea.org/event/258/].

A significant effort has been made to collect and analyze a multi-machine database with data from experiments on ten tokamaks (1)  and two stellarators (2). The database is accessible via an open IAEA web page.

In practice, achieving continuous high-performance operation is extremely challenging since it requires high power to be injected to heat the plasma and then extracted continuously without exceeding the technological limits of the surrounding components. To characterize and compare the heat exhaust capability of different facilities around the world, a simple indicator was defined as the ratio of plasma heating power, P, normalized to plasma surface area, S, P/S. This ratio was plotted against the duration of the high-performance fusion phase (graph opposite). The analysis highlights the challenge in terms of heat exhaust capability for long pulse operation, which is being addressed in France on the WEST Tokamak in preparation for ITER operation.

In addition, the fusion power gain represented in the figure below by the normalized plasma pressure, called βN (plasma thermal pressure divided by magnetic field pressure times a normalization factor), has been calculated and plotted as a function of the duration of the high-performance fusion phase. For continuous operation and high fusion power gain, it is required to operate at high values of βN while remaining stable. The figure shows a significant reduction in fusion performance with increasing duration. The ITER reference scenario targets a βN of 1.8, while fusion power plant projects often aim for βN>2.5. It is worth noting that JT-60SA, recently commissioned, has the specific task of exploring high βN scenarios over extended durations, aiming to optimize the performance of fusion power plants

The analyses, detailed in the publication, demonstrate that ITER's objectives in terms of duration and performance have been achieved and even exceeded independently. However, they also highlight the distance yet to be covered to achieve these goals simultaneously—a challenge that ITER must address.

The CICLOP group has also identified gaps in physics and engineering between current results and the objective for ITER operation and future power plants. This includes the need to explore stationary highly radiative plasmas and to study the effects of the aging of materials surrounding the burning plasmas on fusion performance. These topics will be central to future discussions, anticipated to be rich and exciting at the upcoming IAEA thematic conference scheduled for 2024, dedicated  to long-term operation in magnetic confinement fusion devices.