Nuclear – Introduction

Tokamaks and stellarators are the most promising kinds of fusion machines presently being developed by the worldwide fusion community (both public and private). The common characteristic is the way these machines try to keep the fusing plasma in a stationary position: they use the Lorentz force to create a stable magneto-hydrodynamic configuration (i.e. the way a charged fluid – like the plasma – flows when immersed in a magnetic field) which confines the plasma into a toroidal region of space, avoiding contact between the extremely hot fluid (~ 100 million Kelvin) and reactor walls. The difference between the two concepts lies in the way the stable configuration is achieved, with tokamaks exploiting a strong current running in the plasma (about 1 ÷ 15 MA), and stellarators avoiding such current by (much) complicating the shape of the electromagnets that generate the magnetic field.

In the complex recipe for controlled fusion, mechatronic systems (and their designers!) play important roles. For example, special actuators and sensors are designed to resist the harsh working conditions of the fusion machines. Often, the design of such systems is customized for the specific fusion machine, which demands insightful choices to fulfil all design constraints and requirements. Two examples of mechatronic applications are the movable mirrors of ECRH systems and the remote handling robots for reactor maintenance. We will now have a closer look at ECRH technology, its relevance, and the associated challenges for mechanical, mechatronic and automation engineers. However, the reader should consider that in any case, applications for nuclear fusion require a multi-physical approach to design, which is what makes it so challenging.

Electron Cyclotron Resonance Heating (ECRH) is one of the main methodologies used to heat the plasma to fusion temperatures (~ 10⁸ K). Its working principle is based on injection of high-power (~ 1 MW) microwave beams into the fusing plasma. The microwaves excite the gyromotion of electrons around the magnetic field lines and in this way increase their kinetic energy, which translates macroscopically into a temperature increase. The main advantage of ECRH over other heating methods is the possibility of locally heating a small region of the plasma, allowing for precise control of plasma instabilities and magneto-hydrodynamic (MHD) performance.  This target is achieved by the combination of the following key subsystems:

• Gyrotrons: a special class of high-power vacuum tubes that generate microwaves in the millimeter wavelength range.
• Transmission lines: responsible for transporting the generated microwaves from the gyrotrons to the reactor vessel with minimal losses. The transmission line includes movable mirrors for adjusting microwave beam polarization.
• Steering launchers: these mechatronic systems control in real time the injection angle of the wave into the plasma, which is crucial for optimizing electron heating. Next-generation ECRH systems will involve synergic control of steering angle and beam polarization for the maximization of energy deposition into the plasma.

As mentioned above, tokamaks and stellarators can reach a stable MHD configuration by suitably arranging their magnetic field. However, perturbations of plasma particles trajectory can trigger unstable MHD phenomena if not properly controlled. Real-time control of the ECRH system is used in some of the most advanced fusions machines worldwide to suppress such instabilities. In particular, this will be the case in the Divertor Tokamak Test (DTT) facility, a next-generation Italian fusion reactor currently under construction in Frascati (RM). DTT will host the most powerful ECRH system of its generation, offering a unique chance to contribute to cutting-edge research and development in fusion technology. Our research group is in strict collaboration with the DTT team and Eni S.p.A., which also contributes to DTT development. It is in the context of this collaboration that the following activities are proposed.

Figure 2 shown a CAD realization of the steering launcher under design for the DTT machine. The mechanism includes flexible joints (purple and blue), flexible pipes (red) and piezoelectric actuators. This solution guarantees high compactness (allowing for a high power density to be injected into the plasma), wide steering ranges and precise motion.

Figure 2. (Left) CAD realization of the DTT ECRH launcher, characterized by piezoelectric actuators and flexible joints. (Right) view of a section of the DTT tokamak with the last part of the DTT ECRH transmission line, showing the different possible injection angles of microwave beams inside the vacuum chamber of the machine, by means of 8 ECRH launchers. The mechanism shown in the (Left) picture faces the hot plasma contained in the vacuum chamber from less than 1 m.