Gyrotrons are a high-power source of coherent microwave radiation 1 . Their oscillation mechanism is a cyclotron-resonance maser effect, in which a fraction of the rotational kinetic energy of a mildly relativistic magnetized electron beam is converted into electromagnetic energy. The most active area of gyrotron development is their potential use for heating magnetically confined fusion plasmas to the point of thermonuclear ignition. A major obstacle to this endeavour is that during high-power millimetre-wave operation 2-9 competing modes and mode shifts seriously degrade a gyrotron's stability and efficiency 10-13 . Here, we show that these problems can be overcome by active control of the electron-beam parameters during the oscillation. In doing so, we successfully demonstrate the robust steady-state operation of a 170 GHz gyrotron producing a continuous 1 MW output power with an unprecedented efficiency of over 55% in a hard-selfexcitation region. Moreover, we find that an adjacent resonant mode previously expected to compete with and adversely affect the principal operating mode does not in fact jeopardize but rather helps this mode as a result of nonlinear effects. The result improves the outlook for using these devices for heating and instability control in future experimental fusion reactors, such as ITER [14][15][16][17][18][19] .A basic configuration of the high-power gyrotron oscillator used in the experiment 7 is shown in Supplementary Information, Figs S1,S2. The nominal operation mode is TE 31,8 , in a cylindrical open resonator, whose radius is 17.9 mm. An annular electron beam of 9.13 mm in radius is injected into the resonator along the axial magnetic field to excite TE 31,8 . The oscillation millimetre-wave power P osci is converted to a gaussian-like beam using a quasi-optical launcher 20,21 attached to the resonator, and transmitted through an edge-cooled diamond window as P out . Here, P out ∼ 0.92P osci due to the ohmic loss and the diffraction loss P loss . The collector of the gyrotron is earthed. By applying a positive voltage V d.c. to the resonator section against the collector, the energy recovery of the spent electron beam is available to enhance the overall efficiency 22 .After a demonstration of 1 h oscillation at P out = 0.6 MW with fixed parameters, the operation parameters are actively controlled with a slow timescale to investigate the oscillation characteristics in the continuous-wave state. Figure 1a shows the dependence of the output power on the magnetic field in the resonator, B c . Here, V b ∼ −72.5 kV and V d.c. ∼ 25.5 kV. After the electron-beam I b had stabilized at ∼30 A completely, which takes ∼1 min, the B c scan started from 6.72 T. The frequency is ∼170 GHz. The power increases as the B c decreases, that is, the cyclotron resonance mismatch factor Δ = (1 − (f ce /γf )) increases. Here, f and f ce are oscillation and non-relativistic cyclotron frequencies, respectively, and γ is a relativistic factor of the initial electrons. The maximum power of 0.8 MW is obtained...

The design progress in a compact low aspect ratio (low A) DEMO reactor, ‘SlimCS’, and its design issues are reported. The design study focused mainly on the torus configuration including the blanket, divertor, materials and maintenance scheme. For continuity with the Japanese ITER-TBM, the blanket is based on a water-cooled solid breeder blanket. For vertical stability of the elongated plasma and high beta access, the blanket is segmented into replaceable and permanent blankets and a sector-wide conducting shell is arranged inbetween these blankets. A numerical calculation indicates that fuel self-sufficiency can be satisfied when the blanket interior is ideally fabricated. An allowable heat load to the divertor plate should be 8 MW m−2 or lower, which can be a critical constraint for determining a handling power of DEMO.

Propagation of an ionization front in the beam channel was observed after plasma was generated using a 170GHz millimeter-wave beam in the atmosphere. The propagation velocity of the ionization front was found to be supersonic when the millimeter-wave power density was greater than 75kWcm−2. The momentum coupling coefficient Cm, a ratio of the propulsive impulse to the input energy, was measured using conical and cylindrical thruster models. A Cm value greater than 350NMW−1 was recorded when the ionization front propagated with supersonic velocity.

The concept for a compact DEMO reactor named 'SlimCS' is presented. Distinctive features of the concept are low aspect ratio (A = 2.6) and use of a reduced-size centre solenoid (CS) which has the function of plasma shaping rather than poloidal flux supply. The reduced-size CS enables us to introduce a thin toroidal field coil system which contributes to reducing the weight and perhaps lessening the construction cost. Low-A has merits of vertical stability for high elongation (κ) and high normalized beta (β N ), which leads to a high power density with reasonable physics requirements. This is because high κ facilitates high n GW (because of an increase in I p ), which allows efficient use of the capacity of high β N . From an engineering aspect, low-A may ensure ease in designing blanket modules robust to electromagnetic forces acting on disruptions. Thus, a superconducting low-A tokamak reactor such as SlimCS can be a promising DEMO concept with physics and engineering advantages.

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