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...

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.

Recent progress on the high power gyrotron development in JAEA is presented. The gyrotron is featured to have a triode-type magnetron injection gun, a cylindrical resonator working at 170 GHz with TE31,8 mode, a water-cooled diamond window and a depressed collector. After the demonstration of the ITER basic performance, the gyrotron has been operated for 3 years, and recorded ∼200 GJ of total output energy. Next, a gyrotron which oscillates in higher order resonator mode, TE31,12, is designed and fabricated to study the long pulse oscillation at greater than 1 MW. In parallel, feasibility studies of a CW-power modulation for neoclassical tearing mode stabilization, a dual frequency gyrotron and a rapid frequency control are carried out. It is shown that these gyrotrons will be available with current technology.

IFMIF, the International Fusion Materials Irradiation Facility, is presently in its engineering validation and engineering design activities (EVEDA) phase under the Broader Approach Agreement. The engineering design activity (EDA) phase was successfully accomplished within the allocated time. The engineering validation activity (EVA) phase has focused on validating the Accelerator Facility (AF), the Target Facility and the Test Facility (TF) by constructing prototypes. The ELTL at JAEA, Oarai successfully demonstrated the long-term stability of a Li flow under the IFMIF’s nominal operational conditions keeping the specified free-surface fluctuations below  ±1 mm in a continuous manner for 25 d. A full-scale prototype of the high flux test module (HFTM) was successfully tested in the HELOKA loop (KIT, Karlsruhe), where it was demonstrated that the irradiation temperature can be set individually and kept uniform. LIPAc, designed and constructed in European labs under the coordination of F4E, presently under installation and commissioning in the Rokkasho Fusion Institute, aims at validating the concept of IFMIF accelerators with a D+ beam of 125 mA continuous wave (CW) and 9 MeV. The commissioning phases of the H+/D+ beams at 100 keV are progressing and should be concluded in 2017; in turn, the commissioning of the 5 MeV beam is due to start during 2017. The D+ beam through the superconducting cavities is expected to be achieved within the Broader Approach Agreement time frame with the superconducting cryomodule being assembled in Rokkasho. The realisation of a fusion-relevant neutron source is a necessary step for the successful development of fusion. The ongoing success of the IFMIF/EVEDA involves ruling out concerns about potential technical showstoppers which were raised in the past. Thus, a situation has emerged where soon steps towards constructing a Li(d,xn) fusion-relevant neutron source could be taken, which is also justified in the light of costs which are marginal to those of a fusion plant.

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