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...
This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3 × 1023 m2 s−1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ~30 MW m−2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m−2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ~1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors.
Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density ne_fir = 1 × 1019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of ne_fir. The plasma stored energy of 530 kJ with ne_fir = 3.2 × 1019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.
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