A plane parallel model of multipactor is studied in detail using both an analytical approach and numerical simulations. The analytical analysis is carried out within a widely exploited approximation, which assumes a fixed value for the initial velocity of secondary electrons. It is shown that the commonly accepted picture of the multipactor zones is not quite complete and should be modified by taking into account the existence of hybrid resonance modes and the important consequences of a secondary emission yield that significantly exceeds unity. Numerical simulations demonstrate that the chart of the multipactor zones is also very sensitive to a spread of the initial velocity of the electrons. In particular, the full effect of initial electron velocities cannot accurately be modeled by using a single fixed value only.

This paper presents a novel theory for describing the initial stage of a single-surface multipactor discharge on a dielectric surface in the presence of a dc electric field, which returns secondary emitted electrons to the surface. The calculations employ a statistical method based on an exact analytical solution for the probability density of the arrival times of the secondary electrons. A general integral equation determining the steady-state distribution of the emission phases of the secondary electrons and the threshold of the multipactor growth is formulated. A computer program has been developed to implement this theory for realistic secondary yield curves and arbitrary, nonuniform, distributions for velocities and angles of emitted electrons. Susceptibility diagrams, applicable to a wide range of materials, are obtained in terms of the rf and dc electric fields and are found to be relatively independent of the emission distribution of the electrons.

Detailed numerical simulations of the two-sided multipactor have been carried out within a plane-parallel model. The main aim of the simulations is to clarify the uncertainty that still exists in the literature concerning the overlapping of multipactor zones. Three different codes (Monte Carlo, particle-in-cell, and statistical) were used to calculate the multipactor charts within a wide range of parameters such as spread of initial velocities of the secondary electrons and maximum value of the secondary emission yield. It was found that an increase in the spread of initial velocities results in overlapping of the multipactor zones, provided the secondary emission is high enough. In the opposite case, an increase in the spread of initial velocities leads to a suppression of the higher multipactor zones.

Electron cyclotron current drive (ECCD) calculations for the case of the ITER electron cyclotron resonant heating upper port launcher are presented making use of a quasi-optical (QO) code (Balakin et al 2008 Nucl. Fusion 48 065003). The QO code describes accurately the behaviour of the wave beam in the electron cyclotron resonance layer, taking into account spatial inhomogeneity and dispersion. The ECCD efficiency is obtained using the adjoint calculation as presented by (Lin-Liu et al 2003 Phys. Plasmas 10 4064). The results show a broadening of the EC driven current density profiles in the range 15–30% as compared with beam-tracing calculations using TORBEAM. Consistently, peak driven current density values are found to be decreased by 10–20%. These results have significant consequences for the determination of the requirements on ECCD power to control magnetohydrodynamic instabilities such as neoclassical tearing modes and sawteeth.

Advanced numerical models used for climate prediction are known to exhibit biases in their simulated climate response to variable concentrations of the atmospheric greenhouse gases and aerosols that force a non-uniform, in space and time, secular global warming. We argue here that these biases can be particularly pronounced due to misrepresentation, in these models, of the multidecadal internal climate variability characterized by large-scale, hemispheric-to-global patterns. This point is illustrated through the development and analysis of a prototype climate model comprised of two damped linear oscillators, which mimic interannual and multidecadal internal climate dynamics and are set into motion via a combination of stochastic driving, representing weather noise, and deterministic external forcing inducing a secular climate change. The model time series are paired with pre-specified patterns in the physical space and form, conceptually, a spatially extended time series of the zonal-mean near-surface temperature, which is further contaminated by a spatiotemporal noise simulating the rest of climate variability. The choices of patterns and model parameters were informed by observations and climate-model simulations of the 20th century near-surface air temperature. Our main finding is that the intensity and spatial patterns of the internal multidecadal variability associated with the slow-oscillator model component greatly affect (i) the ability of modern pattern-recognition/fingerprinting methods to isolate the forced response of the climate system in the 20th century ensemble simulations and (ii) climate-system predictability, especially decadal predictability, as well as the estimates of this predictability using climate models in which the internal multidecadal variability is underestimated or otherwise misrepresented.

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Statistical models are known to be simple and effective tools for interseasonal predictions of ENSO dynamics (Barnston et al., 2012; Jan van Oldenborgh et al., 2005). The IRI/CPC ENSO Predictions Plume (Barnston et al., 2012)-an ensemble forecast of the Niño 3.4 index defined as the average sea surface temperatures (SST) in the region (5°N-5°S, 170°W-120°W)-demonstrates that both statistical and dynamical models yield close prediction skills at lead times up to 12 months. This similarity likely reflects the near-linearity of the seasonal tropical Indo-Pacific SST predictability studied by Newman and Sardeshmukh (2017). The main factor limiting statistical forecasts is the spring predictability barrier (SPB), also called the spring persistence barrier, that is, the empirically observed loss of autocorrelations in the tropical Pacific climate dynamics in May-June (Barnston et al., 2012; Torrence & Webster, 1998). Since many statistical models rely on SST anomalies (SSTAs) in the tropics, the SPB impacts statistical models more than dynamical models during forecasts beginning in spring (Barnston et al., 2012). Basically, the SPB phenomenon can be explained as a manifestation of ENSO seasonality related to the phase locking of ENSO dynamics with a seasonal cycle (Liu et al., 2018). In the tropical SSTA variability, there is a distinct one-year temporal pattern, hereinafter referred to as the ENSO cycle, that lasts from June to May of the following year, with persistent SST anomalies developing in the middle of the cycle (autumn-winter), whereas smaller and noisier anomalies appear at the beginning and end of the cycle (summer and spring, respectively). In particular, Tippett and L'Heureux (2020) recently showed that approximately 90% of the Niño 3.4 index variability can be explained by a one-dimensional deterministic signal defined on the June-May interval multiplied by different amplitudes in different years, with extrema in December and the lowest absolute values in May and June.

The secondary-emission discharge (multipactor) on a dielectric surface irradiated by a plane TEM wave of circular polarization is analyzed theoretically. It is shown that interaction of electrons with the electromagnetic wave field can provide their return to the emission surface, which makes a multipactor possible even without any external static fields. Multipactor conditions for different reflection coefficients of the incident wave are found. The obtained results are used to estimate the throughput of the output window of a high-power vacuum microwave device. Specifically, it is established that decreasing the high-frequency field amplitude on the window surface when the mode of a partially standing wave is realized in the vicinity of the window does not always help raise its electric strength.