Cherenkov probes and runaway electrons diagnostics

The beams of fast runaway electrons (RE), which are often produced during tokamak discharges, are particularly dangerous and can induce serious damages of the vacuum vessel and internal components of the machine. The proper and fast diagnostics of RE beams is essential for controlling the discharge, e.g., by early mitigation of disruptions and potentially dangerous RE beams. The diagnostics of RE beams is usually based on measurements of the radiation emitted either by these electrons, or as a result of their interactions with plasma and/or vessel walls. Such a radiation is usually recorded by the means of probes placed outside the vacuum vessel. The method developed by our team is based on the probe located inside the vacuum vessel. The probe can be used to detect highly localized RE bunches and to determine their spatial and temporal characteristics. During last few years, the NCBJ team have developed and used the RE diagnostics based on the Cherenkov effect observed in diamond radiators coupled with fast photomultipliers. During the investigated discharges, the probe was inserted into the vacuum vessel, and its head was placed at the plasma edge, where fast RE are expected. A correlation between signals recorded using our probes and other diagnostics, e.g., hard x-ray signals, was also studied. In this paper, we present recent results of the RE measurements by means of Cherenkov probes, which were performed in the COMPASS and TCV tokamaks.

The Uniformly Accelerated String and the Bell Spaceship Paradox

We consider the string with the length l, the left end and the right end of which is non-relativistically and then relativistically accelerated by the constant acceleration a. We calculate the motion of the string with no intercalation of the Fitzgerald contraction of the string. We consider also the Bell spaceship paradox. The Bell paradox and our problem is in the relation with the Lorentz contraction in the Cherenkov effect (Pardy, 1997) realized by the carbon dumbbell moving in the LHC or ILC (Pardy, 2008). The Lorentz contraction and Langevin twin paradox (Pardy, 1969) is interpreted as the Fock measurement procedure (Fock, 1964;).

Excitation of Ultrashort Spin Waves via Spin-Cherenkov Effect in Magnetic Waveguides

The excitation of ultrashort wavelength spin waves via the spin-Cherenkov effect in magnetic waveguides is investigated via a micromagnetic modeling. The proposed excitation method is relatively simple and easily tunable. The excitation efficiency of the proposed scheme is obtained for different excitation pulse velocities and widths. A coupled waveguide system is also considered. In this case, the spin waves are excited in the first waveguide and then are transferred to the second one due to the dipolar coupling between waveguides. It is also shown that the excitation and transfer of excited spin waves have some limitations related to the dipolar coupling mechanism between the waveguides.