## 2 Large-Scale Coaxial Line  
The modeling of the interaction of UWB EMPs with partially ionized gas under conditions, whose similarity parameters correspond to the space (or ionospheric) environment, presupposes that the propagation path of an electromagnetic wave in the medium is sufficiently long. The EMP amplitude in the plasma can vary over a sufficiently wide range, in which case one observes both linear effects (dispersion transformation of the waveform) and nonlinear phenomena (anomalous absorption and nonlinear waveform distortions due to nonstationary ionization). It is desirable to ensure that the size of the experimental setup in the polarization plane of the electromagnetic wave is as large as possible, so that the diagnostic tools required to measure the parameters of the EMP and the plasma introduce minimal disturbances to the structure of the plasma and the wave fields. The KROT facility, designed to model physical phenomena in the ionosphere and space environment, is one of the largest plasma facilities available [21]. The core of the facility is a vacuum chamber that has a volume of 180 m\({}^{3}\) and a diameter of 3 m, where the residual gas pressure reaches a level of 1 \(\mu\)Torr. A megawatt-pulsed RF source produces plasma with a volume of up to several tens of cubic meters in various gas media in the chamber. The first experiments on the irradiation of large plasma volumes with EMP, which were described in [20], allowed us to observe, for the first time, the dispersion transformation of the EMP waveform in the plasma, due to the rather long path of the pulse propagation. At the same time, the performed studies revealed a set of fundamental problems that arise in experiments with EMP propagation in plasma chambers. First, due to the limited transverse dimensions of the chamber and the produced plasma, it is possible to actually realize only the waveguide regime of EMP propagation along an extended path, and in this regime the lower frequencies are lost in the pulse spectrum, and the waveform is transformed due to the chamber waveguide dispersion even in vacuum. Second, when the end of the plasma column is irradiated by a horn antenna, electromagnetic waves are strongly refracted. A significant fraction of the EMP energy does not enter the plasma, and the field is strongly attenuated as the distance from the horn increases. As a result, it is difficult to observe nonlinear effects in the plasma along the EMP propagation path. In order to make the experimental conditions closer to the modeling ones, it is desirable to, first, ensure excitation and propagation of the EMP in the form of a homogeneous TEM mode and, second, exclude wave divergence and attenuation along the propagation path. To take into account these requirements, a new guiding structure for pulsed TEM modes in the KROT facility was developed, specifically, a \"gigantic\" coaxial line excited by an EMP generator. Depending on the problems to be solved, the working space of the line is filled by the gas mixture with the necessary composition, in which quasi-uniform ionization can be produced by an inductive RF source. The drawing of the line and the scheme of its installation in the plasma chamber are presented in Figure 1. The line consists of a central cylindrical section and two tapered transitions, one to connect generators of nanosecond and subnanosecond EMPs, and the other to connect measuring tools. The total length of the line is 10 m, and the length of the central uniform section is 3 m. The outer conductor of the coaxial line is formed by aluminum plates installed on a frame of Caprolon rings 140 cm in diameter. The width of the plates is 7.5 cm, and the gap between the plates is 5 cm. This design allows one to make the structure lighter and easier to assemble and disassemble (totally or partially), and makes it possible to install diagnostic tools and antennas, which are used to produce background ionization, into the interior of the line. The internal conductor of the central section is a duralumin tube 8 cm in diameter.