## 5 Conclusions  
We have developed a new instrument for the simulation of the interaction of nanosecond EMPs with an ionized gas medium in a large volume within a wide range of pressures and other initial conditions. The \"gigantic\" coaxial line ensured EMP propagation without refraction losses in the form of a quasi-TEM mode, i.e., under the conditions close to those of the EMP propagation in the partially ionized atmosphere of the Earth. The large longitudinal size of the line (10 m) makes it possible to study both the nonlinear (ionization)  
Figure 11: Experimentally measured input and transmitted signals with unipolar EMP with duration of 600 ps (**a**), and 200 ps (**b**).  
Figure 10: “Gigantic” coaxial line during its testing in KROT facility room.  
phenomena in the EMP field, and the dispersion transformation of a nanosecond EMP in the plasma, which was confirmed by the results of the simulation and preliminary tests. The wide working space (about 60 cm between the inner and outer conductors) allows one to install ionization sources, plasma diagnostic instruments, and electromagnetic probes in the coaxial line without significant perturbations in the structure of the EMP fields and the plasma. The parameters of the manufactured line, obtained from the results of experimental testing without plasma, are in satisfactory agreement with the results of preliminary numerical simulation. The differences are mainly associated with a decrease in the actual EMP transmission coefficient, compared with the simulated one. These differences are due to the neglect of a number of dielectric parts of the line and reflections associated with them in the model, the finite conductivity of the line conductors, as well as imperfect contacts in the connectors. Pulses with durations from 300 ps to several ns are transmitted with shape retention.  
The coaxial line will be used to simulate the effects of absorption and transformation of the EMP waveform in the upper atmosphere and ionosphere of the Earth, due to the presence of the background and nonstationary additional ionization, as well as perform laboratory studies of nanosecond discharges in large volumes, test and calibrate diagnostic instruments, and verify the models of EMP interaction with the near-Earth plasma.  
Conceptualization, M.G. ([PERSON]) and V.T.; methodology, A.P., A.S., Y.R., S.K. ([PERSON]), A.K. ([PERSON]), A.K. ([PERSON]); software, S.K. ([PERSON]); investigation and validation, I.Z., [PERSON] ([PERSON]), and A.G.; formal analysis, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); resources, K.L.; writing--original draft preparation, [PERSON] ([PERSON]) and A.P.; writing--review and editing, V.T.; visualization, [PERSON] ([PERSON]), A.G. and V.K.; supervision, V.T. and S.K. ([PERSON]); project administration, [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
The experiments were performed on the Unique Scientific Facility \"Complex of Large-Scale Geophysical Facilities (KKGS)\". The study was supported by the Ministry of Science and Higher Education of the Russian Federation, project 075-15-2020-790.  
Not applicable.  
The data have not been collected using public funds.  
The authors declare no conflict of interest.