## 2 Description of the Experiment
The maximum of the density distribution was around \(r\) = 35-40 cm, and its width decreased slightly as the magnetic field strength increased from 0 to 90 G (Figure 2a). The dynamics of the decaying plasma density or electron plasma frequency measured at the maximum of the radial distribution and the magnetic field or cyclotron frequency, which was realized in the experiment, are presented in Figure 2b. The EMP was excited by the PPM0731 pulse generator module (manufactured by Megaimpulse Ltd., St. Petersburg, Russia), which formed unipolar 6 kV pulses, whose rising time was \(\tau_{f}\) < 200 ps, and the half-height duration was \(\tau_{w}\)-1 ns. The generator was protected against the reflected signal and the interference of the ionizing pulse by means of an attenuator (Figure 1). After the attenuator, the pulse fell on the delta resistive splitter, forming a reference signal, which was fed to the detecting equipment via the attenuator, and a main pulse fed to the GCL input. The amplitude of the pulses in the line was attenuated down to a level of 75 V. The test experiments demonstrated that, at this level, the nonlinear effects in plasma had not started developing yet. The reference signal contained both the incident pulse and GCL-reflected pulse. The incident and reflected pulses were separated in time by \(\sim\)30 ns, which allowed analyzing them independently. The scheme, which explains the formation of the pulsed signals and delays between them, is shown in Figure 3. On passing through the line, the main pulse was attenuated by means of the attenuator and fed to the input of the Tektronix MDO64 digital oscilloscope with a passband of 8 GHz and a sampling rate of 25 gigasamples/s. The attenuator at the GCL output reduced the pulse amplitude to the level which allowed detecting it with the oscilloscope, on the one hand, and protected the oscilloscope against the interference of the ionizing pulse, on the other. Figure 2: (**a**) Radial distributions of the decaying plasma density filling the GCL with or without a magnetic field. (**b**) Time dependences of the electron plasma frequency and the electron cyclotron frequency in the case of magnetized plasma. The time is reckoned from the end of the ionizing pulse. The EMPs in the experiments were excited in the line as a periodic sequence with a duration of 40 ms and an interval of 100 \(\upmu\)s between the pulses. The specific values of the plasma density, at which the data about the shape and spectrum of the ultrawideband pulse (UWB EMP) were obtained, corresponded to different delay times \(T\) of EMP transmission relative to the end of the ionizing pulse. When preparing the experiments, special attention was paid to the effects of parasitic reflection of EMPs from dielectric parts of the GCL structure and rf inductors. It was found that radial dielectric spokes lead to the appearance of reflected signals at a level of several percent of the amplitude of the transmitted signal; these signals arrive with significant (on the order of several ns) delays and do not interfere with measurements. The level of EMP reflection from rf inductors is even lower, possibly due to the small effective area of the wires, and also because the inductor wires are perpendicular to the electric field vector of the EMP propagating in the GCL.