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The fast Z-pinch plasma formation, exploding dynamics, and the evolution of the instability can be controlled experimentally by making special structures on metal surface layer to change the initial state of material, which is valuable for studying the Z-pinch physics. Experiments on the explosion of thin flat foils which have been etched into a periodical structure on surface are performed on the QG-1 facility (～1.4 MA peak current, ～100 ns rise time) in order to study the effects of different surface conditions on explosion and control the evolution of the instability in fast Z-pinch plasma. A kind of inverse load configuration is used in experiment in which the return current post is set at the central axial-position and two modified flat foils are strained outside symmetrically as the main load. So the corresponding J × B force directs outward from the return current post orthogonal to the foil plane, creating an acceleration and pushing the foil plasma away from the center in this configuration. Different surfaces of the foil are also investigated in different conditions because of the asymmetric magnetic field distribution which is useful to study the different evolutions of instability. The foils used in the experiment mainly are the 30-μm-thick aluminum foil. The wavelength of groove perturbations seeded on the surface is 2 mm wide and ～10 μm deep. The plasma explosion dynamic behaviors around conditioned area are diagnosed by laser shadowgraphy, laser interferometry, multiframe optical self-emission imaging and B-dot. It is found that the initially etched periodical structure on surface can control the plasma structure in exploding process which can be concluded as follows. Developing plasma structure shows a periodic character similar to the initial surface structure and the eigenwavelength of the Al is suppressed. In the meantime, the surface without etched perturbations is also influenced by the etched side, showing a similar instability structure but with a lower amplitude. The correlation between two surfaces turns stronger than the case of normal foils. A faster expanding rate occurs in the deep region of the initial periodical groove structure which causes a reverse structure to form. In the discontinuous area of the conditoned structure, a narrow stream of plasma jets perpendicularly from the metal surface which causes a half-wavelength to occur in spectrum analysis. The magneto-hydro-dynamic theory analysis shows that the change of electrothermal instabilities is caused dominantly by the modulation of current density flowing around the periodical structure.

Electron beam pinching is a common physical phenomenon in the working process of high-current electron beam diodes. The radial collapse velocity (Va) of the beam is an important index to determine the beam pinching and the working characteristics of the diode. The current research methods are based on optical diagnosis and theoretical estimation formulas for a specific diode. The radial collapse velocity of Qiangguang-I accelerator’s tight-pinched short γ diode can be obtained by the following three methods in this paper: 1) a theoretical formula, which is used to calculate the radial collapse velocity on the basis of the existing research results, and can very quickly determine the pinching situation because in this case this formula just needs a diode pinching current; 2) the method of calculating Va, which is established based on particle-in-cell simulation. The simulation model includes the anode ion current, thus can simulate the pinching of electron beam more precisely; 3) a method of calculating Va, which is given by measuring the pinch center offset and the γ-ray PIN waveform, because the Qiangguang-I γ diode is inconvenient for optical diagnosis. The radial collapse velocities obtained by the above three methods are 8.43, 8.70 and 7.89 cm2/ns respectively, and the relative difference among the three methods is < 10%. The third method obtains a slightly smaller value because the ion current assumed in the theory and simulation is H+. The ion current composition in the actual diode is complex, the diffusion speed is slower, then the radial collapse velocity is smaller. Compared with the typical Va value (2–4 cm2/ns) of the Gamble II accelerator diode given by the Blaugrund team, the Va value of the short γ diode of the Qiangguang-I accelerator is nearly doubled. The diode on Qiangguang-I, which works after a plasma opening switch (POS), has a very short rising time (less than 10 ns), and pinches quickly. In contrast, the rising time of the Gamble II accelerator diode is about 40 ns, which is different from the working status of the Qiangguang-I diode. This paper provides a new way to study the radial collapse velocity of high-current diodes.

The trigger characteristics of a multi-gap gas switch with double insulating layers, a square-groove electrode supporter and a UV pre-ionizing structure are investigated aided by a high sensitivity fiber-bundle array detector, a UV fiber detector, and a framing camera, in addition to standard electrical diagnostics. The fiber-bundle-array detector is used to track the turn-on sequence of each electrode gap at a timing precision of 0.6 ns. Each fiber bundle, including five fibers with different azimuth angles, aims at the whole emitting area of each electrode gap and is fed to a photomultiplier tube. The UV fiber detector with a spectrum response of 260-320 nm, including a fused-quartz fiber of 200 μm in diameter and a solar-blinded photomultiplier tube, is adopted to study the effect of UV pre-ionizing on trigger characteristics. The framing camera, with a capacity of 4 frames per shot and an exposure time of 5 ns, is employed to capture the evolution of channel arcs. Based on the turn-on light signal of each electrode gap, the breakdown delay is divided into statistical delay and formative delay. A decrease in both of them, a smaller switch jitter and more channel arcs are observed with lower gas pressure. An increase in trigger voltage can reduce the statistical delay and its jitter, while higher trigger voltage has a relatively small influence on the formative delay and the number of channel arcs. With the UV pre-ionizing structure at 0.24 MPa gas pressure and 60 kV trigger voltage, the statistical delay and its jitter can be reduced by 1.8 ns and 0.67 ns, while the formative delay and its jitter can only be reduced by 0.5 ns and 0.25 ns.

A set of fiber detector which consists of an optical fiber array detector and a solar-blind plug-in optical sensor was developed for multi-gap gas switch optical diagnosis. The detailed discharging process was monitored using the detector and the influence of trigger voltage, gas pressure and UV pre-ionizing on the performance of the switch was also analyzed. The timing precision of the fiber array detector is better than 0.6 ns, and its time response is 3.2 ns. The solar-blind sensor responds to light whose wavelength lies between 260 nm and 320 nm. The following results were obtained by the diagnostic system: the order of the gaps breakdown is 3rd-2nd-1st-4th-5th-6th, and statistic delay and over-voltage breakdown delay take the majority of the whole time delay. The jitter of the switch can be reduced by raising the trigger voltage, lowering the gas voltage and stabilizing the intensity of the ultraviolet light.

This paper summarizes an experimental study of the multi-gap switch for fast linear transformer driver. The self-breakdown voltage testing has been conducted and the relation between self-breakdown voltage and gas pressure was obtained when the switch was filled with dry-air. As the pressure changed from 0 to 0.15 MPa, the self-breakdown voltage increased from 80 kV to 200 kV. An experimental study has been carried out on the triggering feature and discharge distribution property，and the uniform discharge distribution has been acquired after 5 000 shots, which can prolong the life of the switch. The switch filled with 0.15 MPa dry-air had 34.4 ns delay with 1.6 ns jitter operating at 78.5% of self-breakdown voltage and trigger voltage of 70kV. The switch is very simple in structure and as