Dense plasmas can be produced by means of transient electrical discharges. In particular, a pinch is a transient plasma column conducting electrical current, which becomes self-confined by the associated magnetic field. Plasma pinches reproduce the scenario of high-energy-density, intense beams of charged and neutral particles, with radiation emission. Thus, they become a suitable laboratory tool for fundamental and applied research on fusion and neutron production, among other phenomena.

The aim has been to produce a stable gas-embedded Z-pinch at currents of thermonuclear interest. Z-pinch experiments relevant to fusion studies require generators capable of achieving currents up to about MA in 100 ns. 

Experiments in gas-embedded Z-pinches have been carried out in Chile since years ago. In the first experiments a small pulse power generator was used, a Marx bank (400 kV) coupled to a water transmission line (1.5, 300 kV, 120 ns double transit time). The current rate was approximately 2 × 10¹² A s⁻¹ and the peak current achieved was between 150–180 kA. The discharges were performed in H2 and He at 1/3 atmospheres, and several preionization schemes were studied [1–4]. 

The most interesting results were obtained in a double column pinch. This configuration uses a preionization scheme based on a combination of an annular micro-discharge followed by a laser pulse. This scheme produces a double column pinch at the early stage that coalesces into a single plasma column at 60 ns, showing again a period of enhanced stability with no MHD instabilities developing during the current rising (150 ns) and achieves 180 kA. 

Following the above achievements, the aim of this research is to study a double column pinch at currents of thermonuclear interest, i.e., greater than 1MA. The SPEED2 generator is being used with this purpose.

SPEED2 is a generator based on Marx technology [5] (4.1 μF equivalent Marx generator capacity, 300 kV, 4 MA in short circuit, 187 kJ, 400 ns rise time, dI/dt ∼ 10¹³ A s⁻¹). SPEED2 was transferred from the University of Düsseldorf to CCHEN and it has been in operation at our laboratory since 2001. Currently, the device is being operated at CCHEN at 70 kJ stored energy, producing a peak current of 2.4 MA in short circuit. 

Figure 1. The SPEED2 generator at CCHEN P4-laboratories.

The SPEED2 generator is suitable for producing wire array discharges over ∼20 wires. Then, on the one hand, in order to produce a metallic pinch plasma radiating soft x-ray for studies relevant to inertial confinement, a zero-dimensional model was used to design wire arrays suitable to be driven by SPEED2. On the other hand, studies on Z-pinch stability are being carried out in gas embedded Z-pinch at mA current driven by the SPEED2 generators.

For linear Z-pinch experiments related to fusion research, the SPEED2 is being used. Zero-dmiensional simulations for gas-embedded Z-pinch and wire arrays were developed to produce and design experiments. An apparently stable pinch was obtained in a gas-embedded Z-pinch operating in deuterium, and neutrons were detected. The production mechanism of those neutrons should be clarified (thermonuclear versus beam-target). Studies of different initial conditions will be performed using a small multipurpose capacitor bank as a complement to the experiments in SPEED2. The experiments on wire arrays are planned to start in the near future.

Experiments in the SPEED2 generator in a D2-embedded Z-pinch at 33 mbar were performed. The electrodes configuration scheme used shows feasibilities and security in order to use the SPEED2 generator in a configuration different to the original design. In the new electrode configuration, the SPEED2 delivers ∼2.4MA of maximum current and produces a voltage, in the central collector, of ∼80kV. An apparently stable plasma column was obtained and neutrons were detected. The line density measured, (2–4) × 10¹⁹ m⁻¹ , corresponds to what expected from the 0D model and is consistent with the finite Larmor radius stability effects. These preliminary results are interesting enough to motivate further experiments. 

There are both theoretical and experimental evidence indicating that composite coaxial pinches (plasma on wire, plasma focus plus gas puffed, sheared flow on Z-pinch, and double column gas-embedded Z-pinch) are more stable than single column pinches. In the near future, experiments including the double column preionization scheme, combining the electrode configuration with a pulsed laser onto the cathode, will be carried out. In addition, a complementary diagnostics to measure the total current through the plasma should be developed.

[1] Chuaqui H, Soto L, Favre M and Wyndham E 1993 Proc. 3rd Int. Conf. on Dense Z-Pinches (London, UK) AIP Conf. Proc. ed M Haines (Melville, NY: American Institute of Physics) p 27.
[2] Soto L, Chuaqui H, Favre M and Wyndham E 1994 Phys. Rev. Lett. 72 289.
[3] Soto L, Chuaqui H, Favre M, Saavedra R, Wyndham E, Skowronek M, Romeas P, Aliaga-Rossel R and Mitchell I 1998 IEEE Trans. Plasma Sci. 24 1162.
[4] Soto L, Chuaqui H, Favre M and Wyndham E 1994 Proc. Int. Conf. on Plasma Physics ICPP (Foz do Iguazú, Brazil) p 216.
[5] Decker G, Kies W, Mälzig M, Van Valker C and Ziethen G 1986 Nucl. Instrum. Methods A 249 477.