International Center for Dense Magnetised Plasmas (original) (raw)

The large Plasma-Focus facility (PF-1000) is a main device operating at the ICDMP. The plasma-focus facility can be divided into three main units:

• the condenser bank and pulsed electrical power circuit driving the plasma discharge,
• the mechanical, vacuum and gas system consisting of the vacuum chamber, coaxial electrodes, vacuum and gas handling system,
• the data acquisition and diagnostics system.

The vacuum vessel of stainless steel surrounds the electrode structure. (Fig.1)

Fig.1

The vacuum chamber has the large volume (1400 mm in diameter and 2500 mm in length). Ordinary vacuum technology, utilizing oil-diffusion pumps and anticreep liquid N2 traps is employed. The condenser bank of 1200 kJ, 40 kV system consists of twelve condenser modules each comprising twenty four 50 kV, 4.625 mF low inductance condensers connected in parallel (Fig.2).

Fig.2

The electric energy is transferred to a collector and electrodes by means of low-inductance cables. The importance of low-inductance cables, condensers, and switches in power supply cannot be overstressed if the large tube currents are to be achieved.

Figure 3 is a photo of the coaxial plasma focus electrode.

Fig.3

The design shown in Fig. 3 consists of two coaxial electrodes and an alumina insulator, across which the initial breakdown occurs. The outer electrode (OE) consist of 24 stainless steel rods. The rod diameter is 32 mm. The outer electrode (OE) and solid center electrode (CE) diameters are 200mm and 115.5 mm, respectively, with CE length of 600 mm. The minimum annular spacing is dr = 68.5 mm. The OE is attached to a circular grounded cable header. The CE attaches to a central header that provides an electrical connection and vacuum seal. The cylindrical alumina insulator sits on the CE (Fig.3). The main part of the insulator extends - 113 mm along the CE into the vacuum chamber. This insulator prescribes the shape of the initial current sheath between the CE and the back plate of the OE.

Summarizing, parameters of the PF-1000 generator are:

• the charging voltage - U0 = 20 4 40 kV,
• the condenser bank capacitance - C0 = 1.332 mF,
• the initial capacitor bank energy - E0 = 266 4 1064 kJ,
• the nominal inductance - L0 = 8.9 nH,
• the quarter discharge time - T1/4 = 5.4 ms,
• the short-circuit current - ISC = 15 MA,
• the characteristic impedance - Z0 = 2.6 mV,

At present we start to work on 500 kJ to 800 kJ energy levels and, the experimental program concentrates on the training procedure and optimization of operational conditions. Up to now we have designed and put in the operation several diagnostic system.

The equipment for electrical pulses measurement and triggering systems consists of detectors situated on the experimental chamber and multi-channel fast, transient pulses capturing, control and analysis system (Fig.4). To avoid the electrical discharge noises on cables, signals from detectors are transmitted by means of opto - links, and also the main control stand has it�s own electrical feeding source switched on during condenser bank discharge. The geometrical position of detectors is as follows:

Fig.4

• current derivative probe (dI/dt) is mounted inside the outer collector
• PIN1 visible light detector is situated in the diagnostic window and its observation region on the plasma-focus axis is placed at a distance of 20 mm from the central electrode face (Fig.4);
• PIN2 - X-ray detector observes through 100 mm. pinhole (covered with 20 mm Be) the same plasma region as PIN1 from another diagnostic window.

The registration set up of the plasma sheath luminosity was utilized to obtain the main triggering pulse to start all fast plasma diagnostics. In this way in our experiment all detectors were switched on independently of the discharge current peculiarities and its stochasticity. It was also easy to change the triggering point in various discharges or to repeat the same diagnostic conditions in a subsequent discharge (if it was necessary, e.g. when the condenser bank energy was changed).

Three independent streak and frame cameras are used in one shot to observe the plasma-focus region:

• the frame, optical camera QUADRO - which gives two subsequent frames at a time distance of 50 ns
• slow streak camera FENIX II (time base equal to 2 ms);
• fast streak camera IMACON type (time base equal to 300 ns).

Neutron and hard X-ray emission were measured by means of three independent types of detectors:

• four silver activation counters situated spatially at various distances from the source and various angels to the axis of the electrodes;
• two indium activation counters situated along the axis and perpendicular to it, respectively;
• a scintillating probe located at a distance of 15m from the inner electrode face to measure time-resolved signals induced by neutrons and hard X-ray radiation;

The neutron detectors were calibrated using an Am-Be neutron source of an activity of 1.5�107 neutrons/s.