Simple Logic Guides Advanced Research
At the Australian National University in Canberra, a Rockwell Automation controller is enabling high-temperature futuristic energy research to proceed with greater ease and flexibility.
Panoramic view of the H-1NF plasma fusion research facility; operational safety is ensured by an Allen-Bradley SLC-5/05.
The quest to achieve sustainable 'fusion' - the very process that creates energy in stars - continues to be a hot issue. Scientists and engineers work with some of the most complex and expensive machines ever devised in their efforts to harness the stellar reaction for humankind. Although there have been many breakthroughs, there is still much work to be done to improve the efficiency of the process. At stake is a technology that has the potential to offer the world an almost unlimited supply of clean energy.
Cooperation is crucial for today's fusion developments. Multi-disciplinary teams across the world, from the United States and Europe to Russia and Japan, are sharing results and collaborating in key areas of research. One particular group that is providing a significant contribution is located at the Australian National University (ANU), in Canberra. The focus for their experiments is a state-of-the-art system that generates tremendous energies and data within a fraction of a second, and a Rockwell Automation small logic controller (SLC) to ensure that the equipment is protected from the enormous 12 megawatts (MW) of electrical energy applied.
Experiments with fields
The principles behind fusion are simple: light-weight elements are combined in a nuclear reaction to form heavier elements. The fusion of hydrogen is especially desirable because the compound is abundant and, unlike the 'fission' or splitting of uranium, the end-product (helium) is benign. Fusion, however, will not occur unless the 'fuel' is heated until it becomes a gas of charged particles - known as a 'plasma' - to many millions of degrees Celsius. Whether researchers are trying to achieve fusion or simply study this plasma, the challenges of attaining such temperatures are formidable and involve contending with serious heat containment and control issues.
Plasma is no ordinary state of matter and ensuring that it behaves properly is at the crux of modern fusion research. Over the years, designers have come up with a number of different machines employing 'magnetic confinement', in which the charged-particles are forced by strong magnetic fields into a prescribed volume of space, preventing them from contacting any surfaces. But confinement is only one part of the story - a controlled reaction requires that attention be given to the stability of the plasma as well as the means of extracting the power, replenishing the fuel and removing the end-products.
Studying the behaviour of plasmas was in mind when, in 1997, the existing 'heliac stellarator' at the ANU, known as 'H-1', was upgraded to a major national research facility, the 'H-1NF'. The facility is run by the Australian Fusion Research Group (AFRG) and is essentially a university-based experiment aimed, not at producing nuclear reactions but rather, at uncovering the fundamental physics underlying plasma flow, confinement and stability.
A look around the facility reveals a vast range of analysis equipment, from electro-optics and CCD cameras, to 'soft' X-ray equipment, and a high-power laser. Although smaller than its counterparts in the US, Europe and Japan, the H-1NF complements global fusion research by engaging in more flexible, exploratory work.
The heart of the facility is an evacuated chamber, inside of which a series of magnetic coils and a central conductor form a 'magnetic bottle', confining the plasma while helically twisting it along a path. Clint Davies, head technical officer at the facility, explains the design: "The plasma can be up to two million degrees. If we were to put it in a glass or metal container, the heat interchange with the walls would cool the gas. So we use a magnetic, rather than a rigid container. By using magnetic fields, the ionised gas conducts electricity and acts like metal; it's like a solenoid electromagnet and we can hold the plasma in position."
Magnetic fields alone, however, will not ionise (charge) the gas nor bring it up to temperature. According to Davies, the H-1NF realises this through resonance heating. "We use a 'gyrotron' to generate the high-frequencies needed to agitate the particles. It's an RF heating system that involves focusing a radio antenna straight into the gas. It uses the same principle as a microwave oven - we pump radio waves in at either low frequency, 7 MHz, or high frequency, at 28 GHz, corresponding to the resonance frequency of the ions and electrons," he says.
Power, pulses and PLCs
While the ultimate goal of fusion research is to generate power, many experiments, including the H-1NF, actually consume a large quantity of electrical energy. The H-1NF is no nuclear reactor, but its operations require power to maintain the necessary magnetic configuration as well as to provide plasma heating. To this end the facility has access to mains power as well as its own motor-generator, via its DC power-supply.
The facility is fitted with hard-wired protection to prevent catastrophic failure but, to ensure that the equipment operates only when safe to do so, an Allen-Bradley SLC-5/05 controller forms a critical link in the control of the power supply. Rockwell Automation distributor, Hi-tech Automation, supplied the controller referred to as 'PLC-1', along with an Allen-Bradley PanelView 600 colour keypad operator interface for adjusting system settings. "The PLC handles all the communications between our main computer and the power supply," says Davies. "It monitors power-levels, interlocks and circuit connections to see if they are valid and correct."
An important part of the stellarator machinery, that resides inside the H-1NF evacuated chamber, is a series of magnetic coils to contain the plasma.
The chief advantage, Davies continues, is that, while the power supply will look after itself in terms of its own interlocks, the PLC-1 identifies external problems before the power is even run. If an operator makes a request that is inherently unsafe, the Rockwell Automation controller will deny them access to the supply. "There are so many variables; if a breaker trips or something blows up, the power-supply doesn't care but the PLC monitors it. It's basically protecting the experiment. You don't want to generate physical forces in the machine that destroy it. If you've got one coil the wrong way around - it knows," he remarks.
Power at the H-1NF facility is provided either in a 'continuous' DC mode or in a 'pulsed' mode. Pulsed mode relies on a huge DC-DC convertor which can provide up to 10MW of power at 14,000 Amps. Creating high-energy plasmas often requires the pulsed mode, but the huge amount of derived heat means the experiment struggles to cool quickly enough. As a result, the PLC-1 will limit the available running time, frequently to less than one second in duration. According to Davies this is not a concern; even in the space of 0.1 seconds of operation, the system will extract many megabytes of useful data.
The success of the small logic controller to the power supply has led to further automation developments at the plasma research facility. Future plans include two additional fibre-optic Ethernet-linked Alled-Bradley SLC-5/05s to monitor temperatures and flow parameters in the H-1NF cooling system, as well as critical items for the motor-generator and transmitters. The future of plasma and fusion research, it would seem, is well and truly under control.