Linear transformer driver
||This article provides insufficient context for those unfamiliar with the subject. (April 2010)|
The linear transformer driver (LTD) is a low-voltage variant of the inductive voltage adder (IVA) technology first applied at Arzamas-16 (now Sarov), Russia and later at the Institute for high-current electronics (IHCE) in Tomsk, Russia. The Hermes III gamma-ray generator at Sandia National Laboratories is another example of an IVA design. In IVA designs, each megavolt “stage” is added in series to the proceeding stage using large iron cores to inductively isolate the individual stages. This concept permits a very large voltage to be generated by adding the voltage from each stage in series. Hermes III delivers a peak voltage of 20 MV to an electron beam loaded to the device.
LTDs use the same IVA concept but use low voltage components. Each individual cavity (similar to an IVA stage) operates at an open circuit voltage typically in the range of 100 kV to 200 kV. There is no high-voltage switching or pulse forming. An LTD cavity is composed of a number of "bricks" connected in parallel. The choice of the brick capacitance, resistance, and inductance defines the base pulse shape of the brick, and therefore the pulse shape and current of the cavity and, finally, the module. There are no further pulse-forming components. The efficiency of energy coupling from the storage capacitors to the load is very high, typically over 60–70%.
Pulse width and pulse shape are simple design choices for LTD generators. By tailoring the capacitors and switches one can generate a wide range of electrical pulse shapes.
Additionally, LTDs provide a range of flexible accelerator designs. Adding bricks in parallel per cavity increases the current per cavity. Adding cavities in series increases the voltage per module. Adding modules in parallel gives the final accelerator current. LTDs have been built with pulse widths as short as ~ 50 ns and as long as ~ 1 µs. LTD generators have achieved voltages greater than 700 kV. LTD drivers have reached currents greater than 1 MA. New LTD drivers are under construction at many laboratories and the performance figures quoted above are subject to change.
Fundamentally, LTDs provide a new flexible & compact pulsed-power architecture. LTDs are particularly suited for high-power low-energy applications such as radiography sources, x-ray generators, and gamma-ray generators.
The simplest electrical description of an LTD cavity is shown in Fig. 1 below.
LTDs at SANDIA NATIONAL LABORATORIES
Sandia National Laboratories in Albuquerque, NM is currently investigating a z-pinch as a possible ignition source for inertial confinement fusion. On its "Z machine", Sandia can achieve dense, high temperature plasmas by firing fast, 100-nanosecond current pulses exceeding 20 million amps through hundreds of tungsten wires with diameters on the order of tens of micrometres. A driver based on the LTD concept is currently being investigated for the next generation of high power accelerators.
Sandia's roadmap includes another future Z machine version called ZN (Z Neutron) to test higher yields in fusion power and automation systems. ZN is planned to give between 20 and 30 MJ of hydrogen fusion power with a shot per hour thanks to Russian Linear Transformer Driver (LTD) replacing the current Marx generators. After 8 to 10 years of operation, ZN would become a transmutation pilot plant capable of a fusion shot every 100 seconds.
The next step planned would be the Z-IFE (Z-inertial fusion energy) test facility, the first true z-pinch driven prototype fusion power plant. It is suggested it would integrate Sandia's latest designs using LTDs. Sandia labs recently proposed a conceptual 1 petawatt (1015 watts) LTD Z-pinch power plant, where the electric discharge would reach 70 million amperes.
- Development and tests of fast 1-MA linear transformer driver stages
Notes and references
- Rapid-fire pulse brings Sandia’s Z method closer to goal of developing high-yield fusion reactor, Sandia's press release (April 27, 2007).
- Z-Inertial Fusion Energy: Power Plant Final Report FY 2006, Sandia Report SAND2006-7148 (October 2006).
- W.A. Stygar et al., Architecture of petawatt-class z-pinch accelerators (October 2007).