Z-pinch
In fusion power research, the Z-pinch, also known as zeta pinch or Bennett pinch (after Willard Harrison Bennett), is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it (see pinch). These systems were originally referred to simply as "pinch", but the introduction of the theta-pinch concept led to the need for increased clarity. The name refers to the direction of the current in the devices, the Z-axis on a normal mathematical diagram.
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[edit] Physics
The Z-pinch is an application of the Lorentz force, in which a current-carrying conductor in a magnetic field experiences a force. One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. The Z-pinch uses this same effect; the entire plasma can be thought of as many current-carrying wires, all carrying current in the same direction, and they are all pulled toward each other by the Lorentz force, thus the plasma contracts. The contraction is counteracted by the increasing gas pressure of the plasma.
In Z-pinch machines the current is generally provided from a large bank of capacitors and triggered by a spark gap. The power flows into electrodes on either end of a cylindrical vacuum tube. The experimental gas in the tube is pre-ionized to make it electrically conductive. When the current flows into the plasma, it pinches into a smaller cylinder, until the gas pressure and magnetic force balance. As the conductivity of plasma is fairly good, about that of copper, the energy stored in the power source is quickly depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature.
Z-pinch machines contrast with their better studied toroidal counterparts. As the plasma is electrically conductive, a magnetic field nearby will induce a current in it. In practical devices this was normally arranged by placing the plasma vessel inside the core of a transformer, arranged so the plasma itself would be the secondary side. When current was sent into the primary side of the transformer, the magnetic field induced a current into the plasma. As induction requires a changing magnetic field, and the induced current is supposed to run in a single direction in most reactor designs, the current in the transformer has to be increased over time to produce the varying magnetic field. This places a limit on the product of confinement time and magnetic field, for any given source of power.
[edit] Usage
[edit] Early machines
Pinch devices were among the earliest efforts in fusion power. The concept traces itself to work in the UK in the immediate post-war era, but a lack of interest led to little development until early 1950. Then, the announcement of the Huemul Project in early 1951 led to fusion efforts around the world, notably pinch devices in the UK and pinch and stellarators in the US. A number of small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel. The problem became known as the "kink instability".
[edit] Stabilized pinch, the race to fusion
A number of solutions were proposed, and by 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added a set of external magnets that created a toroidal magnetic field inside the chamber. When the device was fired, this field added to the one created by the current in the plasma. The result was that the formerly straight magnetic field was twisted into a helix, which meant that particles attempting to kink outward would soon find themselves on the inside of the vessel, where the outward force was the "proper direction".
Researchers in the UK planned a major assault on the stabilized pinch field, and started construction of ZETA in 1954. ZETA was by far the largest fusion device of its era, and equipped with all of the latest equipment. At the time, almost all fusion research was classified, so progress on ZETA was generally unknown outside the labs working on it. However, in 1956 the walls started to come down, and when they visited ZETA at Harwell, US researchers became aware that they were about to be trumped. A race broke out as teams on both sides of the Atlantic rushed to be the first to complete their stabilized pinch machines.
ZETA won the race, and by the summer of 1957 it was producing bursts of neutrons on every run. Although the scientists working on the device, and similar ones in the US and UK, were careful to point out that it was not proven, the results were nevertheless released with great fanfare as the first successful step on the path to commercial fusion energy. However, further study soon demonstrated that the measurements were misleading, and none of the machines were near fusion levels. Interest in pinch devices faded, although ZETA and its cousin Sceptre would serve for many years as experimental devices.
[edit] Tokamak
Although it remained relatively unknown for years, the Soviets used the pinch concept to develop the tokamak device. Unlike the stabilized pinch devices in the US and UK, the tokamak used considerably more energy in the stabilizing magnets, and much less in the plasma current. This reduced the instabilities due to the large currents in the plasma, and led to great improvements in stability. The results of their experiments were so good that other researchers were skeptical of them when they were first announced in full force in 1968. Members of the still-operational ZETA team were called in to verify the results. The tokamak has since gone on to become the most studied approach to controlled fusion.
[edit] Various Z-pinch machines
They can be found in various institutions such as Cornell University (USA), University of Michigan (USA), Sandia National Laboratories (USA), Nevada Terawatt Facility (USA), Ruhr University (Germany), Imperial College (United Kingdom), École Polytechnique (France), and the Weizmann Institute of Science (Israel).
[edit] See also
[edit] External links
- Z Machine (Sandia Labs)
- An Inertial-Fusion Z-Pinch Power Plant Concept (Sandia Labs)
- Development path for Z-pinch IFE
- "Physics of 'Ocean's Eleven'"
- The MAGPIE project at Imperial College London is used to study wire array Z-pinch implosions.
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