Two rapidly expanding plasma bubbles collide.
Bringing Stars and Galaxies Down to Earth. Scientists at Imperial College London have developed a new technique to produce in the laboratory centimeter-sized versions of the powerful jets of plasma observed in young stars, active galaxies, and in supernovae explosions. The results provide new insights into the physics of jet formation and open the door to laboratory studies of some complex space phenomena.
The experiment is designed to reproduce the interaction of magnetic field loops with a plasma environment. Coupled with state-of-the-art simulations, the experiments show that the sudden release of energy produces a magnetic bubble inside the hot plasma, enveloped by a relatively thin shock-layer. Magnetic field and pressure distributions confine the magnetic bubble into a cylinder-like structure that grows taller in time: a “magnetic tower.”
Within the bubble, a current-carrying jet appears with the magnetic field lines tightly wrapped around it. The plasma is accelerated to speeds of over half a million miles an hour and compressed to temperatures of a million degrees; then, instabilities in the jet and the “bursting” of the magnetic bubble lead to the break-up of the system.
Surprisingly, it was observed in both experiments and simulations that well collimated “blobs” of plasma are left behind, forming a relatively narrow channel of energy and mass reminiscent of the clumpy structure observed in many astrophysical jets. Additional important physical effects, such as poloidal magnetic fields and rotation have resulted in the first rotating jets ever produced in the laboratory.
Boiling Plasmas. Hot, magnetically confined plasma is much like a boiling pot of water, and the intensity at which the plasma is boiling often determines how well it is confined. The chaotic motions of the “boiling” plasma are called turbulent eddies. Recent collaborative experiments on the DIII-D National Fusion Facility by researchers from the University of California, Los Angeles, University of California, San Diego, University of Wisconsin-Madison, and General Atomics have provided detailed measurements of the turbulence, allowing for the creation of more precise models to predict the performance of future fusion reactors.
One of the most interesting developments in our understanding of plasmas has been the discovery that the turbulence actually “stirs” itself by generating flows which limit the turbulence and its associated transport. This stirring has now been measured for the first time by researchers at the UW-M. Using a technique called beam emission spectroscopy that tracks the movement of turbulent eddies, they were able to infer the corresponding flow fields, similar to inferring wind speed by watching cloud movement. These measurements confirmed many of the predicted properties of these flows and were complemented by measurements in the CSDX experiment at the UCSD.
Recent experiments by a research team from UCLA show that “short” wavelength turbulence clearly exists in tokamaks, that the level of this turbulence increases as the electrons are heated, and that the changes in turbulence level correlate with changes in electron heat transport. By contrast, the larger scale, long wavelength turbulence did not change. Understanding small-scale plasma turbulence may hold the key to controlling heat transport in future fusion power experiments. These new observations were made possible by advanced microwave and far-infrared diagnostic systems developed by the UCLA team.
When Dense Plasmas Collide. Astrophysical processes such as supernovae explosions and pulsar wind outflows are often associated with regions of colliding plasmas. Although we cannot create these extreme conditions in a laboratory experiment, a detailed study of interpenetrating dense plasmas can shed light on some of the processes involved. These include magnetic turbulence and the generation of intense localized magnetic fields, electron beams and ion jets, three-dimensional current systems, and magnetic field line reconnection.
In an ongoing basic plasma study performed in the Large Plasma Device (LAPD) the collisions and their aftermath can be studied in great detail. Two carbon targets immersed in a magnetized helium plasma were simultaneously struck by powerful lasers. The plasma and the laser pulses are highly reproducible. Full three dimensional data was acquired for up to a million experimental shots.
In addition to Alfvén waves, lower hybrid, whistler, and ion acoustic waves were observed, all on different timescales and spatial scales. Basic plasma experiments such as this permit highly detailed space-time exploration of phenomena that are related to astrophysical situations. Dimensionless parameters such the magnetic Reynolds number, normalized expansion speed etc., can be used to scale these. (Often these numbers are estimated in astrophysics and cannot be measured). The scaling is never exact but a great deal can be learned in the laboratory, and this can serve as a guide to astronomers and plasma astrophysicists.
Blowing Magnetic “Bubbles.” Researchers at the Princeton Plasma Physics Laboratory have successfully used Coaxial Helicity Injection (CHI) to generate plasma current at the National Spherical Torus Experiment (NSTX).While the CHI method has previously been studied in smaller experiments, such as the Helicity Injected Tokamak (HIT-II) at the University of Washington, the results from the much larger NSTX demonstrate the exciting potential of this method on a scale much closer to that of a fusion reactor.
The generation of the plasma current by CHI involves a process called magnetic reconnection, which is also involved in the eruption of solar flares on the surface of the sun. In magnetic reconnection, the magnetic “film” is initially attached to the edges of a gap with opposite polarity, like the north and south poles of a magnet. Once adequately stretched, the magnetic field has a tendency to attract and reconnect, leading to the formation of a doughnut-shaped magnetic bubble.
This process of reconnection has been experimentally controlled in NSTX to allow this potentially unstable phenomenon to reorganize the magnetic field lines to form closed, nested magnetic surfaces in the shape of a doughnut carrying a plasma current up to 160,000 Ampères. This is a world record for non-inductive closed-flux current generation, and demonstrates the high current capability of this method.
Surfing Electron Waves. Scientists at the Department of Energy's Lawrence Berkeley National Laboratory, in collaboration with researchers at the University of Oxford, have accelerated electron beams to energies exceeding a billion electron volts (1 GeV) in a distance of just 3.3 centimeters. The researchers anticipate that billion-electron-volt beams from laser wakefield accelerators will open the way to very compact high-energy experiments and superbright free-electron lasers. By comparison, the Stanford Linear Accelerator Center (SLAC) boosts electrons to 50 GeV over a distance of two miles (3.2 kilometers). The Berkeley Lab group and their Oxford collaborators achieved 1/50th of SLAC’s beam energy in just 1/100,000th of SLAC’s length.
The Berkeley Lab and Oxford researchers were able to increase the acceleration length by lowering the plasma density in order to increase the wake speed, and by using a capillary
Images of a wakefield produced by a 30 TW laser pulse in plasma of density 2.7 x 1018 cm-3. The color image is a 3D reconstruction of the oscillations, and the grey-scale is a 2D projection of the same data. These waves show curved wavefronts, an important feature for generating and accelerating electrons that has been predicted, but never before seen.
The Berkeley Lab and Oxford collaborators are now working on injection, the insertion of an already energetic beam into an accelerating cavity, and on staging the handoff of an energetic beam from one capillary to the next and subsequently to others, until very high energy beams are achieved. The researchers believe they can reach 10 GeV with an acceleration structure less than a meter long. These results are essential steps to realizing the potential of laser wakefield accelerators to provide high electron energies over distances much smaller than existing machines.
In addition, other experiments indicate that the duration of the high energy electron beams are tens of femtoseconds in duration, orders of magnitude shorter than existing machines, which will allow unique opportunities in ultrafast science. The waves are the fastest matter waves ever photographed, clocking in at about 99.997% of the speed of light, close to 1 billion miles per hour, and give rise to enormous electric fields, reaching fields higher than 100 gigaelectron volts/meter (GeV/m).
Melting Diamond. Diamond is one of the materials being considered as an ablator material in the design of fuel capsules for inertial confinement fusion (ICF) experiments at the National Ignition Facility. ICF uses high-powered lasers to vaporize a target capsule containing fusion fuel, creating an implosion that compresses the fuel in the capsule to the temperatures and pressures necessary for fusion. Understanding diamond’s shock melting properties is critical to designing capsules and radiation drive pulse-shapes that minimize microstructure effects from mixed solid and liquid phases during this implosion phase.
In most materials this shock melting pressure is a few million times atmospheric pressure. In diamond the shock melting pressure was found to be remarkably high; a shock wave strength of 6-7 Mbar was required to reach the onset of melting in diamond. Furthermore, for shock strengths between about 6 and 10 Mbar the resulting material was a mixture of molten carbon and solid diamond. Shock strengths of greater than 10 million times atmospheric pressure were required to fully melt the diamond upon shock compression.
This 3-4 Mbar coexistence region observed for diamond, the pressure regime where the shocked state lies on the melt line, is extraordinarily large. In comparison, the coexistence region for beryllium, another candidate capsule ablator material, was found to be approximately 0.5 Mbar, with the onset and completion of melt at about 2.1 and 2.6 Mbar, respectively. The high pressures required to achieve complete melting in diamond and the very large coexistence region place significant constraints on the design of ICF capsules with diamond ablators.
“Unsociable” Electrons. It is a common assumption that electrons with different energy in plasma “socialize” due to collisions and form an equilibrium Maxwellian electron velocity distribution function (EVDF). Recent experimental, theoretical and numerical studies revealed that this assumption is, in general, incorrect. In low-pressure discharges, electrons do not have time to “socialize”; they retain their “differences” according to origin and individual life-time experience. As a result, for the first time, it has been demonstrated that the EVDF is noticeably anisotropic.
The “unsociability” of electrons in low-pressure plasma devices makes them a remarkable tool for many applications, because it gives the opportunity to selectively control populations of electrons with different energies. Understanding and controlling the electron population help material processing, high-intensity lighting, electric propulsion, and other such devices.
Metallic Water. Scientists at Sandia National Laboratories have significantly altered the theoretical phase diagram of water at high energy densities. Their computational study shows that an electronically conducting phase of water could occur at a temperature of 4000 K and a pressure of 100 GPa, which is significantly lower than the previous estimates (7000 K and 250 GPa, respectively).
In addition, the superionic phase of water (with frozen oxygen atoms and mobile hydrogen atoms) is demonstrated to extend to higher temperatures than previously concluded and, on a pressure versus temperature phase diagram, directly borders the conducting phase. Importantly, these revisions are in the region of the phase diagram that corresponds to conditions that exist inside giant planets like Neptune and could have significant consequences for planetary models.
The motivation driving the research is the desire to better understand, from basic physics principles, conditions on Sandia’s large pulsed-power Z machine which uses water as both an insulator and a breakdown dielectric for switching. The study significantly expands the range where water’s electrical conductivity is known, enabling more accurate simulations of the extreme environments encountered during operation of the Z accelerator.
The Z machine is currently in the process of being upgraded, a large project which is to be completed in July 2007. With new capacitors, the expected amperage sent through the machine to a target placed at its hub is expected to rise from 20 million to 26 million amps. Compression of Z’s amperage in time is the cause of its huge power–equivalent to 50 times the electrical production of all the generating plants on Earth, albeit for a few nanoseconds.
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Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff