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Beaming in on Warm Dense Matter

December 17, 2009
 
Feature

Contact: Paul Preuss

The Neutralized Drift Compression Experiment II (NDCX-II) now under construction at Berkeley Lab will deliver a high-current pulse of lithium ions to a foil target almost simultaneously, momentarily heating it to a state known as warm dense matter. Designing the accelerator to meet these exacting specifications required extensive computer modeling, including the simulations shown here.

The Neutralized Drift Compression Experiment II (NDCX-II) now under construction at Berkeley Lab is an accelerator custom-made to study warm dense matter, a state of matter that’s warm indeed – typically around 10,000 degrees Kelvin – and at the same time so dense it’s almost solid. Found throughout the Universe in places like the cores of giant planets, warm dense matter interests scientists not just for its own sake but also because it’s one of the stages matter passes through on its way to nuclear fusion, notably in the fuel capsules of proposed inertial-fusion power reactors.

To create warm dense matter takes a machine that can deliver a high current (large number) of charged particles to the target almost simultaneously. NDCX-II is designed to do just that. A project of the Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL), a collaboration of Berkeley Lab, Lawrence Livermore National Laboratory, and the Princeton Plasma Physics Laboratory, NDCX-II will accelerate bunches of about 200 billion lithium ions (atoms lacking one or more electrons) to moderate energies of about three and a half million electron volts (3.5 MeV), then compress the pulse so that the entire bunch hits the foil target within a billionth of a second, heating it to warm dense matter.

Neutralized drift compression comes into play during the final steps. Initially the tail of the ion beam is given a higher velocity than its head. But when the beam enters the drift line, which is filled with a plasma of ions and free electrons, it is no longer accelerating, and the tail catches up with the head. Meanwhile the negatively charged electrons in the plasma move and cancel out the electric field induced by the positively charged lithium ions, allowing the beam to be squeezed by strong magnetic fields at the last moment into a dense, millimeter-diameter package that strikes and heats the target.


Designing an accelerator to meet these exacting specifications requires extensive computer modeling. Many potential configurations of the acceleration procedure were explored by William Sharp using a fast, one-dimensional beam simulation code. David Grote later created a series of animated simulations with Warp, a beam simulation program developed by the HIFS-VNL and its collaborators.

“Visualizations have enabled the group to better understand the motion of the beam, especially when it moves off-axis as a result of small misalignments,” says Alex Friedman, head of the HIFS-VNL’s Simulations and Theory Group, of which Sharp and Grote are members. All three are affiliated with Livermore Lab’s Fusion Energy Program, but maintain their principal offices at Berkeley Lab.

Of the two animations on this page, the one at left shows the evolution of the beam in a perfectly aligned accelerator. In the animation at right, the solenoid magnets that confine the beam are assumed to be randomly displaced by up to two millimeters.

Each movie shows the beam emerging from the injector, accelerating, then exiting the accelerator, entering the drift line, and striking the target. The red shapes are the solenoid magnets. The beam tail is accelerated by the last several induction cores, all shown in blue, so that the beam will be compressed in the plasma-neutralized drift line.

Different colors in the beam indicate the kinetic energies of the ions, according to the color bar. For example, the higher energy of the tail of the beam is indicated by color changing from green to yellow to red as the tail overtakes the head. A final eight-tesla solenoid focuses the beam onto the target at the time of minimum pulse duration, heating the target to create warm dense matter.

NDCX-II has received $11 million in funding from the American Recovery and Reinvestment Act. Construction began in July 2009, and completion of the first phase, an initial configuration with 15 induction cells, is anticipated by March 2012.

Additional information
More about warm dense matter and NDCX-II

More about the Warp simulation program

Animations were generated from simulation data using an in-house-developed Python interface to the open-source visualization package OpenDX.


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