From Theory to Reality: Success in the Laboratory

In the late 90s, a small group of researchers at MIT and Columbia University started the Levitated Dipole Experiment (LDX) to test if Professor Akira Hasegawa’s predictions would hold true in a laboratory experiment. LDX ran successfully for 10 years, laying the foundations for future dipole research. Soon after LDX was built, the Ring Trap 1 (RT-1) experiment at the University of Tokyo was constructed. Both experiments were successful in demonstrating the properties predicted by Hasegawa and further developed the understanding of dipole plasmas.

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Core Ideas

Two experiments have been built to test Hasegawa’s predictions: LDX & RT-1.
Both experiments demonstrated the fundamental properties of astrophysical magnetospheres also hold true in a laboratory dipole.
- These first experiments also pushed the understanding of dipole plasmas beyond Hasegawa’s original theory.

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In Detail

The LDX and RT-1 experiments demonstrated that key properties of magnetosphere plasmas are also present in a laboratory dipole. Critically, turbulent pinch and high ‘beta’ plasmas.

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Turbulent Pinch

Plasma turbulence - chaotic motions of the charged particles and associated electromagnetic fields - typically degrades performance and cools the plasma, preventing nuclei from fusing together. In other fusion devices, overcoming this turbulence necessitates complex processes and control systems. Professor Akira Hasegawa proposed the dipole would not experience this phenomena but rather benefit from something he described as the “turbulent pinch” effect.^{1, 2}

In simple terms, turbulent pinch means that even though turbulence stirs the plasma, it actually helps concentrate heat and particles towards the centre, rather than causing them to leak out as it does in other configurations. The figure to the side helps explain why this surprising behaviour occurs.

This schematic shows a cross section of a dipole plasma, where highlighted in red and blue are two “flux tubes” – think of these as flexible tubes filled with plasma. Each tube holds the same number of particles, but their volumes are different: the red tube is smaller and denser (high pressure), while the blue tube is larger and more spread out (low pressure). In a dipole magnetic field, these tubes can swap places – a motion called an interchange. The high-pressure red tube expands and moves outward, cooling in the process. At the same time, the low-pressure blue tube is compressed as it moves inward, heating up. This process is an adiabatic interchange, meaning it happens without losing any energy or heat. This allows turbulence to move plasma around without draining energy from the hot, high-pressure core. This is one of the unique advantages of a dipole plasma system.

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High Beta Plasmas

Betas in plasma refer to the ratio of plasma pressure to magnetic pressure – as pressure increases, fusion reactions are more likely to occur. High-beta plasmas are desirable in fusion devices as high pressures can be achieved from relatively weak magnetic fields. In this way, beta can also be thought of as a measure of how efficiently a plasma is contained by a magnetic field. Tokamaks and stellarators typically have beta-values around 5%. LDX demonstrated peak beta-values of 30%, and RT-1 achieved peak betas over 100%.

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¹A.C. Boxer et. al. Nature Physics 6, 207–212 (2010).

‍²H. Saitoh et. al. Journal of Fusion Energy 29, 553–557 (2010).

From Theory to Reality: Success in the Laboratory

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