1. Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Plasma Physics Laboratory and Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08543, USA
2. Present address: Department of Physics and Astronomy, Pomona College, Claremont, California 91711, USA.

Correspondence to: Hantao Ji¹ Correspondence and requests for materials should be addressed to H.J. (Email: hji@pppl.gov).

Abstract

The most efficient energy sources known in the Universe are accretion disks. Those around black holes convert 5–40 per cent of rest-mass energy to radiation. Like water circling a drain, inflowing mass must lose angular momentum, presumably by vigorous turbulence in disks, which are essentially inviscid¹. The origin of the turbulence is unclear. Hot disks of electrically conducting plasma can become turbulent by way of the linear magnetorotational instability². Cool disks, such as the planet-forming disks of protostars, may be too poorly ionized for the magnetorotational instability to occur, and therefore essentially unmagnetized and linearly stable. Nonlinear hydrodynamic instability often occurs in linearly stable flows (for example, pipe flows) at sufficiently large Reynolds numbers. Although planet-forming disks have extreme Reynolds numbers, keplerian rotation enhances their linear hydrodynamic stability, so the question of whether they can be turbulent and thereby transport angular momentum effectively is controversial^{3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}. Here we report a laboratory experiment, demonstrating that non-magnetic quasi-keplerian flows at Reynolds numbers up to millions are essentially steady. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out purely hydrodynamic turbulence, our results indirectly support the magnetorotational instability as the likely cause of turbulence, even in cool disks.

1. Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Plasma Physics Laboratory and Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08543, USA
2. Present address: Department of Physics and Astronomy, Pomona College, Claremont, California 91711, USA.

Correspondence to: Hantao Ji¹ Correspondence and requests for materials should be addressed to H.J. (Email: hji@pppl.gov).

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