Experimental confirmation of the standard magnetorotational instability mechanism with a spring-mass analogue

Abstract

The Magnetorotational Instability (MRI) has long been considered a plausibly ubiquitous mechanism to destabilize otherwise stable Keplerian flows to support radially outward transport of angular momentum. Such an efficient transport process would allow fast accretion in astrophysical objects such as stars and black holes to release copious kinetic energy that powers many of the most luminous sources in the universe. But the standard MRI under a purely vertical magnetic field has heretofore never been directly measured despite numerous efforts over more than a decade. Here we report an unambiguous laboratory demonstration of the spring-mass analogue to the standard MRI by comparing motion of a spring-tethered ball within different rotating flows. The experiment corroborates the theory: efficient outward angular momentum transport manifests only for cases with a weak spring in quasi-Keperian flow. Our experimental method accomplishes this in a new way, thereby connecting solid and fluid mechanics to plasma astrophysics.

From helical to standard magnetorotational instability: predictions for upcoming liquid sodium experiments

“…Originally discovered by Velikhov in 1959 [1], and then "forgotten" for nearly three decades, in 1991 it was "rediscovered" and successfully applied to the long-standing problem of turbulence and angular momentum transport in accretion disks around protostars and black holes [2]. While extensively studied analytically and numerically over the last three decades (see recent reviews [3,4] and references therein), standard form of MRI (SMRI) in a classical setup -rotating cylindrical flow of a conducting fluid threaded by a purely axial magnetic field -where it was originally discovered theoretically, has yet eluded any clear experimental confirmation, despite great efforts and encouraging first results [5][6][7][8]. This is mainly due to the fact that for the onset of SMRI both magnetic Reynolds (Rm) and Lundquist (Lu) numbers should be high enough O (10), which makes dedicated liquid metal experiments challenging, because the very low magnetic Prandtl numbers P m = ν/η = 10 −6 − 10 −5 of strongly resistive liquid metals (ν is viscosity and η resistivity) requires very high Reynolds numbers Re = Rm/P m 10 6 .…”

Persistent mysteries of jet engines, formation, propagation, and particle acceleration: have they been addressed experimentally?

“…There have been efforts to measure the onset of the "standard MRI" (referring to case of initial vertical field) in the laboratory using liquid metals [160], but complications associated with purely fluid effects such as Ekman circulation challenge this interpretation [161]. The mechanical analogue of the standard MRI mechanism has however been demonstrated experimentally [162]. The inductionless low magnetic Reynolds number helical MRI or toroidal field MRI has been measured in liquid metals [163].…”

Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability

“…Unlike other fundamental plasma processes such as Alfvén waves [14][15][16] and magnetic reconnection [17][18][19] which have been detected and studied in space and in the laboratory, SMRI remains unconfirmed long after its proposal 8,20,21 other than its analogs [22][23][24][25] , despite its widespread applications in modeling including recent black hole imaging 26 . Due to its microscopic nature and the limitations of telescope resolution, SMRI cannot be captured by current astronomical observations.…”