The “Junior” experiment, built by OpenStar Technologies, is a new generation of dipole confined plasma experiment which builds on the work of LDX at MIT [1] and RT-1 [2] at the University of Tokyo. The Junior experiment is the first levitated dipole to use a core magnet made from 2nd generation REBCO HTS which also has an onboard superconducting power supply housed on board the core magnet. This power supply (known as a “flux pump” [3]) is used to maintain current during levitation with very low heat leak into the magnet.

The Junior facility is an attractive platform for the investigation of fundamental plasma physics phenomena including, but not limited to: multi-scale plasma turbulence and energy cascades, self-organization phenomena, high-β (β>1) plasma stability regimes, “artificial radiation belt” formation, non-linear Alfvén wave dynamics, wave-particle and wave-wave interactions in magnetospheric geometries, the effect of anisotropy on stability and confinement, energetic particle dynamics with hot electron interchanges modes.

Details:

The Junior levitated dipole experiment consists of a core magnet housed in a 5.2 m diameter vacuum vessel and levitated by an additional magnet on top of the vessel.

Core magnet: A 5.7 T non-insulated REBCO HTS dipole with an inner bore of 420 mm and 0.38 Wb of wall limited flux.

Top magnet: A 0.45 T wax-impregnated REBCO double pancake.

Diagnostic equipment:

·      Four chord microwave interferometer provides line integrated density measurements.

·      UV-vis spectrometer (Avantes, Avaspec-2048).

·      Video cameras (2448px * 2048px@106FPS horizontal view and 4096px * 3000px@44FPS view from top of chamber).

·      Twelve magnetic flux loops (circular planar and concentric with the magnetic axis). Eight loops external to the vessel capture large flux with bandwidth <500 Hz. Four internal to the vessel capture smaller flux with higher (2 kHz) bandwidth.

·      Sodium iodide detector provides integrated x-ray intensity (Bicron LA-1378).

·      X-ray pulse height analyzers: two cadmium zinc telluride measure between 10 keV and 60 keV (eV Products SPEAR), one silicon drift detector measures below 20 keV (AmpTek XR-100SDD).

·      Langmuir probes (being commissioned): edge mounted triple probe and a height adjustable array of 24 probes equally spaced azimuthally at a radius of 1 m and subtending a 90o angle (eight ion saturation, sixteen floating potential).

Computing and networking: A network of Linux machines capture data from various devices and PLC units on an EtherCat network and store the data in MDSplus.

Heating sources: Up to 43 kW of microwave power available for electron cyclotron resonance heating with tunable power outputs all launched by Vlasov antennae in X-mode at the midplane and the plasma is cavity heated.

·      Two 15 kW magnetrons at 2.45 GHz (Rell GEN15KW2I400-50-0)

·      One 3 kW klystron at 6.4 GHz (Varian VA-936R12) (being commissioned)

·      One 10 kW klystron at 10.5 GHz (Varian VA-911 P) (being commissioned)

Additional diagnostics: The large unobstructed surface area of the vessel makes adding additional diagnostics and sources for launching waves relatively straight forward. We welcome collaborators to develop their own diagnostics not included in the list above to be used on the experiment.

Data access:

A collaboration agreement would include open data access through MDSplus (available remotely). Data from the LDX experiment is also available through MDSplus.

[1] A. C. Boxer et al., “Turbulent inward pinch of plasma confined by a levitated dipole magnet,” Nature Phys, vol. 6, no. 3, pp. 207–212, Mar. 2010, doi: 10.1038/nphys1510.

[2] H. Saitoh et al., “High-beta plasma formation and observation of peaked density profile in RT-1,” Nucl. Fusion, vol. 51, no. 6, p. 063034, May 2011, doi: 10.1088/0029-5515/51/6/063034.

[3] J. Geng, C. W. Bumby, and R. A. Badcock, “Maximising the current output from a self-switching kA-class rectifier flux pump,” Supercond. Sci. Technol., vol. 33, no. 4, p. 045005, Apr. 2020, doi: 10.1088/1361-6668/ab6957.