Some call it RAMBO—the Rice Advanced Magnet with Broadband Optics—though its builders often omit the name from the papers they send out for peer review. It’s a benchtop rig that combines single-digit-Kelvin temperatures, 30-Tesla pulsed magnetic fields, and spectroscopy in a compact package. The fields it generates dwarf those of common medical magnetic resonance imaging scanners (1.5 to 3 Tesla), and even that of the INUMAC MRI (11.74 Tesla) Tech Talk recently covered.
Rambo was built to shoulder some of the work previously possible only with the massive magnets of the U.S. National High Magnetic Field Laboratory, instruments weighing in at nearly 1000 times RAMBO’s bulk. These include the 32 000-kilogram constant-field 45-Tesla Hybrid in Tallahassee, Fla., and the new, 8200-kg, 100-Tesla pulsed field installation at Los Alamos. Think of Rambo as a lone warrior challenging a regiment.
As they report in the Review of Scientific Instruments, Rice University’s G. Timothy Noe—with principal investigator Junichiro Kono, and colleagues at Rice, Tohoku University in Sendai, Japan, and the Laboratoire National des Champs Magnétique Intenses in Toulouse, France—built RAMBO to study superfluorescence, a spontaneous flash of confined, incoherently excited atoms that suddenly and simultaneously release their color-coordinated photons in a sort of chain reaction of light.
The Kono laboratory’s superfluorescence work currently focuses on properties of condensed matter, including optical properties of exotic materials and magneto plasmas at terahertz frequencies, and the “many-body interactions and Coulomb correlations” of super-cooled semiconductors, whose conductivity becomes very exactly quantized—the quantum Hall effect.
RAMBO was designed with spectroscopy in mind: the benchtop is specifically an optical bench. In addition to the laser and a 9-kilojoule capacitor bank, the device consists of two cryostats (relatively slim cylinders less than three-quarters of a meter long, mounted on a heavy plate that precisely maintains their positions). One cryostat, slightly larger in diameter, holds the magnet, a sliver-and-copper-wire-wrapped, 43-mm-diameter doughnut with an inner bore of 12 mm. The smaller cryostat, cooled by liquid helium down to around 7 Kelvin, extends the sample, mounted on a sapphire “cold-finger” into the maw of the magnet.
The system is designed with two optically clear ports, so that a beam—laser or LED light, depending on the experiment's demands—can pass through the sample into a spectrometer (or other optical device) mounted on the bench outside the cold environment.
After it’s triggered, the magnet takes about 1.9 milliseconds to reach peak field intensity. During this time, the researchers can use the light source in a variety of ways. The Kono lab has tested several configurations, including:
- a time-resolved mode, in which the sample is laser-illuminated several times as the magnetic field ramps up, to produce data for a range of magnetic field strengths;
- a time-integrated mode, which uses a single laser pulse at maximum field strength; and
- a transmission mode, using an 880-nanometer infrared LED.
Using RAMBO, the Kono team has been able to extend their studies of high-density electron-hole plasma in semiconductor quantum wells to encompass higher magnetic field strengths with better time resolution. They see the technique as opening doors to many new kinds of experiments on condensed matter in high magnetic fields.