An experimental microwave amplifier delivered more than 10 decibels of gain in magnetic fields above 1 tesla when it was placed nearly parallel to the field. A second device, oriented perpendicular to the field, remained resilient at about 5 to 10 millitesla; by 20 millitesla, nearly all the settings explored produced no net gain. Those sharply different results are the headline finding in an arXiv version 1 preprint dated 26 August 2026.
A sharp split by orientation
The study examines how KTWPAs perform under harsh magnetic fields and how field strength, orientation, bias current and pump settings affect operation. It compares a perpendicular KTWPA with a parallel KTWPA relative to the bore's vertical magnetic field. The perpendicular device was explored from 0 to 35 mT, with bias currents of 120 to 320 microamps. The parallel device was explored from 0 to 1.5 T, with currents of 260 to 350 microamps. Pump frequencies ranged from 12.5 to 13.5 GHz for the perpendicular device and 13.9 to 14.6 GHz for the parallel device. Both used 21 to 23 dBm of pump power.
Measurements used three-wave mixing with bias-tees, directional couplers and filters. The two orientations were set by mechanical assembly, without in-situ magnetic orientation sensors. Critical current was estimated from discontinuities in direct-current current-voltage responses. Microwave transmission was modeled with a heuristic sum of two sigmoid, or S-shaped, curves.
From millitesla to tesla
Critical current showed a weak low-field enhancement followed by a monotonic decline at higher fields.
In the perpendicular setup, the device was resilient to about 5 to 10 mT, after which gain dropped significantly. By 20 mT, nearly all explored settings produced no net gain.
By contrast, the near-parallel configuration achieved gain above 10 dB in fields over 1 T. The abstract reports peak gain at 0.25 to 0.5 T. The manuscript also contains an internal unit inconsistency elsewhere, where the same numeric range is later given in millitesla, so the peak-field figure should be read with caution.
The amplifier survived repeated field exposure
After multiple magnet ramps up to 2.5 T, the devices still provided gain above 20 dB, without thermal quenching to expel trapped magnetic flux. Field exposure did, however, move the preferred operating point: increasing the field shifted the optimal pump frequency higher. At 0.5 T, the reported operation required approximately 400 MHz of tuning adjustment.
The tuning moved with the field
The researchers also examined kinetic inductance, a parameter linked to the amplifier's electrical response. Measurements and calculations bounded its change at no more than 2.5 pH per square or less than 8 percent.
A proposed explanation for the lost gain
The authors propose that field-related microwave dissipation consistent with vortex depinning, meaning magnetic vortices are no longer held in place, counteracts amplification, lowering peak gain while roughly preserving the gain-profile shape.
What comes next
The work is an arXiv version 1 preprint dated 26 August 2026, based on one perpendicular device and one parallel device. The orientations were set by mechanical assembly without in-situ magnetic orientation sensors. At manuscript time, a follow-up study was underway to test the design in axion-haloscope applications, including system gain and noise under repeated multi-tesla ramps and trapped-flux quenches.
The acknowledgments identify primary support from the U.S. Department of Energy Office of Science Office of High Energy Physics, with additional support from NIST, NASA and DOE research programs.
Paper data and sources
Original title: Operation of Unshielded Kinetic-Inductance Traveling-Wave Parametric Amplifiers in Multi-Tesla Fields
Authors: Christian Boutan, Erik Lentz, Corwin Shiu et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text