We present a compact current sensor based on a superconducting microwave lumped-element resonator with a nanowire kinetic inductor, operating at 4.2 K. The sensor is suitable for multiplexed readout in GHz range for large-format arrays of cryogenic detectors. The device consists of a lumpedelement resonant circuit, fabricated from a single 4 nm-thick superconducting layer of niobium nitride. Thus, the fabrication and operation is significantly simplified in comparison to state-of-theart current readout approaches. Because the resonant circuit is inductively coupled to the feed line the current to be measured can directly be injected without having the need of an impedance matching circuit, reducing the system complexity. With the proof-of-concept device we measured a current noise floor δI min of 10 pA/Hz 1/2 at 10 kHz. Furthermore, we demonstrate the ability of our sensor to amplify a pulsed response of a superconducting nanowire single-photon detector using a GHz-range carrier for effective frequency-division multiplexing.

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The interaction of a plane, electromagnetic wave with a flat, uniform freespace -plasma interface in a static magnetic held has been co~lsidered for arbitrary angles of incidence. The dispersion relation for the plasn~a is a co~nplex quartic equation which reduces to a q~~a d r a t i c if the static magnetic lielcl and plasma boundary are oriented along any one of the rectangular co-ordinate axes.(These axes need not simultaneously be the same for the plasma and the magnetic -Y~u~~lerical resultsare presented for the attenuation ancl phase c o n s t a~~t s for each of the two possible waves in the plasn~a, for each orientation of the static magnetic held. Data are given for various angles of incidence, plasma properties, and orientations of the static magnetic held relative to the plasma boundary.Inspection of the fields in the plasma reveals some interestingaspects. 111 certain cases, \\.aves which appear to move upward to\vards the plasrna interface exist. Since these xiraves may carry energy into the plasma, they have been referred to as "back\\ard" waves. Totally reflected waves which have both finiteattenuation and finite phase coefficients can also esist in the plasma. These have been termed "modified Sommerfeld" waves.