Abstract. A magnetic cannon consists of a line of steel balls which are in contact with a permanent magnet. If another steel ball collides with the system on one side, then the ball at the other end of the line is ejected at high speed. A theoretical model is proposed for the system by neglecting energy losses and making the approximation that the magnetic field is uniform near each ball. Our model can account for any magnet shape and is able to accurately predict the most efficient system configuration, as verified by experiments. Additionally, the predicted values of the ejection velocity are in most cases found to be in reasonable agreement with the experiment.

Cryogenic low noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in applications such as radio astronomy, deep space communications, and quantum computing, and the physical mechanisms governing the microwave noise figure are therefore of practical interest. In particular, the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. Here, we report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using a Schottky thermometry method. At temperatures ∼ 20 K, we observe a nonlinear trend of thermal resistance versus power that is consistent with heat dissipation by phonon radiation. Based on this finding, we consider heat transport by phonon radiation at the low-noise bias and liquid helium temperatures and estimate that the thermal noise from the gate is several times larger than previously assumed owing to self-heating. We conclude that without improvements in thermal management, self-heating results in a practical lower limit for microwave noise figure of HEMTs at cryogenic temperatures.

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Accurate measurement of angular positions on the sky requires a well-defined system of reference, something that in practice is realized by the International Celestial Reference Frame (ICRF) with observations of distant (typical redshift ∼1) Active Galactic Nuclei (AGN). At such great distances a subset of these objects exhibit as little as 10−50 μas/year observed parallax or proper motion, thus giving the frame excellent spatial and temporal stability. Until fairly recently the majority of AGN centered imaging was accomplished in the S (2.3 GHz) and X (8.4 GHz) radio frequency bands, however S-band observations for reasons such as sensitivity "plateauing", increased source structure (jets), and radio frequency interference (RFI) have become less productive. Following spacecraft telemetry moves to higher frequencies and a desire to strengthen JPL's leadership in defining the next-generation of celestial reference frames has motivated the development of a "Quad-band" prototype receiver that operates in X, Ku, K, and Ka band in both right hand (RCP) and left hand (LCP) circular polarization. The goal of this receiver is to achieve less than a 20 % increase in noise over the Jansky Very Large Array (JVLA, NRAO) performance specification, which in such a wide bandwidth represents a revolutionary capability. To evaluate the various technical developments of the 8 GHz−40 GHz receiver the feedhorn optical beam was designed to interface to the US based Very Long Baseline Array (VLBA). The receiver's intermediate frequency (IF) spans 4 GHz−8 GHz, giving rise to up to eight 4 GHz IF channels for a fully populated instrument. This paper outlines the technical development of a 2 1 / 2 octave wide (8 GHz−40 GHz) X-Ka band prototype receiver, fulfilling a need for super broadband technology within the VLBI network. An important additional benefit of the wideband receiver approach is its simplicity and low cost of operation.