HARP: Mixer Bias Performance Investigation and Mixer Spares Production

'HARP' is a 16-element 350 GHz focal plane array receiver that was delivered to the JCMT in late 2005. HARP was originally funded largely by STFC in the form of its contribution to the JCMT instrument development programme, which covered the capital elements, plus, to cover the staff effort, research grants to Cavendish Astrophysics as main contractor (the Group making this application) and the funding of some sub-contract development work at the UK-ATC. HARP is the only array of its kind operating in the world at sub-millimetre wavelengths and has a performance that is state of the art, allowing large scale mapping of the sky at hitherto unheard of rates. It has been formally accepted as a facility instrument by the JCMT and is now routinely producing data for many diverse observational programmes. In the period since installation, commissioning and acceptance of the instrument (and hence closure of the original Cavendish Astrophysics grant) a number of relatively minor issues have arisen in relation to the ongoing operation and maintenance of the instrument on the JCMT that require attention. This application proposes a programme of work to address these at a modest cost. The bulk of the funding requested is to cover the costs of building four new HARP mixer blocks to act as 'spares' and repairing three original mixer blocks that were damaged or have become faulty during the commissioning process. Funding is also requested to enable the investigators to undertake a study into the nature and cause of spurious oscillations that have been found to occur at some d.c. bias conditions of the HARP mixers. This would be done by setting up a near identical experiment mixer/LO/IF system to that of HARP in the Astrophysics Group laboratories. The goal would be to attempt to understand the issues and hence suggest improvements to the HARP bias electronics system that would allow the superconducting junctions to be biased at d.c. levels that are currently not viable. It is believed that these will allow lower receiver noise temperatures to be achieved and hence greater sensitivity.