Filling the Gap: An Overview of Recent Developments Towards ALMA Band 2+3
With the delivery of ALMA Band 1 the only remaining gap in ALMA's frequency coverage from 30 to 950 GHz is from 67 GHz up to 84 GHz. In the original design for ALMA, this frequency range, Band 2, was to covered by a LNA receiver cartridge, with the remainder of the frequency range in the atmospheric window up to 116 GHz being covered by the Band 3 SIS cartridge. However, since the initial frequency band definitions for ALMA in 1998, there have been significant improvements in LNA design and manufacture. Recent work in Europe, partially funded by ESO, has demonstrated that a single LNA-based ALMA cartridge covering 67 to 116 GHz (Bands 2 and 3) is now feasible. Scientifically, the primary importance of the Band 2 frequency range is in the lines it contains which trace the evolution of cold, quiescent dense gas and the enhanced access it provides to redshifted lines. This presentation will discuss both the key science drivers for access to the 67 GHz to 90 GHz frequency range and the recent technical advances which now allow this to be achieved on ALMA with a single LNA-based, Band 2+3 cartridge.