Propositions for Japan participation to an enhanced ALMA project.

Draft by S.Guilloteau

From the December 4, 1999 meeting of the ALMA Liaison Group in Grenoble.

Revised after the February 16, 2000 meeting of the ALG in Mitaka.

This document present a number of possible contribution of Japan to an enhanced ALMA project. The enhancements resulting from the various proposition are quite varied : adding new capabilities (e.g. supra-THz capability), shortening the project duration, increasing the sensitivity, simplifying the maintenance or reducing global cost in some areas. Each proposed contribution is presented in 4 steps : a description, the expected improvement on ALMA, the method proposed to value the contribution, and a short term consequence on Phase I activities.

In addition to the specific items mentionned below, which result in visible improvements, it is of course also expected that Japan shares the basic infrastructure and running costs of ALMA.

1) An addition of a number of 12-m antennas.

Description :

Involvement of Japanese industry is essential for Japan participation in the project.

Japanese industry could build up to 1/3^rd of the total number of 12-m antennas. These antennas would be built to the same or better specifications than the US-European antennas, with a possibly different design, but plug-in compatible in the same stations.

ALMA benefit :

Antennas of similar performances would increase the ALMA sensitivity (or speed). Antennas of better performances would significantly increase the highest frequency capabilities of ALMA.

ALMA value :

Unless antennas performance are significantly increased, it is proposed to evaluate this contribution on an equal value for each antenna, unrelated to its origin.

Phase I implication :

Coordinate antenna specifications and interfaces, including foundation specifications.

2) An addition of a number of smaller, high accuracy, antennas in a compact array.

Description :

Japan could built a compact array of small, but high surface accuracy, antennas.

This array could take the form of e.g. a compact hexagon with 7 antennas of 6 to 8 m in diameter. The outer antennas could be moveable on rails to allow fast reconfiguration in order to tailor the shape of the array to the source declination, to avoid shadowing effects. The antenna mount and receiver cabin could be identical to those of the 12-m antennas, allowing them to move on larger configurations also, and to have the highest compatibility for receiver interfaces. Expected antenna surface accuracy is of the order of 15 microns rms or better.

ALMA benefit :

Such a compact array would enhanced ALMA capabilities for short spacing measurements, specially at the highest frequencies where the 12-m antenna performances and the atmospheric properties make the problem most difficult. It would also allow to operate the 12-m antennas with an under-illumination pattern at the highest frequencies to select a better compromise field-of-view and pointing performance versus sensitivity. The reconfigurable option would open the possibility for ALMA to explore the highest frequencies, perhaps even above 1 THz, with appropriate field of view and angular resolution.

ALMA value :

Assuming the same complement of receivers as the 12-m antennas, we would give each small antenna the same value as a 12-m antenna. The increased complexity of the proposed « quick » repositioning system compensates the savings expected from the smaller dish diameter.

Phase I implication :

Watch out mount and foundation design to allow close packing of 6 to 8 m antennas.

3) A participation to the junction effort

Description :

A new facility for SIS junction production is being developed in Mitaka. This facility could be used for ALMA junction production. Japan is also developing innovative technologies for junctions (e.g. the distributed junction scheme from Dr. Noguchi)

ALMA benefit :

This contribution could alleviate a possible bottleneck in the ALMA project

ALMA value :

This contribution would be evaluated on the basis of US-EU agreement for junction production value.

Phase I impact :

Developments should be integrated in ALMA Phase I, to be incorporated in receiver design and production plans.

4) Fabrication of one/several receiver frequency channels

Description :

Japan could provide e.g. the 490 GHz receivers for ALMA (or other bands).

ALMA benefit :

This could speed up the completion of the ALMA project, and avoid excessive retrofit actions to the receiver packages.

ALMA value :

This contribution would be evaluated on the basis of US-EU agreement for receiver band value.

Phase I impact :

Japan receiver experts should become involved in the Joint Receiver Design activity if they are going to be suppliers of some frequency modules.

5) Cryogenics

Description :

Japan is probably the biggest supplier of cryocoolers, with proven reliability.

ALMA benefit

Basic contribution

ALMA value :

Market prices

Phase I impact :

Implementation of a real scale test at 5000 m is very valuable.

6) Photonics

Description :

NTT is actively developping high frequency photomixers, which may be suitable for the full photonic LO system for ALMA

ALMA benefit :

Photonic LO system is simpler than the photonic reference approach.

ALMA value :

Value based on estimated cost of the photonic reference + multiplier solution and purely photonic approach, whichever is the most expensive. The gain in simplicity justifies extra cost (if any).

Phase I impact :

Evaluation of photodetectors needs to be performed actively. Good liaison between the Tucson group and NAOJ is important here.

7) Correlator

Description :

Japan is developping a wideband, 128 000 channels FX correlator. The goal is ultimately to cover the full 2 GHz bandwidth (perhaps even 4 GHz) with this number of channels. This removes the extra complexity of input filtering (analog or digital).

ALMA benefit :

Line surveys could be carried out more effectively. Serendipitous discoveries are to be expected (e.g. molecular masers in stars). Multiple-line observation and continuum subtraction from narrow lines could be executed more precisely and effectively. Heavy molecules could be detected by pattern-matching integration of the line forests.

ALMA value :

The value should be based on the observing time savings that such a correlator will provide for the astronomy projects requiring narrow lines observations. The induced computing cost resulting from the huge number of channels should be evaluated and accounted accordingly. The effective sensitivity should also be asserted.

Phase I impact :

Invite Japanese expert to correlator PDR in January. Develop scientific evaluation based on typical observing scenarios.

8) More digital bandwidth

Description :

Japan could provide enhanced transmission bandwidth from antenna to central building, and from central building to Operation Support Facility in San Pedro.

ALMA benefit :

Current nominal bandwidth of standard fibers does not allow 3-bit samples to be transferred on single fiber from the antenna. This would become possible. Ultra high speed link with OSF would allow to relocate the correlator and all its associated computing equipment in San Pedro.

ALMA value :

Based on cost.

Phase I impact :

Maintain contacts for information.

9) Other proposals mentionned

A few other options were discussed, among which a large single-dish antenna, a super-computer for enhanced data mining capabilities, etc… These were felt impracticable, or to far from the scope of an « Enhanced ALMA » project.