Cold Cathode Ion Gage Experiment

Description

The Cold Cathode Gage Experiment (CCGE, also referred to as the Cold Cathode Ion Gage, or CCIG) was deployed as part of the Apollo Lunar Surface Experiments package (ALSEP) on Apollos 12, 14, and 15. It was designed to measure the density of neutral particles to determine the amount of gas present at the lunar surface. This data helps constrain the source and loss mechanisms of the tenuous lunar atmosphere. A cold cathode ionization gage was used for this purpose. The gage can determine the amount of gas present, expressed as concentrations of particles per cubic cm or pressure in torrs, but not its composition. The CCGE gage head was roughly 18 x 10 x 12 cm and used 6.5 W. The instrument consisted of the cold cathode ionization gage and associated electronics.

The CCGE gage unit was carried in a compartment of the SIDE instrument and was removed and set up on the lunar surface by the deploying astronaut. The gage unit was connected by a wrapped wire cable to the SIDE package, because the CCGE and SIDE electronics comprise an integrated system. On deployment, the stiffness of the cold wrapped cable caused problems for proper location and orientation of the gage head, but it was finally left with the sensor opening generally upward.

The cold cathode ionization gage is a cylindrical sensor unit in a stainless steel envelope consisting of stainless steel coaxial electrodes. The cathode is a spool running through the center axis of the cylinder. A cylindrical anode surrounds the cathode, and both electrodes are in an axial magnetic field of 0.09 Tesla (900 Gauss) produced by a permanent magnet. A magnetic shield is mounted around the gage and its magnet. A potential of +4500 volts was applied to the anode by a power supply consisting of a regulator, convertor, voltage-multiplier network, and feedback network to the low voltage supply. This created a self-sustained Townsend discharge in the gage in which electrons remained largely trapped in the magnetic field with enough energy to ionize any gas particles they would strike. The ions would then be collected at the cathode, producing a current. The cathode was connected to an auto-ranging, auto-zeroing electrometer that measured currents in the range 1.0E-13 to 1.0E-16 amps. A temperature detector was included to enable conversion of the readings to equivalent pressure.

Baffles are mounted in the cylinder between the opening and the electrodes. A dust cover closed but did not seal the opening before deployment and was pulled aside by the squib motor and spring on command. Because the CCGE was not evacuated, adsorbed gases produced an elevated response at turn-on. The gases escaped from the gage rapidly at first and then slowly baked out during the lunar day.

The device was sensitive to gas density rather than pressure. The response varied somewhat with gas composition, but errors due to the uncertainties in composition were expected to be within a factor of two. The data from the experiment have been expressed as equivalent density for a nitrogen lunar atmosphere. The CCGE had three auto-switched overlapping sensitivity ranges enabling detection of the lunar atmosphere from 2.E+5 to 1.E+11 particles/cubic cm (equivalent nitrogen). In the normal operational mode the basic cycle repeated five measurements (separated by 2.4 s), three measurements (separated by 40 s), and 16 seconds of calibration and auto-zeroing every 2.5 min. Temperature and other engineering functions were also sampled within this 2.5-min cycle. In a ground commanded special mode, one measurement was obtained every 2.4 s, with no other measurements being performed. More details are available in "Apollo scientific experiments data handbook," NASA TM X-58131, August 1974 (revised April 1976).

The Apollo 12 ALSEP central station was located at 3.0094 S latitude, 23.4246 W longitude. The CCGE was deployed approximately 15 meters southwest of the central station and 185 meters NW of the Lunar Module. It was turned on 19:18 UT on 19 November 1969. The readings were initially saturated at full scale because of gas trapped within the gage. After about 1 hour the reading was below saturation (4.0E-7 torr) and dropping. The instrument recorded LM depressurization and an astronaut approach and reached a level below 1.0E-8 torr before a catastrophic failure occurred roughly 14 hours after turn-on. Ground tests verified that a transistor failed in the high voltage control circuit causing loss of the 4500-V power supply.