Instrument

P-BACE is a neutral gas mass spectrometer of the time-of-flight type for the analysis of ambient atmospheric gas. It was originally developed to study the Martian atmosphere from a landed spacecraft. The mass range of P-BACE is from 1 to 300 amu (1000 amu), and mass resolution m/Δm is >1000. P-BACE produces a complete mass spectrum every 65 seconds, which actually is a histogram of 65'000 mass spectra recorded during this time interval.

P-BACE sensor head (large image)

The P-BACE instrument consists of a gas inlet system feeding into the sample chamber, a electron impact ion source with an electrostatic pulsing system and an ion acceleration stage, several field-free drift tube sections, a reflectron type ion mirror and a multi-channel plate detector. Several high and low voltage power supplies, front end electronics and a data acquisition, control and data storage system complete the instrument. Time-of-flight is measured from the start pulse in the ion source to the arrival of the ion(s) at the detector. Using the known drift path length, ion energy from the acceleration section, and the charge state, the ion mass is determined. The gas inlet is a tube extending from the balloon by 3.7m, that feeds external gas via a small orifice (10 micro-m) into the sample chamber of the mass spectrometer. The orifice reduces the external pressure of 10 mbar to about 1e-7 mbar inside the instrument. During the flight P-BACE is pumped by an ion pump of 20 l/s. During pre-flight activities P-BACE is pumped by an external vacuum system.

In order to use comercial of the shelf (COTS) components for supporting electronics, P-BACE is mounted inside a pressurized box. Only the gas inlet system and connections for power and communication protrude outside.

Pressurized box containing the P-BACE instrument along with control computer, high voltage supplies, communication equipment and vacuum pump.