Background
Cryocoolers have been used for a variety of space, military, medical, semiconductor fabrication, and superconducting electronics applications. For example, cryocoolers are used to cool infrared (IR) sensors in satellites (military), superconducting magnet coils in MRI systems (medical), and astronomical detectors (space). Another application in space exploration is the cryotrapping of trace level volatile gases followed by desorption of said gases for increasing sensitivity and improving the probability of detection.
The MAss Spectrometer for Planetary EXploration (MASPEX) on the Europa Clipper mission (hereafter referred to as MASPEX-Europa) is equipped with a cryocooler to cryotrap gases during the course of a Europa flyby. After the flyby, MASPEX-Europa is sealed, meaning the valve is closed, and the cryocooler is turned off to desorb the cryotrapped gases and ionize and mass analyze the gaseous components. However, major differences in concentration and composition of gases from the percent level to parts per billion (ppb) levels and mass spectral overlap of potential components complicate this type of measurement. This project aims to investigate the use of thermal desorption and/or laser desorption for selective, or step-wise, desorption of cryotrapped species liberated for mass spectral analysis, thereby improving sensitivity and limits of detection of all volatiles.
Approach
Figure 1: Photograph of the components mounted on the cryocooler cold finger for the thermal desorption experiments. The components are the adsorber, the temperature sensors, and the heating resistors.
To test the capability of the cryocooler for selective, or step-wise, desorption a high surface area Mott filter, referred to as the adsorber, is attached to the cryocooler coldfinger for large trapping capacity. After the adsorber head, carbon resistors are mounted to resistively heat the adsorber head for thermal desorption experiments. The cryocooler experimental setup is incorporated into a vacuum chamber housing a mass spectrometer for mass spectral analysis of the desorbed components. A close-up photograph of the components mounted on the cryocooler are displayed in Figure 1.
The mass spectrometer employed for these experiments is a time-of-flight (TOF) mass spectrometer referred to as the W-TOF-MS for the W-shaped flight path, or trajectory, that the ions take in a folded ion optics, dual reflectron design. This W-TOF-MS is being developed under NASA’s Heliophysics Technology and Instrument Development for Science (HTIDS) program and is used in this IR effort to demonstrate the applicability of the cryocooler to future instruments and potential space missions beyond MASPEX-Europa. The TOF MS is essential with its fast repetition rate and collection of the entire mass spectrum with each extraction (Fellgett Advantage) to enable the detection of the rapidly changing desorbed gas composition.
For testing of laser desorption of cryotrapped species, a separate plate (surface) will be designed for cryotrapping volatile species followed by laser irradiation of the cryotrapped species to investigate selective desorption of species based on the laser wavelength employed.
Figure 2: Overlay of the individual alkane desorption profiles from pentane to decane.
Accomplishments
Over the first quarter of this project, a number of accomplishments have been achieved and are listed below:
- Incorporation of all components on the cryocooler, including the adsorber, temperature sensors, and heating resistors.
- Temperature calibration of the temperature sensors
- Data obtained for cryotrapping and desorption of alkanes (pentane to decane)
- Preliminary resolution values and temperature of desorbed alkanes
- Preliminary linearity of response experiments for varying concentration of pentane
Cryotrapping and desorption of a series of alkanes (pentane to decane) are displayed in Figure 2. The thermal desorption experiment shows resolution of the alkanes and that step-wise desorption of components is possible from controlled heating of the adsorber. Further investigation of the selected ion current of targeted m/z ions representative of individual alkanes from the mass spectral data provided by the W-TOF-MS confirm the step-wise desorption and resolution of alkanes (Figure 3). Preliminary linearity of response has been established from varying concentrations (pressure) of pentane being cryotrapped and subsequently desorbed from the adsorber with the data presented in Figure 4.
Figure 3: Selected ion currents, or targeted m/z values, representing pentane (m/z = 72, red trace) and hexane (m/z = 86, green trace) along with overall chamber pressure profile from desorption of pentane and hexane.
Figure 4: Preliminary linearity of response data for pentane obtained by integrating the area of the pentane desorption curve – pressure profile of the evolved gas from the adsorber.