PRAXIS Application for AL50-S-WCCL Laser
While many nations don’t have large space programs, most economic zones participate in a centralized, multi-national group for launching and maintaining satellites (such as the European Space Agency). These near-earth satellites, and especially deep space probes (Deep Space 1 – NASA & SMART 1 – ESA), heavily rely on plasma thrusters for navigation during their missions. One of the best tools available for evaluating the efficacy of these thrusters is a measurement known as PRAXIS, or PRopulsion Analysis eXperiments via Infrared Scattering.
This technique measures electron density fluctuations by a phase shifted interferometer. When an electromagnetic wave propagates in a plasma discharge, it undergoes a phase shift given by:
where φ is the phase shift, λo is the wavelength of the radiation and Ne(Z) is the chord dependent electron density. According to the formula above, the phase shift is proportional to the wavelength of the radiation used and the chord-averaged electron density. However, since the signal obtained is proportional to the cosine of φ, it cannot be determined whether φ is increasing or decreasing.
This can be addressed by splitting the signal with an acousto-optic modulator (AOM) as the first-order beam acts as the local oscillator leg of the interferometer, while the zero-order beam is frequency shifted before it propagates through the plasma where its phase modulated by an angle φ. This inspection beam is then combined with the local oscillator, illuminating a detector, which after some signal processing provides two signals, one proportional to the sine of φ and the other proportional to the cosine of φ. Thus, an unambiguous calculation of the phase shift φ can be made. Knowing φ, the chord-averaged electron density can be calculated. However, to calculate the radially dependent electron density, a transformation must be applied which requires information from several chord paths, thus a multi-channel interferometer should be used. Since the phase shift, φ, is directly proportional to the wavelength of the radiation used, longer wavelengths will produce larger φ values, thus making this phase shift easier to detect. Due to this relationship CO2 and FIR THz sources are preferentially selected.
The figure below shows typical spectra obtained from the PRAXIS experiment, after FT signal averaging (left) with plasma scattered signal in blue. The experimental dispersion relation of the instability is (right), showing the variation in peak frequency as a function of the observation length.
Since the laser is passing through an AOM before being split into several channels, a source is needed that combines very high frequency stability, good power stability, high mode quality and enough CW power (~40W) for each chord to have a high signal to-noise ratio. The AL50-S-WCCL, available from Access Laser, is a suitable candidate for this application which generally requires the AOM to be downstream from the laser. However, the AOM option can of course be investigated or simply supplied in addition to the laser.
To request more information or a quotation for this or other Access Laser products, contact IL Photonics.