A 1 mm interferometer for time and space resolved electron density measurements on pulsed plasmas

摘要

A quasi-optical interferometer operating at the frequency f = 274.2 GHz has been employed for measuring electron densities of pulsed plasmas in a wide range. Line-integrated densities down to nearly 10(14) m(-2) can be measured with temporal and spatial resolutions of 1 mu s and less than 10 mm, respectively. A wavelength of about 1 mm requires a quasi-optical set-up making use of grid and dielectric beam splitters as well as elliptic mirrors for focussing the probing and reference beams. Due to the Michelson-like set-up, the probing beam traverses the plasma twice thus doubling the sensitivity as compared with the Mach - Zehnder configuration. The large difference between the probing and reference paths enables one to shift the phase by varying the frequency in a narrow band. When the detector output signal of the interferometer is recorded thereby shifting the phase stepwise, one obtains a cosine function for each sample time from which the phase shift by the plasma and, thus, the electron density, is deduced. The performance of the interferometer is demonstrated on a pulsed high-density helicon discharge as well as the large-volume plasma in a diffusion chamber connected with the helicon source.