Three-dimensional analysis of microwave generated plasmas with extended planar laser-induced fluorescence

Abstract

We present the development and application of a diagnostic system for the analysis of microwave generated low-pressure plasmas, which might also be used for the investigation of the edge regions in magnetically confined fusion plasmas. Our method uses planar laser-induced fluorescence, which is produced by excitation of neutral metastable atoms through a short, intense, pulsed laser. The beam expansion optics consist of an uncommon setup of four lenses. By controlled shifting of an element of the optics sideways, the location of the laser sheet in the plasma is scanned perpendicular to the excitation plane. Together with a spectrometer observing different observation volumes along the beam path, we are able to map absolute three-dimensional (3D) population density distributions of the metastable ((2)P(12) (o)) 3s[12](0) (o) state of Ne I in an electron cyclotron resonance heating (ECRH) plasma. This optical tomography system was used to study the influence of the microwave power and mode on the spatial structure of the plasma. The results show that the population density of the neutral neon in this metastable state is found to be in the range of 10(16) m(-3), and that its spatial distribution is associated with the 3D structure of the magnetic field. We also report that the spatial distribution strongly varies with the mode structure, which depends on the microwave power.

One- and two-dimensional laser induced fluorescence at oblique incidence

“…Since LIF probes particular quantum states, LIF can also provide measurements of ambient electric and magnetic fields in plasmas [3][4][5][6]. LIF schemes are available for a variety of plasma species [7][8][9]; two-photon absorption, optical-optical double resonance spectroscopy and four wave mixing approaches are available for direct probes of the ground state of many species [10][11][12][13]; planar LIF can provide two-dimensional (2D) measurements of plasma density [14][15][16]; LIF tomography can provide twodimension velocity space measurements [17][18][19] and multiple approaches to time-resolved LIF yield temporal resolutions from milliseconds to microseconds [20][21][22][23].…”

Detecting an extended light source through a lens

“…That is, for f < < 14 cm the central region of V s is beyond the end (z = 30 cm) of the cylindrical volume. By the time = 14 cm, there is enough of V s inside the cylinder to result in these features 3 . As the detector distance increases beyond > 14 cm, the angles of the rays incident on the detector tend towards zero.…”

“…Optical systems of lenses in experimental apparatus are often used to collect light from extended fluorescent sources such as foil-excited atomic beams [1], electronically bombarded molecular gases [2], laser-induced fluorescence from a plasma [3], fluorescence generated by discharge from a plasma [4], or light sources found in teaching laboratories. When considering the placement of the lens-detector combination, a basic question naturally arises: what is the amount, extent and spatial distribution of what the system detects from within the source volume-in short, what does a lens-detector combination 'see'?…”

Similarities and Particularities of Low‐ and High‐Temperature Plasma Technologies

“…The device is shown in Figure 4. For this microwave generated, low temperature plasma source, a new three‐dimensional analysis of the neutral particle density distribution was developed by extending a planar LIF device, which was successfully applied in ref 36. Also, this diagnostic system reflects the similarity with the LIF‐systems applied for the measurements of particle and energy distributions in the boundary layers and divertors for high temperature, fusion plasmas 37–39…”

“… Scheme of the ECRH‐induced microwave plasma generator for plasma diagnostics using emission and absorption spectroscopy, and laser‐induced fluorescence 35, 36…”