Cavity ring-down spectroscopy (CRDS) works on the principle of absorption of laser radiation of a given wavelength by atoms and molecules in plasma. The laser beam generated in a tunable dye laser, which is excited by a pulsed pump laser, is fed into a cavity resonator consisting of two nearly impermeable spherical mirrors placed coaxially opposite each other. In this cavity, the laser beam undergoes multiple reflections, which significantly lengthens its path through the absorbing plasma environment. The absolute concentration of atoms and molecules in the plasma can then be determined from the gradual decrease in radiation intensity in the cavity. In addition to determining the concentrations of ground states, this diagnostic method can also be used to determine trends in the concentration of negative ions or to monitor the occurrence of metastable states.

The CRDS system consists of an InnoLas SpitLight 400 pump laser and a Radiant Dyes NarrowScan tunable dye laser.

InnoLas SpitLight 400 pumping laser

• Radiation source: Nd:YAG rods with pulse pumping
• Emitted wavelengths: 1064 nm (basic), 532 nm, and 355 nm
• Pulse frequency and duration: 10 Hz, 6–7 ns
• Pulse energy: 400 mJ (1064 nm), 300 mJ (532 nm), and 120 mJ (355 nm)
• Beam diameter: 6 mm

Radiant Dyes NarrowScan dye-tunable laser

Configuration:

• Radiation source: Nd:YAG rods with pulse pumping
• Emitted wavelengths: 1064 nm (basic), 532 nm, and 355 nm
• Pulse frequency and duration: 10 Hz, 6–7 ns
• Pulse energy: 400 mJ (1064 nm), 300 mJ (532 nm), and 120 mJ (355 nm)
• Beam diameter: 6 mm

The electrostatic Langmuir probe works on the principle of measuring the electric current passing through a thin wire immersed into the plasma depending on the difference in electric potential between the probe and the surrounding plasma. Unlike optical emission spectroscopy, this is an invasive method. From the measured volt-ampere characteristics, it is then possible to derive important local characteristics of the plasma, such as the concentration of electrons and ions, the temperature of electrons or their distribution function depending on energy, or the floating and plasma potential. These characteristics are important both for a thorough understanding of the physical and chemical processes taking place in the plasma and for the purposes of computer simulations.

Specifications:

• Probe tip: cylindrical, diameter 0.15 mm, length 10 mm
• Voltage range: −200 V to +100 V
• Current range: −100 mA to 1000 mA
• Time resolution: approximately 2.5 µs

Hiden Analytical EQP-6 is an energy and mass analyzer of ions. It allows to measure energy and mass spectra of ions extracted from a plasma source or created from neutral particles by the instrument's ionisation source. The instrument can be used, for example, to measure the composition of discharge plasmas, partial pressure of gases in a vacuum chamber, energies of ions incident on a growing film or to estimate the plasma potential. The instrument allows time-resolved measurements, so it enables the investigation of  transient phenomena.

The instrument is equipped with the following functional parts:

• system for extraction of ions from plasma
• ionisation source (electron energy up to 150 eV)
• electrostatic field analyser for ion energy filtering (range up to 1000 eV, positive or negative ion detection)
• quadrupole mass filter enabling to filter the particles by mass to charge ratio (up to 300 amu)
• ion detector – secondary electron multiplier (SEM)
• sampling orifice shift (100 mm range)
• DC-biasable sampling orifice

One of the suitable tools for plasma diagnostics is optical emission spectroscopy, whose great advantage over other methods is its non-invasiveness – it does not interfere with the process itself. It only detects plasma radiation. The measured spectral intensities provide information about the occupancy of various excited states of atoms and molecules in the plasma, from which important plasma parameters can be determined (e.g., the degree of ionization of atoms and molecules or the degree of molecular dissociation). The main components of an optical spectrometer are a monochromator and radiation intensity detectors.

 

 

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The easily portable OceanOptics HR4000 spectrometer can also be used for rapid analysis of plasma radiation.

 

 

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