Editing Plasma diagnostics (section)

==Active spectroscopy==
Active spectroscopic methods stimulate the plasma atoms in some way and observe the result (emission of radiation, absorption of the stimulating light or others).

===Absorption spectroscopy===
By shining through the plasma a laser with a wavelength, tuned to a certain transition of one of the species present in the plasma, the absorption profile of that transition could be obtained. This profile provides information not only for the plasma parameters, that could be obtained from the emission profile, but also for the line-integrated [[number density]] of the absorbing species.

===Beam emission spectroscopy===
A beam of neutral atoms is fired into a plasma. Some atoms are excited by collisions within the plasma and emit radiation. This can be used to probe density fluctuations in a turbulent plasma.

===Charge exchange recombination spectroscopy===
In extremely high-temperature plasmas, such as those found in magnetic fusion experiments, light elements become fully ionized and do not emit line radiation. However, when a beam of neutral atoms is fired into the plasma, a process known as [[charge exchange]] occurs. During charge exchange, electrons from the neutral beam atoms are transferred to the highly energetic plasma ions, leading to the formation of hydrogenic ions. These newly formed ions promptly emit line radiation, which is subsequently analyzed to obtain information about the plasma, including ion density, temperature, and velocity.

One example of this is the Fast-Ion Deuterium-Alpha (FIDA) method employed in tokamaks.<ref>{{Cite journal |last1=Heidbrink |first1=W. W. |last2=Luo |first2=Y. |last3=Muscatello |first3=C. M. |last4=Zhu |first4=Y. |last5=Burrell |first5=K. H. |date=2008 |title=A new fast-ion Dα diagnostic for DIII-D |url=https://pubs.aip.org/rsi/article/79/10/10E520/1070883/A-new-fast-ion-D-diagnostic-for-DIII-Da |journal=Review of Scientific Instruments |language=en |volume=79 |issue=10 |pages=10E520 |doi=10.1063/1.2956828 |pmid=19044502 |issn=0034-6748|url-access=subscription }}</ref><ref>{{Cite journal |last1=Jansen van Vuuren |first1=A. |last2=Geiger |first2=B. |last3=Jacobsen |first3=A. S. |last4=Cavedon |first4=M. |last5=Dux |first5=R. |last6=Köhnlein |first6=H. |last7=ASDEX Upgrade Team |date=2019 |title=An edge fast-ion D-alpha system installed at ASDEX Upgrade |url=https://pubs.aip.org/rsi/article/90/10/103501/360770/An-edge-fast-ion-D-alpha-system-installed-at-ASDEX |journal=Review of Scientific Instruments |language=en |volume=90 |issue=10 |doi=10.1063/1.5121588 |bibcode=2019RScI...90j3501J |issn=0034-6748|hdl=21.11116/0000-0004-CCFD-A |s2cid=209844219 |hdl-access=free }}</ref> In this technique, charge exchange occurs between the neutral beam atoms and the fast [[deuterium]] ions present in the plasma. This method exploits the substantial Doppler shift exhibited by [[Balmer series|Balmer-alpha]] light emitted by the energetic atoms in order to determine the density of the fast ions.<ref>{{Cite journal |last=Heidbrink |first=W. W. |date=2010 |title=Fast-ion Dα measurements of the fast-ion distribution (invited) |url=https://doi.org/10.1063/1.3478739 |journal=Review of Scientific Instruments |volume=81 |issue=10 |pages=10D727 |doi=10.1063/1.3478739 |pmid=21033920 |bibcode=2010RScI...81jD727H |issn=0034-6748}}</ref>

===Laser-induced fluorescence===
{{main|Laser-induced fluorescence}}
Laser-induced fluorescence (LIF) is a spectroscopic technique employed for the investigation of plasma properties by observing the fluorescence emitted when the plasma is stimulated by laser radiation. This method allows for the measurement of plasma parameters such as ion flow, ion temperature, magnetic field strength, and plasma density.<ref>{{Cite journal |last1=Boivin |first1=R. F. |last2=Scime |first2=E. E. |date=2003 |title=Laser induced fluorescence in Ar and He plasmas with a tunable diode laser |journal=Review of Scientific Instruments |language=en |volume=74 |issue=10 |pages=4352–4360 |doi=10.1063/1.1606095 |issn=0034-6748|doi-access=free |bibcode=2003RScI...74.4352B }}</ref> Typically, [[Tunable laser|tunable]] [[dye laser]]s are utilized to carry out these measurements. The pioneering application of LIF in plasma physics occurred in 1975 when researchers used it to measure the ion velocity [[Distribution function (physics)|distribution function]] in an argon plasma.<ref>{{Cite journal |last1=Stern |first1=R. A. |last2=Johnson |first2=J. A. |date=1975 |title=Plasma Ion Diagnostics Using Resonant Fluorescence |url=https://link.aps.org/doi/10.1103/PhysRevLett.34.1548 |journal=Physical Review Letters |language=en |volume=34 |issue=25 |pages=1548–1551 |doi=10.1103/PhysRevLett.34.1548 |bibcode=1975PhRvL..34.1548S |issn=0031-9007|url-access=subscription }}</ref> Various LIF techniques have since been developed, including the one-photon LIF technique and the [[two-photon absorption]] laser-induced fluorescence (TALIF).<ref>{{Cite journal |last1=Amorim |first1=J |last2=Baravian |first2=G |last3=Jolly |first3=J |date=2000 |title=Laser-induced resonance fluorescence as a diagnostic technique in non-thermal equilibrium plasmas |url=https://iopscience.iop.org/article/10.1088/0022-3727/33/9/201 |journal=Journal of Physics D: Applied Physics |volume=33 |issue=9 |pages=R51–R65 |doi=10.1088/0022-3727/33/9/201 |s2cid=250866136 |issn=0022-3727|url-access=subscription }}</ref>

====Two-photon absorption laser-induced fluorescence====
TALIF is a modification of the laser-induced fluorescence technique. In this approach, the upper [[energy level]] is excited through the absorption of two photons, and subsequent fluorescence resulting from the radiative decay of the excited level is observed. TALIF is capable of providing precise measurements of absolute [[ground state]] atomic densities, such as those of hydrogen, oxygen, and nitrogen. However, achieving such precision necessitates appropriate calibration methods, which can be accomplished through titration or a more modern approach involving a comparison with a noble gases.<ref>{{cite journal |last1=Niemi |first1=Kari |title=Niemi, K., V. Schulz-Von Der Gathen, and H. F. Döbele. "Absolute calibration of atomic density measurements by laser-induced fluorescence spectroscopy with two-photon excitation |journal=Journal of Physics D: Applied Physics |date=2001 |doi=10.1088/0022-3727/34/15/312 |s2cid=250805163 |url=https://pure.qub.ac.uk/ws/files/551190/niemi01JPD34_2330.pdf }}</ref>

TALIF also offers insight into the temperature of species within the plasma, apart from atomic densities. However, this requires the use of lasers with a high [[spectral resolution]] to distinguish the Gaussian contribution of temperature broadening against the natural broadening of the two-photon excitation profile and the spectral broadening of the laser itself.

===Photodetachment===

Photodetachment combines Langmuir probe measurements with an incident laser beam. The incident laser beam is optimised, spatially, spectrally, and pulse energy, to detach an electron bound to a negative ion. Langmuir probe measurements are conducted to measure the electron density in two situations, one without the incident laser and one with the incident laser. The increase in the electron density with the incident laser gives the negative ion density.

===Motional Stark effect===
If an atom is moving in a magnetic field, the [[Lorentz force]] will act in opposite directions on the nucleus and the electrons, just as an electric field does. In the frame of reference of the atom, there ''is'' an electric field, even if there is none in the laboratory frame. Consequently, certain lines will be split by the [[Stark effect]]. With an appropriate choice of beam species and velocity and of geometry, this effect can be used to determine the magnetic field in the plasma.