A variety of diagnostics are used in the edge plasma, the three most common techniques, Langmuir probes, spectroscopic imaging and Thomson scattering are described below.
Langmuir probes
A sufficiently negatively biased surface in contact with the plasma will repel all electrons, positive ions will flow to the surface causing a current jsati that
is independent of the applied voltage. Assuming Ti = Te this saturation
current is given by
jsati = enecs (1.12)
As the bias is increased electrons begin to reach the surface and the net current decreases until it reaches zero at the floating potential Vf when
ji = je. The measured current is given by
I = (ji+ je)A = ji(1 − exp(e(V − Vf)/Te))A (1.13)
where A is the projected area of the probe, V is the bias voltage relative to the plasma and ji, je are the current densities due to electrons and ions.
As the voltage is further increased ions are prevented from reaching the surface and an electron saturation current is reached. By sweeping the bias voltage one can determine the electron temperature and density. This is the single electrode Langmuir probe, double and triple probe configurations are also possible where probes are biased to different levels with respect to each other.
Langmuir probes are cheap and robust and provide one of the most im- portant tokamak diagnostics. Several hundred may be found located in the typical tokamak divertor and they provide valuable information for experi- ment and modelling. Reciprocating probes also allow the use of Langmuir probes inside the SOL, however the probe itself will perturb the plasma and can only be in contact with the plasma for short periods of time.
A pair of Langmuir probes aligned with B and with the collection faces pointing outwards along the field allow one to make an estimate of parallel flow in the SOL. This technique relies on the premise that the ion current will be greater for the probe facing into the plasma flow. The ratio of the
upstream to downstream current being a function of the ratio of the flow velocity to the ion sound speed. Langmuir probes in this configuration are known as Mach probes. Mach probe measurements are however subject to significant errors and care must be taken in their interpretation[43].
Thomson scattering
Several non-invasive methods exist for probing the SOL. One of the most common of these is Thomson scattering, powerful infra-red or visible lasers are fired into the plasma and the light scattered from electrons is detected by spectroscopically filtered cameras. The total intensity of the scattered light is proportional to the plasma density and the electron temperature is calculated from the observed Doppler broadening of the scattered light due to the elec- tron thermal motion. Thomson scattering systems exist on many tokamaks and typically provide electron temperature and density profile measurements at resolutions better than 1cm [44, 45].
Spectroscopic imaging
At the relatively cool temperatures in the SOL plasma emission is predom- inantly in the visible spectrum. This makes spectroscopic investigation of the SOL particularly useful. First the deuterium confinement time can be obtained by measuring the Dα line intensity and assuming that the rate of
recombination is much less than the ionisation rate in the SOL. The effec- tive plasma atomic number Zef f can also be determined by measuring the
background Bremsstrahlung radiation, which is a function of electron temper- ature, density and Zef f. Aside from these two techniques edge line emission
provides valuable information. Charge exchange between plasma ions, impu- rities or heating or diagnostic beams provides a measurement of Ti. Doppler
broadening and shifting of emission lines provides information on Ti, plasma
rotation and SOL flow. Emission from impurities allows spatially resolved measurements of both impurity temperature and density as well as providing information on impurity transport and plasma flow. Helium and lithium are typically used in diagnostic beams while carbon has several strong visible lines, particularly CII at 514nm and CIII at 465nm. The presence of carbon in standard plasmas and the existence of these lines makes carbon a partic- ularly useful tool in studying plasma flow. Modern fast cameras are able to image the emission on microsecond timescales and several experiments have been carried out using impurity injection to probe plasma flow, see section 1.3.2.
Other edge diagnostics
Aside from the diagnostics mentioned above a whole range of options exist for diagnosing the edge plasma. These include bolometer arrays for the mea- surement of radiated power, charge exchange recombination spectroscopy for density and temperature profiles of low Z impurities, reflectometry for edge density profiles and turbulence measurements, interferometry for integrated electron density measurements and divertor pressure gauges. Spectroscopic imaging of Lithium and Helium beams also provide measurements of tem- perature and density
Coherence imaging spectroscopy
Accurate measurements of plasma flow are particularly hard to achieve, Mach probes, already described here in 1.3.1 perturb the plasma and have a large measurement error. A method for passive measurements of plasma flow has been developed on the DIII-D tokamak using coherence imaging [46–48]. This
technique uses an imaging 2 beam interferometer; a fixed delay is introduced between the two beams at all positions on a 2D image of the plasma. An additional delay is then added which varies along one image direction and results in a set of parallel fringes superimposed on the image. Any phase change due to Doppler shifts arising from plasma flow cause a distortion in these fringes and comparison with an un-distorted calibration pattern using FFT techniques yields flow information. It should be noted that the spatial resolution differs between the horizontal and vertical directions, assuming the variable delay is introduced horizontally the vertical resolution is defined by the scale of the fringes which can be many pixels.