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Diagnostics

Lidar - Thomson Scattering

 

Focus On : Lidar-Thomson Scattering

 

The LIDAR-Thomson Scattering diagnostic (Fig. 1) measures the plasma electron temperature and density. In the JET plasma the temperature of the electrons can range from about 200 eV (2 million degrees) near the edge to over 10 keV (100 million degrees) in the centre. One way to measure such high temperatures is to shine an intense laser pulse into the plasma and to detect the back-scattered light from the electrons (light scattered from the ions can usually be ignored because the mass of a proton is around 2000 times that of an electron).

This diagnostic has the advantage that it is non-perturbative but it is technically challenging to implement and operate. At JET we run two such LIDAR diagnostics - the "Core" system looks at the bulk of the plasma, and the "Divertor" system looks at the edge plasma.

The monochromatic laser light is scattered and doppler shifted by the fast moving plasma electrons producing a broad spectrum of scattered light (Fig. 2).

By measuring the width of this scattered spectrum the velocity distribution and hence the electron temperature (Te) can be determined and by measuring the total intensity of the scattered light the density of the electrons (ne) can be deduced. This is the basis of the Thomson scattering technique.

At JET we also want to know how the temperature and density vary across the plasma. To get this information we send a short laser pulse (0.3 nano-seconds duration which, at the speed of light, is only 10 cm long) across the plasma diameter. By using a fast detection and recording system, we can observe its progress by capturing the changes in the back-scattered spectrum. We can then analyse these changes as the pulse passes from the relatively cool edge, through the hot core and out again through the opposite plasma edge. Since we know by the time of flight, or LIDAR, principle where the laser pulse is in the plasma at each instant, we can compute from the instantaneous scattered spectrum the local values of temperature and density in the plasma, ie. from the time of flight of one laser pulse through the plasma we can obtain the temperature and density variations across the whole diameter.

This is the basis of the Core LIDAR-Thomson scattering diagnostic which we have developed at JET. We use a 1J ruby laser (wavelength 694nm) as the light source pulsed at four times a second, and for detection we have six microchannel plate photomultipliers (rise time 0.3ns) each connected to a type of fast storage oscilloscope (1 GHz Bandwidth). The scattered spectrum is dispersed into the six detection channels using a set of dielectric edge filters which act together to make a high throughput spectrometer. All the sensitive equipment is located outside the biological shield in the JET roof laboratory, only the large and relatively simple input and collection optics assembly is required to be located next to the machine in the torus hall.

The result obtained by firing the laser several times during a JET plasma pulse is shown in Figure 3. The changes in the temperature and density profiles due to 18MW of Neutral Beam heating are clearly seen.

A second LIDAR system, the Divertor diagnostic, operates on the same principle but has a 3J laser with a pulse repetition rate of 1Hz. It uses four photomultipliers and detection channels.

schematic

Fig. 1  Schematic of JET's Lidar-Thomson Scattering Diagnostic

 

 

 

 

photo of a JET plasma taken with an IR camera

Fig. 2  Input and Scattered Spectra

 

 

 

 

diagram showing the cross-section of plasmas with limiters and divertors

Fig. 3  Temperature and Density profiles

   

Author : CW Gowers

Photo of Chris Gowers

Notes :

LIDAR = LIght Detection And Ranging - similar to radar but using laser light instead of radio waves

Thomson Scattering = the classical theory of the scattering of electromagnetic radiation by a charged particle was developed by Joseph John Thomson (1856-1940) - an English physicist generally credited as the discoverer of the electron

Doppler Shift = A shift in the observed frequency of a wave (electromagnetic or sound) which occurs when the source and observer are in motion relative to each other. The frequency increases when the source and observer approach each other and decreases when they move apart.