Friday, 23 January 2015

Spectroscopy – more than meets the eye

There must be hundreds, if not thousands, of diagnostic techniques used by scientists around the world, but one that I quite admire is spectroscopy. 

Now there are some scientists who would automatically assume I’m talking about mass spectroscopy, which is a valuable tool for measuring the mass of particles in a system, so you can figure out which atoms or molecules are present.   I, however, am talking about the splitting of light into its constituent wavelengths.

There are stories you hear of tailors who can accurately guess your size from one brief glance at you, and other tradesmen who take one quick look, and can see the problem.  For me, the beauty of spectroscopy is that is it essentially just taking a look, all be it with some slightly more fine-tuned instrumentation than the human eye.  Spectroscopy is, on the whole, passive (I’ll come to some examples considered non-passive later):  Light is emitted by a system, and we just put a device in there to collect some of it.  It’s what we can do with this information which I think is rather impressive.
Figure 1 - Spectroscopy principles
How do we use Spectroscopy at JET?

On JET we have spectrometers covering a large range of the electromagnetic spectrum, from infra-red (~1000 nm) through to the visible (400-700nm) down to the UV (300nm), VUV (Vacuum Ultra Violet ~10-200nm)), XUV (<10nm), XRay(<1nm).  Due to the high temperatures inside the JET plasma, most of the visible emission comes from the edge region.  As you move further into the centre of the plasma, it gets hotter, and the wavelengths seen are usually much shorter, until there’s very little spectral emission we can detect.  Spectrometers can actually take a number of guises.  Some have a grating and will separate light over a specific wavelength range, others may just be a narrow-band filter (a few nm) and a detector.

The first and most obvious use for spectroscopy is to identify the atoms and molecules emitting the light.  The differing electronic structures of particles emit light with a unique set of wavelengths, acting as a signature.  In tokamaks like JET or MAST, we know that the plasma is mostly made of deuterium which gives a pinkish colour due to the main spectral lines at 656nm (in the red), 486 (blue) and 434 (violet). However there are a number of other elements that can be seen in the plasma, and spectroscopy allows us to look for the weaker sources of light.
Figure 2 - Hydrogen Balmer spectrum

In some JET plasma pulses, it has been possible to see carbon, beryllium, tungsten, copper, iron, helium, oxygen, nitrogen and neon.   A lot of these appeared due to their presence in the vessel components, and are often monitored for each pulse.  If the JET plasma is experiencing problems, one possibility is that impurities are causing it, so being able to identify the impurity through spectroscopy is very useful.  During a recent failure of a vacuum isolation valve between JET and one of the neutral beam injection (NBI) beamlines, impurities were seen when the NBI plasma heating system was firing.  This was quickly identified as being copper thanks to one of the spectrometers, and although it didn’t identify the source completely, it showed operators the level of the problem. When copper is being put into the plasma by NBI, it means something is getting damaged, and this could cause a water leak from the extensive cooling system on JET.
Figure 3 - Spectra for various elements

Is it really so simple?
Sometimes the impurities present in the plasma aren’t so easy to identify.  If an element has been seen in the plasma previously and has a simple reason for being there, one or two of the stronger lines it emits can be monitored.  If, however there is an impurity problem but none of the usual suspects is showing high levels, things can get a bit more complicated.  When monitoring just a couple of lines, you don’t need to look at the whole wavelength range captured by the spectrometer and recorded on a CCD (Charge coupled device) camera or PDA (Photo-diode array).  You just have a program to analyse the sections of the data that you know the line falls on.   When you don’t know what the problem is, you have to start analysing the whole spectrum by eye, looking for unusual lines that aren’t accounted for by the normally seen impurities.  If you find a line, there is still a lot to do to identify the source of it.  You need to find which materials emit lines at that particular wavelength, and then look for the other lines they would emit, to confirm the identification.  This can be quite a tedious task when you don’t know what you’re looking for, and all the different elements, with all their different ionisation stages can produce a lot of lines throughout the electromagnetic spectrum.

Not quite 10 years ago on JET, there was an impurity problem, again when NBI was running.  This time, however, the copper levels remained unchanged, leaving a number of people scratching their heads.  Eventually, using spectroscopy, the impurity was identified as titanium, which has no business being inside JET!  It was actually found to be from titanium oxide, which is used as a whitening agent in masking tape, a small piece of which had been left on a beamline component, and was slowly being eroded by the neutral beam, sending particles into the JET plasma.

Sometimes trace amounts of impurities are injected into JET on purpose, using either a gas puff, or laser ablation for metallic elements.  Using spectrometers to monitor the locations of these elements helps our understanding of how materials move about in the plasma.

Now, I’ve gone on about the matter of identification of elements a bit longer than I intended, but just wanted to highlight that it’s not easy if you don’t know exactly what you’re looking for.  Now I’m going to talk about the really cool (I only say that slightly ironically) stuff we can get from spectroscopy, in particular on tokamaks.

What else can spectroscopy tell us about plasmas?

The Doppler effect is a pretty well-known effect that if something emitting light is moving fast enough, the wavelength seen will shift red if it’s moving away, blue if it’s moving closer.  This is a pretty key idea in astrophysics and cosmology.  In plasma physics, we have a hot gas emitting light.  This gas will be moving in all directions, so what you see is a broadening of the spectral line, with the width of the line corresponding to the energy and temperature of the emitting material in the plasma.  If your resolution is good enough, and the plasma hot enough, this width can easily be measured, and voila, you have some useful plasma parameters.

Each element has numerous spectral lines associated with it, and the relative intensities of these lines can tell us quite a lot.  If, with computer power, we can accurately model the mechanisms that excite the electrons in an emitting atom, we can often calculate plasma electron temperatures and densities, ionisation and recombination rates, and densities of other species that interact with the emitting species.  The types of mechanism considered will be thermal excitation, collisions with plasma electrons, excitation by light, or collisions with other elements.  Some parameters such as the electron temperature and density can be measured by alternate methods, but most of these will be invasive to some degree, which means we may affect the behaviour of whatever we are measuring.  As I said earlier, the beauty of spectroscopy is that it can be passive. 

Figure 4: Spectrometer on the Small Negative Ion Facility (SNIF) at CCFE. Looks at molecular and atomic hydrogen spectrum.

If we look at the spectrum emitted by a molecule, there are a lot of small lines due to the vibration and rotation of the molecule, and again, line ratios can help find the energy in these.
Figure 5: Example of hydrogen molecular spectrum on SNIF

Similar techniques, along with some very impressive telescopic instrumentation, which just require us to look with our eyes, have given us so much information about what is taking place out in the solar system and the universe beyond, as well as inside a tokamak, such as understanding the power produced by the sun – vital if we want to recreate it!

Figure 6: Example of how Thompson Scattering spectra are used
Not so passive spectroscopy…

Just a quick note on some of the non-passive techniques that use spectroscopy:  In tokamaks these can be grouped into Thompson Scattering, and Charge Exchange Spectroscopy.  The former fires a laser through the plasma, and as the light is scattered by the plasma electrons, it is Doppler shifted.  Analysis of the shifts was one of the first techniques used on a tokamak to obtain the electron temperature.  The intensity of light scattered can also give the density.  Charge Exchange spectroscopy, and some derivatives such as Motional Stark Effect spectroscopy, make use of the interaction of the neutral beam heating particles with the plasma particles.  The light emitted can be analysed to give velocity and rotation information for the plasma.

So, in conclusion, I just want to say that it’s impressive that we can learn so much by essentially just looking. 

Thanks for reading.

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