 Hello everybody and welcome to video number 35 of the online version of the fusion research lecture We are on chapter 6 turbulent transport and you might remember that in the last video we talked about Actually, we talked about the turbulent transport and in this video We will talk about turbulence measurements as this is a very important topic in fusion so turbulence Turbulence measurements Is the topic of this video turbulence measurement? Measurement and this is actually a field of active research this is a field of active or very active research and Just to give a bit of an historical introduction we come to the picture on the right in a minute the first proper turbulence measurements were conducted not before the 1980s really rather late. So the first proper turbulence Measurements Oops Measurements Sorry measurements We're performed in the 1980s in In small talker mugs and what was found was basically a broadband turbulence spectra So a broad band Turbulence spectra was found meaning that we have Components with different size over a large range of sizes being present in a plasma and It was also clear then that the turbulence was responsible for the observed transport so turbulence is responsible for the observed transport and The picture on the right-hand side actually is is a spectra of Turbulent density and potential fluctuation. This is a spectra measured in the TJK Stelerator and you can here nicely see in the spectra how the Turr how the dual how a dual cascade is found. So what was basically what I said Will is going to happen for a two-dimensional turbulent system that we have a dual cascade that we have here the Inverse energy cascade here and that here we have the direct endstrophy cascade with with different Slopes This is actually a measurement. So this was found. This was measured with lung reprobes in the in the TJK Stelerator and low-temperature Stelerator So this is a nice Example where the experiment a nice example for a low-temperature device being able to Reproduce what was predicted by by the theoretical expectations The density fluctuations coming back to the measurements the density fluctuations are up to a few 10% So the density variations They are up to It can be even up to a few 10% at the plasma boundary Up to a few percent more on the plasma center So it really depends where you are but the density the amplitude of the variations can be really large and You might have seen that already in the picture the Boltzmann relation is fulfilled. So the Boltzmann relation is fulfilled and the Boltzmann relation basically says that the Density the normalized density variations is approximately given approximately equal to the normalized potential variation And this is what is also called that there is an adiabatic response So this is I'm also called Adiabatic response Okay, another example or another important result is and We already had that in I think it was the last video that the amplitude of the density fluctuations scale basically with the gradient of the plasma, which is not surprising and the size of the Density structure the size of the vortices where ln here is The inverse gradient length over and not and This corresponds To the so-called mixing length model. So this is corresponding to the mixing length Model and the universality of that law so that the Fluctuation amplitude scales inversely with the gradient This is shown in the graph on the right-hand side where you can see here the amplitude of the density variations Fluctuations as a function of the inverse structure size of the product of the size of the structures times the inverse density gradient length, sorry And you can see how there is a number of variety of experiments being arrived and being Arranged around that line of a slope one here We have Stellarators and tokamaks there. So this really and and spending like three orders of magnitude. So this is really Universal law So in addition it was found that turbulent structures are strongly elongated along the magnetic field line So it was found that turbulent Structures are strongly elongated along the magnetic field This was found or can be made visible for example when you Make a gas a gas puff at the edge and then look at the Resighting H alpha emission that is a typical experiment for that So looking a gas puff at the edge and then looking at the H alpha emission What does that mean? This means that the perpendicular structure size is usually on the order of centimeters Whereas the parallel structure size is usually on the order of meters. That's so strongly elongated along the magnetic field line Now a very useful tool for turbulence measurements are Langmuir probes But Langmuir probes can only be used in low-temperature devices. So Langmuir probes can only be used in low temperature devices or When or when you take extremely care only in the very edge of hot fusion plasmas when you like have very fast reciprocating reciprocating probes that might also be possible in principle only in low-temperature devices And this is why these low-temperature devices are often which these these are also often referred to as wind tunnel experiments Wind tunnel experiments where you have a reduced set of plasma parameters Reduced set of plasma Parameters for example the density being reduced by a factor of hundred and by reduced I mean as compared to a fusion to an actual fusion plasma the temperature Being reduced by a factor of ten where the factor of ten here refers to the edge of the fusion plasma Come so yeah to the edge of the fusion plasma the magnetic field Being reduced by a factor of 30 or something like that Despite the reduced the set of plasma parameters. There are some similar dimensionless parameters. So there are some similar Dimension less parameters and These are for example the the plasma beta which can be approximated The dimensionless parameter by n times the temperatures of the plasma pressure over the square magnetic field and also the New which is here given by the density over the temperature to the power of three half Then the characteristic scales these are usually somewhat larger So the character characteristic or some characteristic scales Are often somewhat larger and An important characteristic here to describe the turbulence is row s row s Which is the sound speed Well the sound speed CS Over the iron cyclotron frequency Corresponding approximately to the electron temperature or the square root of the electron temperature over the magnetic field Defining roughly the size of the turbulent structures And The advantage of Langmuir probes is that they offer very good spatial and temporal resolution So this is the big advantage of Langmuir probes. So Langmuir probes offer very good spatial and Temporal resolution Which is good from an experimentalist point of view and is also good if you want to compare something with codes compare with codes Because then you can use Basically these kind of tabletop experiments to benchmark your numerical codes Which you can then use to make more solid predictions or descriptions for an actual fusion device An example for Langmuir probes here are Langmuir probe arrays in the cell radar TJK again the example Here are Langmuir probes and these are actually Langmuir probe arrays so Langmuir probe Arrays in the cell radar TJK located in Stuttgart And then the left hand side you can see these these here are the probe tips and The probe tips are arranged on one flux surface with a spatial resolution here of approximately 0.7 centimeters and Here you have them arranged in a matrix basically in a matrix order with a spatial resolution of one centimeter and As you might know Langmuir probes. These are just electrostatic tips So here as it's written here tungsten tips your tungsten tips then ceramic tubes and At TJK there is a data acquisition system for example used Which allows to acquire 128 channels so 128 probes simultaneously with a bit of a resolution of 16 bit and Sampling rate of 1 megahertz so microsecond time resolution So that is the sampling rate and that is a very nice system to describe turbulence both in good temporal and spatial resolutions In such probe arrays allow to measure the polo structure of transport for example so such Probe Rays allow For example to measure poloidal structure of transport Loyal structure of transport and you can see here a drawing so in The center. This is a poloidal cross section. This is a poloidal cross section of a plasma here. You can see the vessel wall and You might remember from the interchange instability as depicted in these two drawings on the right and on the left hand side that the interchange in stability Gets unstable on the low field side and is stabilized on the high field side Meaning if we put such a probe array To to this cross section Then we should be able to see this kind of asymmetry meaning that we would expect more Transport on the low-field side than on the high-field side. This is actually what we see here So here you can see the transport as a function of the poloidal angle Where we have here the low-field side where the maximum is located then here the high-field side where the minimum are located Meaning that this nicely shows that the transport is higher on the low-field side so this nicely shows that the transport is higher on the low Field side So this is a very nice experiment Okay, now how is The measurement of plasma density turbulence actually performed in fusion experiment, so the plasma density variation measurement in Fusion experiments we have for example beam emission spectroscopy Yeah, for example beam emission back trust Kopee in Beam emission spectroscopy We inject neutrals and then measure the light emitted from these and neutrals and The intensity variation then reflect the local density Fluctuation strengths. Okay, so you inject some kind of neutrals measure the light emitted by them and measure them very Precisely such that you can deduce from the intensity variation something to the local density fluctuation Then we have reflectometry reflectometry You might know standard reflectometry Which you can use to measure the profile of your plasma density, but when you have a Doppler reflectometry You actually measure the back scattered microwave the microwave which is back scattered on density variation And if this is Doppler shifted then from this Doppler shift you can reconstruct From measuring the K component the density fluctuation. So it's basically Doppler reflectometry Then there is also laser scattering laser scattering so measuring The light being scattered by small scale density variation and due to the short wavelengths of the laser This is more sensitive to very small density structures So these are just a few examples and a very important comment is also to be aware of that magnetic so far We talked about electrostatic turbulence and magnetic field Fluctuations has been measured so it is possible to measure that but No clear evidence has been found that this contributes significantly to the overall transport So the magnetic field magnetic field variation of fluctuation has been measured, but there is no clear Evidence that They Contribute significantly Significantly to the overall to the global transport Okay, that's it for this video which was a brief overview about turbulence measurement Remember, this is a field of active research. I showed you an example for a dual cascade Actually measured in a teacher case accelerator We talked about the strength of the density variation the density fluctuation Being up to a few 10 percent at the plasma boundary Then I briefly talked about the mixing length model where the density which says that the density fluctuation amplitude scales with a size of the Density structures over the size of the vortices and inversely with the size of the gradient which is quite of expected and That turbulent structures are strongly elongated along magnetic field line and that longer probes can be very useful But only be used to low-temperature devices and fusion you have to use as outlined here on this slide for example Be mission spectroscopy Doppler reflectometry or laser scattering and that magnetic field variations of fluctuations have been measured But there's no key evidence that they contribute significantly to the overall transport Okay, that's it for this video. Hope to see you the next video