1. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 1/35 A Brief tutorial to Thomson Scattering With a focus on LIDAR By Mark Kempenaars For the EFTS/EODI training, 12 th June 2009 at Culham Science centre

2. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 2/35 Outline of Talk 1.Introduction 2.Thomson scattering theory – the highlights 3.Conventional TS 4.LIDAR TS 5.Towards ITER

3. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 3/35 Introduction Thomson scattering was first described in 1903 by J.J. Thomson, many years before lasers existed. Thomson discovered electrons in 1897. First application to a laboratory plasma in 1963 by Fünfer (First ruby laser in 1960) First measurements in hot plasmas by Peacock et al., in 1969 at the Russian T3 Tokamak

4. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 4/35 Thomson scattering theory Thomson scattering is nothing more than the interaction of EM radiation with an electron any light will do. We can use Maxwell’s equations (1873) to describe the forces on and movements of the electrons. The highlights… Let’s consider this experimental setup: Incident EM wave with amplitude E 0, propagation vector k 0, and angular frequency  0, so electric field at the electron is given by: dd ksks R k k0k0   E0E0 Scattering Electron Origin rjrj

5. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 5/35 This electric field will then apply a force on the electron (with mass m and charge e at position r j ) and following Maxwell’s equations, we get the acceleration of the electron: Theory cont’d - 1 This equation clearly shows us that the electron will be oscillating up and down, together with the electric field of the light wave. Since this electron is now a moving charged particle it will create an EM field of its own, with the same wavelength as the incoming light!

6. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 6/35 Theory cont’d - 2 When a moving electron like this is observed from a large distance (R>> ) its radiation can be described as dipole radiation: This equation shows us that the radiation from ions is negligible compared to that of electrons, since r 0 ~ 1/m: Where k is the differential vector (k s -k 0 ). and r 0 is the classical electron radius:

7. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 7/35 Theory cont’d - 3 In this lay-out d  is the solid collection angle, it basically describes the fraction of scattered radiation we collect. If we then divide the total scattered power by this solid collection angle we get the differential scattering cross section: Which tells us that the re-radiation is maximum perpendicular to E 0 ; d  T /d  =r 0 2. And that the scattering cross section is very small This makes it clear that every photon is important! And we want as big a window as possible.

8. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 8/35 Theory cont’d - 4 Obviously the scattered power depends on the number of electrons caught by the laser, but also on the interaction between them. This interaction start to happen above the Debye length. So this depends on the density and temperature of the electrons If  << 1:then the scattering is from individual electron: Incoherent TS If  ≥ 1:then scattering by electrons surrounding ions; (Ion) Coherent Thomson Scattering If  ~ 5-20:Scattering by electron density fluctuations, or Bragg-scattering Coherent Thomson scattering For T e = 10 keV, n e = 5×10 19 m -3 : D ~ 100  m (typical for JET) The so-called Salpeter-parameter tells us whether the scattering we are observing is coherent or not:

9. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 9/35 The scattered light is clearly proportional to the density. The form function is given by: where f( ) is the velocity distribution In this equation the delta function tells you about the Doppler shift: Theory cont’d - 5 WithP 0 :Incident (laser) power n e :Electron density in the plasma  L:Length of scattering volume S(k,w):Scattering form factor describes frequency shifts from electron motion as well as correlation between electrons. The total scattered power is given by:

10. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 10/35 Theory cont’d - 6 If the velocity distribution f(v) is Maxwellian (i.e. low density, no interaction between particles) then: with ‘a’ the thermal velocity: One then finally finds an equation that contains wavelengths: With 0 the incident wavelength and s the scattered wavelength Where we then find the spectral width of the scattered light, which has a Gaussian shape: If we were to take a Ruby laser (694.3nm) and 90º scattering then this would give:

11. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 11/35 Theory cont’d - 6 Once the electrons get really hot (i.e. really fast) we have to start including relativistic effects, which effectively change the scattering cross section of the electrons, by a factor 1/  2 where  is the Lorentz factor, which shows that for a 1% deviation we need an electron temperature of 2.56 keV. Also there is a “search light” effect or relativistic aberration, which means that the electrons radiate preferentially in their forward direction. E.g. moving at 10% of c, then power in forward direction increases by 36%, it decreases by 26% in backwards direction. This leads to a blue shift of the spectrum…

12. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 12/35 TS spectra So, what does this look like? / laser

13. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 13/35 TS Spectrometer What does a spectrometer look like? If we cut our scattered light “broadband” light into sections: 1234 1 2 3 4 Incoming collected light

14. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 14/35 So, what do we need? A powerful pulsed laser –Typically one would get 1 photon in every 1×10 14 back, e.g. if we use a 3GW laser pulse we get 30  W back on a high density plasma (10 20 m -3 ) and 100% transmission. Fire this laser into the plasma –A window on the machine that can stand the high laser power and does not get dirty –plus the optics to deliver it there. Collect as much light as possible –A large window is needed that can see the laser line –This window can’t get dirty, or if it does we must be able to clean it. –The other optics need to be aligned and stable (also during disruptions etc.)

15. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 15/35 90º TS - Single point In the early days of TS on Fusion devices all TS systems were “single point” diagnostics, i.e. the optics were only looking at one point. This seems archaic but it was still one of the better and more reliable diagnostics. This was also the case on JET, where a ruby laser was fired vertically into the plasma. A large set of windows and mirrors was used to relay the light to a spectrometer. Collection optics Plasma Laser

16. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 16/35 90º TS - Multi point More modern systems have a range of points. Where each spatial point is imaged onto an optical fibre. Laser Collection optics Plasma Each fibre then represents a spatial point in the plasma, a high spatial resolution can be achieved by using a lot of fibres. Keep in mind however that a smaller volume will scatter fewer photons. And this setup means one needs a spectrometer for each spatial position, so can get very expensive

17. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 17/35 90º TS on JET – HRTS A new system was installed on JET in 2004. High Resolution Thomson Scattering. -High power (5J, 15ns) Nd:YAG laser. -Fire at 20Hz, horizontally -Scattered light is then collected from a window at the top of the machine. In order to collect as many photons as possible we need a big window, largest on JET 20cm diameter. 63 spatial points on the LFS, at approximately 1.5cm resolution

18. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 18/35 90º TS on JET – HRTS cont’d 1

19. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 19/35 MAST 90º TS A set of lasers can be fired in sequence or in a burst, giving a high temporal resolution ~1  s

20. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 20/35 LIDAR – 180º TS Now we go to  = 180º, or back scattering Light detection and ranging, we fire a laser pulse and count the elapsed time before we get a signal back, like in radar. Plasma Laser (short pulse) Mirror labyrinth Of course we have to count very quickly, since light travels at ~3×10 6 m/s (or 1m every 3ns) Major advantages: ‘Point and shoot’ method, which requires minimum access Very short laser pulse ~250ps Only one spectrometer needed, but it has to be fast!

21. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 21/35 LIDAR – 2 The main advantages of LIDAR: The alignment is relatively easy Only one spectrometer Because of the previous two, much easier to calibrate and maintain The main disadvantage of LIDAR: Time is of the essence! If anything is slow it will contribute to the spatial resolution. HOWEVER! Time is on our side:

22. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 22/35 Note that the profile length in time is dt=2L/c. Effectively 15cm/ns! Instead of normal 30cm/ns Detector and laser response defines spatial resolution Plasma, Length L Laser Pulse Scattered Light Scattered Light LIDAR – 3 7cm (ITER requirement) is equivalent to ~460ps combined laser and detector response time (so det/laser response ~300ps FWHM)

23. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 23/35 LIDAR on JET - 1 JET is the only fusion machine in the world that has LIDAR. LIDAR only really works on big machines due to its limitations in spatial resolution. Two LIDAR systems on JET, the Core LIDAR and the Edge LIDAR. The edge LIDAR has recently been upgraded with new detectors and digitiser, so it has better resolution. Core LIDAREdge LIDAR Laser pulse length~ 300ps Laser power3J/pulse = 10GW1J/pulse = 3GW Detector response~ 300ps~ 650ps Digitiser response8GHz, 20GSa/s1GHz, 4GSa/s Spatial resolution~ 6.5cm~ 12cm

24. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 24/35 LIDAR on JET - 2 The total distance the laser beam has to travel is about 50m, important to keep the beam “nice”. Light is collected through a set of 6 windows

25. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 25/35 LIDAR on JET - 3 Collected light is relayed via a set of mirrors and lenses to the spectrometer. The Core LIDAR spectrometer has 6 detectors in a 3D layout. Each detector generates its own trace, these are then combined to form a temperature and density profile

26. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 26/35

27. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 27/35 NEXT: ITER LIDAR 10 ms (100 Hz) r/a < 0.9 ~7см (a/30) T e 0.5 – 40 keV (10%) n e 3x10 19 -3x10 20 m -3 (5%) Target requirements Core LIDAR (C.01 group 1b – advanced plasma control) Short line indicates the required measurement resolution of a/30. This is equivalent to approximately 7cm in real space. Note: the full profile from -0.9r/a to 0.9r/a is required ~2m

28. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 28/35 ITER LIDAR - 1

29. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 29/35 ITER LIDAR - 6 Low impact diagnostic access required In vacuum mirror protection (passive/active) Detectors (sensitivity, response time, wavelength) Materials (neutrons/radiation)--fit purpose Long term, low maintenance reliability Laser development Beam dump Access to anywhere inside this area is similar to accessing a satellite-very infrequent Mirrors Lasers enter machine boundary Mirrors Large mirrors collect suitable amount of light Exposed to plasma ~2m

30. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 30/35 ITER LIDAR - 2 Need to have radiation below 100uS/Hr 14 days after a shutdown in area behind plug

31. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 31/35 ITER LIDAR - 3 Influence of optical labyrinth Minimising activation of components just outside the tokamak will be key to easier maintenance in the future From Attila code

32. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 32/35 ITER LIDAR - 4 PhotocathodeResponse time, nsWavelength coverage S-200.2 ns(below)UV, visible up to 500 nm GaAsP0.3 ns (as above)visible up to 750 nm GaAs0.35ns (estimated)visible up to 850 nm InGaAs?NIR S-20 GaAsP GaAs NIR Several options, but none good enough yet.

33. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 33/35 ITER LIDAR - 5 Needs reasonable energy and short pulse simultaneously Options to chose from: –Nd:YAG (1064nm) –Ruby (694nm) –Ti:Sapphire (~800nm) –Nd:YLF (1056nm) Wide temperature range Time repetition expected from laser(s) – 100Hz Also need to consider –Space envelope/ Maintainability/ Power consumption/ Data quality

34. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 34/35 ITER LIDAR - 6 Laser specifications wavelength~ ~1.06microns (1ω +2ω +cal ) laser energy ~5J/pulse laser pulse ~250-300ps (20GW) Proposing 7 lasers at ~15Hz More achievable technology Compact footprint Measurement capability maintained even if 1,2,3... lasers malfunction Burst mode available to exploit plasma physics e.g. very fast MHD events

35. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 35/35 At the end… This tutorial is intended as a first introduction in to Thomson scattering and not as an exhaustive review Only some typical examples were given (mostly JET), every fusion machine has TS I’ve only focused on incoherent TS The aim was mainly on demonstrating how it works and how powerful a technique it can be Epilogue Thank you for your attention

36. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 36/35 Any extras… ? Any extras… ?

37. UKAEA Thomson Scattering Tutorial for EFTS/EODI, 12 th June 2009, M.Kempenaars 37/35 Space time domain