Atomic beam diagnostics on fusion devices

1. Sándor Zoletnik Department of Plasma Physics KFKI Research Institute for Particle and Nuclear Physics (KFKI RMKI) Association EURATOM-HAS KFKI-RMKI Atomic beam diagnostics on fusion devices Association - HAS

2. Fusion research Page 2. S. Zoletnik Atomic beam diagnostics on fusion devices • Magnetic confinement fusion research reached a state where plasma conditions are close to the ones required for a fusion reactor. • This is made possible by a huge improvement of • plasma technology in the past decades: • Magnetic configurations, improved confinement • Heating, current drive • Diagnostics and plasma control

3. Plasma diagnostics X-mode O-mode Page 3. S. Zoletnik Atomic beam diagnostics on fusion devices • Plasma diagnostics uses a set of special and extreme physical methods using a wide range of physical phenomena and technologies: • Magnetic and electric probes • Electromagnetic wave measurements: • active and passive • from microwave to gamma ray • Particle detectors, analyzers • Particle beams • …. • Probes can be inserted only • into the cold edge plasma • In the corelocal measurements • are possible only by intersecting two lines: • Incoming beam and observation (detection) • The only exception is a category of • microwave measurements where a critical • surface exists in the plasma

4. Understanding fusion plasmas Turbulence Primary unstable waves Radial profiles External coil currents Transport Secondary (meso) structures MHD equilibrium and parameters Flow instabilities Plasma edge Turbulence Radiation loss Transport Plasma current drive Beam heating modeling Microwave heating modeling Fusion reactions, alpha heating RF heating modeling Fuelling Impurities Page 4. S. Zoletnik Atomic beam diagnostics on fusion devices A fusion plasma is an extremely complex system with interactions on a wide range of scales (10 micron – 100 m, 0.1 microsec – 100 sec) Plasma turbulence is an especially challenging field which self-consistently determines the plasma state: A nonlinear system of waves, flows at multiple scales Special turbulence diagnostics are needed: well localized and fast measurements

5. Beam diagnostics Page 5. S. Zoletnik Atomic beam diagnostics on fusion devices Beam diagnostics are powerful techniques for measuring several plasma parameters: density, temperature, magnetic field, potential and their fluctuations. There are two basic possibilities: Injecting an ion beam and detecting the same or a secondary ion beam Need large Larmor radius  Heavy ions, high energies  Heavy Ion Beam probe (HIBP) TJ-II HIBP diagnostic (CIEMAT, Madrid) Injecting an atomic beam and observing its line radiation Beam Emission Spectroscopy (BES), Motional Stark Effect (MSI)

6. Various beams Page 6. S. Zoletnik Atomic beam diagnostics on fusion devices • Different beams are used for BES: • Gas jet (0.1 eV) • accesses only very edge of plasma (Scrape-Off Layer) • Laser blow-off (10 eV) • Somewhat inside plasma • Pulsed beam • Alkali beams (50 keV) • Up to core plasma in small devices (measures almost directly density) • Heating beams (50 keV) • Large diameter, powerful H, D, He beams reaching core plasma • Often needs special observation gemotery • MSI uses heating beams only

7. Beam Emission Spectroscopy program at KFKI RMKI Page 7. S. Zoletnik Atomic beam diagnostics on fusion devices • KFKI RMKI has started a multi-device BES program aiming at comparative measurements on a series of different fusion devices in the EU fusion research programme. • Wendelstein 7-AS: Quasi 2D turbulence measurement with Li-beam • (Garching, Germany 1997-2002) • TEXTOR-94: Li-beam diagnostic • (Jülich, Germany, 2002-) • JET: Li-beam upgrade • (Culham, UK, Li-beam (2003-) • MAST: Core turbulence measurement with • BES on heating beam(Culham, UK, 2006-) • COMPASS: Various BES schemes • (Prague, Czech Rep., 2008-) • TCV: Gas jet injection • (Lausanne, Switzerland, 2007-) • ASDEX Upgrade: Turbulence measurement in Li-beam • (Garching, Germany, 2007-)

8. BES technology at the Hungarian fusion Association Page 8. S. Zoletnik Atomic beam diagnostics on fusion devices BES needs technically challenging components, KFKI RMKI has most of them: Simulation: Atomic physics modelling, design of schemes Ion source: Solid state ion source or heating beam Acceleration and beam control system: Focussing, beam chopping, gas injection Detection: High QE, fast detectors to detect all the photons and reduce background Data evaluation: Statistical methods, correlation analysis

9. Beam simulation, design of schemes Page 9. S. Zoletnik Atomic beam diagnostics on fusion devices • Comprehensive simulation tools enable design of BES systems • Standard BES with Li, Na, … beams • Details of light collection, smearing, … • Mixture of BES and HIBP is being studied: • injecting an atomic beam and detecting ions • Vibrating beam, quasi-2D measurement • (KFKI RMKI invention, 2000) • Beam sweeps measurement region within a few microseconds • Sweeping time is shorter than turbulence decorrelation time • Spatial location maps to time in detectors: • 2D information can be extracted by time-slicing data Li-beam simulation for COMPASS

10. Ion source Page 10. S. Zoletnik Atomic beam diagnostics on fusion devices • Ion sources for alkali beams are developed at KFKI RMKI • High temperature (1200-1500 °C) ceramics materials • W or Mo filament heating, heat shields • Extraction with 5-8 kV in Pierce geometry • Limited lifetime, needs replacement after 1-100 hours beam operation • (a few second/discharge in current experiments) Extracted Li-beam Li ion source testing at KFKI RMKI The RMKI ion source for JET

11. Acceleration, neutralization, beam control Page 11. S. Zoletnik Atomic beam diagnostics on fusion devices • Alkali beams are accelerated to 30-80 keV and formed by a standard 3 electrode system • Beam position/vibration/chopping is controlled by deflection plates • Neutralizer is a Na gas cell (not provided by KFKI RMKI yet) • Beam can be checked by Faraday cup, and by imaging on metal plates • Alternative acceleration schemes are being simulated. The TEXTOR Li-beam

12. Light detectors Page 12. S. Zoletnik Atomic beam diagnostics on fusion devices • BES is limited by photon statistics, needs high Q.E. 1 MHz bandwidth and low noise • The RMKI Avalanche detector system: • Compact 8 channel array using large area • Hamamatsu APDs: 5x5 mm • State-of-the-art 3-stage low noise amplifiers • Vacuum enclosure • TEC cooled/stabilised • 16 channel system being built for TEXTOR • Very close to ideal detector from 1010 photons/s Test LEDs Detectors Amplifiers Ideal detector with QED=100% and 85% Typical PM range Calibration Comparable in S/N to much larger and more expensive US system

13. Next generation BES detectors Page 13. Page 13. S. Zoletnik S. Zoletnik 2D BES turbulence imaging diagnostic for MAST Atomic beam diagnostics on fusion devices • The detector for the upgraded MAST BES system will use a 4x8 APD matrix • Analogue electronics from previous system • Digital electronics based on KFKI RMKI camera design: • Event Detection Intelligent CAMera (EDICAM) • Will appear as high-speed (>1 MHz) low-resolution camera HamamatsuS8550 • EDICAM itself is a new camera concept for the next generation fusion experiments and industry: • 500 Hz @ 1.3 Mpixel, 100 kHz @ 32x32 pixel • Digital eye concept: • Low frequency readout on 1.3 Mpixel sensor • Automatic fast readout on changing regions on interest • Ultra high-speed (10G) industry-standard interface The EDICAM sensor head under testing These specialised cameras will have industrial applications. A company is being set up for this purpose.

14. Data evaluation Page 14. S. Zoletnik Atomic beam diagnostics on fusion devices • Comprehensive evaluation programs and statistical analysis tools have been developed for turbulence BES over the past 12 years: • Correlation and spectral analysis with all tricks to get rid of noise and background • Special methods to detect temporal and spatial changes in measurements • Related numerical technique: tomography • At present KFKI RMKI provides all tomography • simulations for ITER: • bolometer, neutron, X-ray diagnostics Related tool: Tomography laboratory demonstration device: TOMOLAB ITER bolometer diagnostic lines of sight as designed by KFKI RMKI

15. Page 15. S. Zoletnik Atomic beam diagnostics on fusion devices Results: Li-BES on Wendestein 7-AS Density profile, temporal and spatial correlation of fluctuations are reconstruced from SOL to edge/core Only rought estimate of Te and Zeff is needed Non-perturbing diagnostic

16. 2D density profile Page 16. S. Zoletnik Atomic beam diagnostics on fusion devices Results from 2D diagnostic 2D correlation (radial-poloidal) function Poloidal velocity is determined from shift of correlation along magnetic surface.

17. Results from trial core BES system on MAST Page 17. S. Zoletnik Atomic beam diagnostics on fusion devices Detection limit for fluctuations is 1-2% SOL-edge turbulence is well seen: • Fluctuation level > 10% • Autocorrelation time ~50 μs • Radial propagation • Spatial correlation determined by detection • Magnetohydrodynamic modes seen, density fluctuation correlated with magnetic field Detection limit in upgraded system will be ~0.2%

18. Conclusions, outlook Page 18. S. Zoletnik Atomic beam diagnostics on fusion devices • Our improved and new BES systems will come on-line in the next 1-2 years: • TEXTOR, ASDEX Upgrade, JET, MAST, COMPASS, TCV • KFKI RMKI has the most up-to-date technologies for BES on fusion devices • A bunch of turbulence phenomena are detected at the plasma edge: • partly explained by theory partly not • Core turbulence measurement is marginal with alkali beams •  MAST core turbulence should provide data • Improved detectors/cameras are suitable for industrial applications.