Atomic beam diagnostics on fusion devices

Sándor ZoletnikDepartment of Plasma Physics

KFKI Research Institute for Particle and Nuclear Physics (KFKI RMKI)

Association EURATOM-HAS

Association - HASKFKI-RMKI

Fusion research

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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

Plasma diagnostics

Plasma diagnostics uses a set of special and extreme physical methods using a wide range of physical phenomena and technologies:

Magnetic and electric probesElectromagnetic wave measurements: active and passive from microwave to gamma rayParticle detectors, analyzersParticle beams….

Probes can be inserted only into the cold edge plasma

In the core local 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

O-mode

X-mode

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Understanding fusion plasmas

Beam heatingmodeling

External coil currents

MHD equilibriumand parameters

Microwave heating

modelingRF heatingmodeling

Plasma current drive

Turbulence

Transport

Plasma edge

FuellingRadiation loss

Fusion reactions,

alpha heating

Impurities

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

Primary unstable waves

Secondary (meso)

structures

Flowinstabilities

Radial profiles

Transport

Turbulence

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)

Beam diagnostics

Injecting an atomic beam and observing its line radiation

Beam Emission Spectroscopy (BES), Motional Stark Effect (MSI)S. Zoletnik Page 5. Atomic beam diagnostics on fusion devices

TJ-II HIBP diagnostic (CIEMAT, Madrid)

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

Various beams

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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-)

Beam Emission Spectroscopy program at KFKI RMKI

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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

BES technology at the Hungarian fusion Association

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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

Beam simulation, design of schemes

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

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Li-beam simulation for COMPASS

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)

Ion source

Extracted Li-beam

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Li ion source testing at KFKI RMKI

The RMKI ion source for JET

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.

Acceleration, neutralization, beam control

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The TEXTOR Li-beam

Light detectors

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 TEXTORVery close to ideal detector from 1010 photons/s

Detectors

Test LEDs

Amplifiers

Calibration

Ideal detector with QED=100% and 85%

Typical PM range Comparable in S/N

to much larger and more expensive US system

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Next generation BES detectors

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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

Hamamatsu S8550

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

These specialised cameras will have industrial applications. A company is being set up for this purpose.

The EDICAM sensor head under testing

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

Data evaluation

Related tool:

Tomography laboratory demonstration device:

TOMOLAB

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ITER bolometer diagnostic lines of sight as designed by KFKI RMKI

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

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2D density profile

Results from 2D diagnostic

2D correlation (radial-poloidal) function

Poloidal velocity is determinedfrom shift of correlation along magneticsurface.

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Results from trial core BES system on MAST

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%

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Conclusions, outlook

• 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.

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