Current Profile Measurement Techniques€¦ · Current density profile in Tokamaks • Current density is a critical parameter, affects: – Plasma stability (“second stable edge”)

Current Profile Measurement TechniquesThe JET MSE Diagnostic

N C Hawkes

Outline

• Current density profile in tokamaks• Equilibrium reconstructions• Motional Stark Effect and Zeeman (heating beams/Li

beam)• Optical system: Low Verdet glass. Amici prism• Opto­mechanical design process• Lens mounting• ITER

Current density profile in Tokamaks

• Current density is a critical parameter, affects:

– Plasma stability (“second stable edge”)– Steady state operation: non­inductive

current drive

Snyder Wilson and Ferron, Phys. Plasma, 9, 2037

Current density profile in Tokamaks

• Often more important to consider the q­profile (winding number)

Current density profile in Tokamaks

• Often more important to consider the q­profile (winding number)

– MHD modes resonant at rationals (reduced confinement or disruptions)

– Rationals also linked with reduced transport (a benefit)

– Low shear also benefits transport

• In many cases the mechanisms are still debated

Schilham Hogeweij and Lopes Cardozo, PPCF 43 1699

Evaluation of Magnetic Equilibrium

• The plasma current distribution and q­profile is obtained from the magnetic equilibrium.

• This is the solution of the 'Grad­Shafranov' equation which describes the distribution of poloidal flux (R,Z), in 2 dimensions, across the plasma cross section:

• Solution can be obtained using only the external magnetic sensors, however it frequently becomes inaccurate in the centre of the plasma.

*R ,Z =0 R2 p ' 0

2 f f '

=0 R j

Evaluation of Magnetic Equilibrium

• Need to include 'internal' measurements:

– Polarimetry: line integrals of B.ne

– Motional Stark Effect: point measurements

MSE Measurement Principles

MSE Measurements

• Deuterium atoms in the neutral heating beams are excited by collisions, emit H­alpha radiation

• Plasma magnetic field is Lorenz transformed to an electric field in the frame of the emitting atoms

• Stark splitting and polarisation of the radiation by this E­field

• Polarisation projected onto detection optics ­ yields information on the magnetic pitch angle

Bv

EvxB

First Identification of Motional Stark Effect, JET

A Boileau et al, J. Phys. B, 22, L145,1989.

JET Geometry

JET Geometry

• Diagram of the diagnostic layout on JET

JET Geometry

• MSE diagnostic is aimed at the track of pini 1, octant 4.

• Other pinis cross the lines of sight, causing interference.

• 25 channels, 6 cm resolution

Diagnostic view of the inside of JET

(JET Geometry)

• Polarisation projected onto detection optics ­ yields information on the magnetic pitch angle

TRZ

TRZm BaBaBa

BaBaBa543

210tan++++=γ

Er sensitivity

• Effect of Er (driven mostly by toroidal rotation) on MSE angles

• For co­injection, Er is positive and has the effect of making γm more positive

rb

m EBv

∆Ω=∆α

γϕ sin

costan

ϑϕϕϑ BVBVneZ

pEr +−∇= Neutral Beam

Er

B

v×B+Er

v×B

Er sensitivity

• Effect of Er (driven mostly by toroidal rotation) on MSE angles

• Qualitative effects can be seen of ITB barrier expansion and contraction.

• Values of Er ~90 KV.m­1 agree with toroidal rotation term.

Lithium beam measurements

• Using heating beams achieves good penetration and line splitting

– The ITER system will be based on the heating beams

– Disadvantage is complex spectrum and mixed polarisation from the different PINIs (in JET) or the large source size (in ITER)

• Disadvantage of Stark measurement is that it is sensitive to Er

– Zeeman effect can also be exploited – requires a dedicated atomic beam

– Good for the edge, where Er can be large (and unmeasured), but line­splitting is small making the measurement extremely difficult.

Thomas et al Phys. Plasmas 12, 056123 (2005)

Overlapping PINIs

• Diagnostic is aimed at pini 1 and uses the pi lines of the tangential bank to get the larges Doppler shift. • Spectra from other tangential­bank pinis overlap pini 1 and pollute the measurement• Voltage of pini 1was increased to 125 kV , other pinis remain at 80 kV. The difference in Doppler shift

allows us to spectrally isolate pini 1.• Worse problem for ITER because of the large NBI source size. Voltage discrimination not possible.

All pinis Pini 1

Design of the JET MSE System

Design of the JET MSE System

• Coordinates of neutral beam and viewing port, and length of port tube, sent to the optical designer in France.

• Optical design carried out in Zemax (Code V is another popular program)• DXF files of the lens layout and raytrace sent to JET to import to CATIA for

checking of optical alignment and mechanical clashes.• Great care needed to avoid conversion error

Design of the JET MSE System

• At least two companies now offer optical design software integrated in to CAD packages (chiefly Solidworks or CATIA):

– Breault Research: ASAP– SPEOS: OPTIS

• Expensive (but powerful) programs, mainly seem to be used by the automotive and aerospace industry for driver/pilot ergonomics.

Design of the JET MSE System

• Coordinates of neutral beam and viewing port, and length of port tube, sent to the optical designer in France.

• Optical design carried out in Zemax (Code V is another popular program)• DXF files of the lens layout and raytrace sent to JET to import to CATIA for

checking of optical alignment and mechanical clashes.• Great care needed to avoid conversion error• Manufacturers tendered, then optical design 'tool fitted'

Design of the JET MSE System

Design of the JET MSE System

• Deviation angle of 45 degrees can't be achieved with a simple mirror without significant vignetting.

• Forced to use a more exotic solution, a solid glass prism: Amici configuration• ... but concerned about large optical path length close to the plasma (radiation

darkening, Faraday rotation) so used a dichroic 'air amici' design.

Design of the JET MSE System

• Blue tube is the torus vacuum boundary. Re­entrant. Carries the primary window.• Red tube is keyed to torus flange at outer end to maintain rotational alignment.

Carries the prism (inner end) and the secondary vacuum window (outer end).Also holds the lens tube (grey)

• Lens tube maintains the correct lens spacing (seated at inner end). Rotationally symmetric.

JET vessel wallLimiter guide tube

Primary windowSecondary window

Window tube

Lens Tube

• Original tube shown. Matt black treated stainless steel• Black coating chemically degraded in use at 300C. Whole lens tube had to be

replaced: Internally coated with colloidal graphite instead.

Prism holder

• Outer tube dowelled against rotation. Carries the prism at front end, secondary vacuum window at other (outer) end.

Window tube

• Primary silica vacuum, tritium, beryllium boundary (and wire driven shutter.)• Never removed since installation (~40000 JET shots)

Low Verdet glass components

• Optical system is immersed in the field from the JET P4 coils. The field direction can change depending on the coil currents

• Low Verdet glass, Schott SFL­6(no longer available) to minimise Faraday rotation of the light.

Low stress lens mounts

• SFL­6 is vulnerable to stress­induced birefringence, hence important not to apply stresses to lens elements.

– Must be stress­free at operating temperatures from 20—350 C

• Optical tube assembled from short 'barrels' each containing one or two lenses. Barrels screwed on to spacer sections.

• Lenses seated on angled shoulders.• Pre­set compression, with spring

loaded screwed retaining ring.Opto­mechanical System Design: Paul Yoder, CRC Press

Low stress lens mounts

• Lenses seated on angled shoulders.• Pre­set compression, with spring

loaded screwed retaining ring.

Secondary vacuum window

• Double window design for tritium compatibility. SFL­6 secondary window

PEM­based Polarimeter

PEMs used for Polarisation Detection

• Plasma light passes through two PhotoElastic Modulators (PEMs), each running at a different frequency (20 and 23 KHz in the JET case).

• ‘imprints’ the light intensity with the PEM frequencies• Signal from detector contains frequency components at 40 and 46 KHz,

corresponds, roughly speaking, to the sine and cosine of the polarisation angle.

Synchronous detection in software

• 4 output signals from the demodulation process: DC, f1, 2*f1, 2*f2Encode the four Stokes components of the light input to the PEMs I, M, C, SUnpolarised, Linear and Circular components

ITER MSE system using mirrors

• Transmissive optics cannot be used close to the ITER plasma

• Instead a folded mirror path is proposed.

• In the initial design the first mirror is very vulnerable to plasma contamination, which will lead to changes of its optical properties.

• If the mirror becomes contaminated it will affect the polarisation of light it reflects.

ITER MSE system

• The JET MSE system is removed from the machine for calibration during shutdowns.

• Because of the likelihood of the calibration of the ITER MSE system changing over time an in­situ calibration source would be required. Extremely difficult to device such a system.

• Instead people are looking at other ways of extracting the Stark information which doesn't rely on polarisation measurements.

• Techniques which need only the spectrum seem more attractive, but are presently untested and have their own difficulties.

• Will still need a light collection system, but just a little less demanding.

Getting Good MSE DataAnd what can be seen

Limits on Operation

• High density operation causes strong attenuation of the beams by the time they reach the centre of the plasma. In addition the background light levels increse.

• Leads to a fall in the polarised light fraction and a drift of the MSE angles.

• Adjacent channels, here tuned to σ and π lines, diverge during a density ramp.

• ELMs• density ramp from ITER talk Adjacent channels of JET system

tuned to σ and π during density ramp

Good data needs pini 1. Notching to avoid background light.• Background light during high

performance can be compensated – up to a point – using modulation of measurement pini.

Notching is unable to recover very bad channels

• Channel 20 is particularly badly affected by ELMs (perhaps because it looks directly at a limiter)

• Because ELMs are irregular the beam­ON and beam­OFF windows are not equally affected.

Ch 20Ch 21

Pini 1

Dα

Accuracy/sensitivity

• Calibrated to about 0.2 degrees (we think).

• May be systematic effects which are larger than this

• Statistical accuracy often 0.1 degrees (20 ms integration)

• Can see Monster ST, but not regular ST• Minimum practical time resolution is

20ms, less than this the accuracy is imparied.

• Raw signals can detect large, slow, MHD modes, but this is rare.

Shot 65252, Sawtooth crash. MSE/Pini 7 data

56.69s pre­crash 56.78s post­crash

MHD mode seen in Beam­emission light

• Spectrogram of channel 4 (near plasma edge) showing ELMs and MHD mode• Diagnostics is not optimised for BES fluctuations, can only detect large, slow

modes. Modes this large are infrequent.• In this example the PEMs have been switched off (normally have horizontal lines

across spectrogram at PEM harmonics and intermodulation frequencies.

#61170

Plasma Er influence

Er sensitivity

• Er correction applied to MSE signals using toroidal rotation from charge exchange measurements

• Correction reduces the q values• (Correction is a different approach to DIII­D where rotation is included in EFIT).

Neutral Beam

Er

B

v130×B+Er

v80×B+Er

Experiments have been tried on JET to attempt to measure Er using the MSE apparatus. These experiments exploit the different beam energies available in octant 4 and rely on the difference in the Er perturbation between 80 kV and 130 kV injection. So far the experiments have not yielded any convincing measurements– the noise levels of such measurements are too large.

Attempts at using as measurement

‘Direct’ Interpretation of Data

Non­EFIT measurements

• EFIT is used to obtain q(R) and J(R)– see talk by M Brix.

• Relies on parameterised (‘smooth’) profiles

• Certain analyses can be done with the raw data

– Allows a more ‘local’ interpretation of features

5

2

5

0

543

210tan

aa

BaBa

BaBaBaBaBaBa

T

Z

TRZ

TRZm

+≈

++++=γ

Current perturbation associated with MHD• Slight deviation of MSE angle from a

smooth profile, expressed as ∆Bz

• Express as local current and q perturbation

• Snake at 3.5m—source of perturbation

• Limit of accuracy of diagnostic

Z

Z

Z

BBqq

Brrr

∆−=∆

∆=∆∂∂ J0

1 µ

ICCD• Similar analysis comparing two shots,

identical except for ICRH phasing (ICCD).

• #51522 (+90) and #51523 (­90)

• Difference in γm interpreted as local current drive.

Current hole

• Profile of MSE measurements from a current hole, compared to Bz=0 calibration shot.

PICTURE

Sweeping of plasma can avoid channel­to­channel calibration errors.

• Used to prove the existence of the current hole

• Use Ampere’s law to estimate current density within flux surfaces directly from MSE data

• Measurements give |j(0)| <200 kA.m­2 from the slope of γm

• Apply radial ‘jog’ to the plasma to eliminate channel to channel variations: |j(0)|<80 kA m­2

Current hole

• Fitting linear slope to MSE data within current hole (temporal and spatial averaging to reduce error bars)

• Current at axis is zero +/­ 40 kA.m­2

Ell Measurement

C B Forest, PRL, 73, 2444 (1994)

• Radial component of Faraday’s Law

in cylindrical geometry

Surface loop voltage is boundary condition to radial integral

• Toroidal electric field drives ohmic current, Bz

and Ampère’s Law yields total current. Difference is non­inductive part (bootstrap and driven)

• Actually need E|| for current drive, which requires EFIT results for ψ(R,Z).

• Most interesting cases have small B­dot (low resistivity) – evaluation of derivative uncertain.

BZ

Eφ

tR

R ∂∂−=∧∇ BE)(

( )φRERRt

BZ

∂∂−=

∂∂ 1