R Felton 1 (22) Session Leader Training November 2010

Plasma Density Control

R FeltonSession Leader TrainingNovember 2010

Torus radius 3.1 mVacuum vessel 3.9m x 2.4m Plasma volume 80 m3 Plasma current < 5 MA Toroidal field < 4 Tesla

Unique technical capabilities : Tritium Beryllium

R Felton 2 (22) Session Leader Training November 2010

Plasma Density ControlObjectives

• Inject Gas for breakdown and initial current rise

• Inject Gas to achieve required density for (i) Experiment (ii) Neutral Beam Injection

• Inject special gasses for experiment e.g. Tritium, Neon, Argon

• Inject special gasses to assist RF / LH couplinge.g. Hydrogen

R Felton 3 (22) Session Leader Training November 2010

Ways and Means

• Gas Bottles, Lines and Matrices

• Gas Introduction Modules

• Plasma Density Measurements

• Plasma Density Controller (PDLM)

• Gas and Density Pulse Schedule

• Programme Co-ordination

• JET Operating Instructions

R Felton 4 (22) Session Leader Training November 2010

Gas Bottles, Lines, Matrices

• 9 gasses in Gas Store

• 4 lines from Gas Store to Vessel

• 11 Injection Modules

• 2 Valve Matrices

• Control:-GIS on VC

R Felton 5 (22) Session Leader Training November 2010

Gas Introduction Modules

• Mid-Plane– 1,2,4

• Top & LH– 5,6,7,8

• Divertor– 9,10,11,12

• Tritium– 15

R Felton 6 (22) Session Leader Training November 2010

GIM Control & Calibration

• Piezo moves valve against spring

• Valve opening depends on voltage across piezo (programmable psu)– Not linear– Stable enough

• PDLM controls– 0 .. 10V ==

0 .. 100% open– Needs calibration

curve• Calibration Pulses

– In Restart

R Felton 7 (22) Session Leader Training November 2010

Density Measurements

• Line-Integrated Density (LID), usually the Vertical Line-Of-Sight 3– KG1 Interferometer (main)– KS3 Bremsstrahlung (back-up)

• Range 0 .. 400 e18/m2

• Occasional problems with– Fringe-jumps– Calibration of Bremsstrahlung

• New signal processing 2011+– KG1 real-time– KG1 fringe counting– KG4 Polarimetry

KG1

KS3

R Felton 8 (22) Session Leader Training November 2010

Plasma Density Validation

• Previously– Main density signal for Density Control selected from KG1 LIDs,

blended in KVS– Backup from KS3 Bremsstrahlung, scaled to the main density

• Future, 2011+– Density signal from several LIDs

• KG1v+KVS, • KG1rt, KG1c (fringe counter), • KG4 polarimetry (Cotton-Mouton)• KS3 Bremsstrahlung

– Density signals for • Density Control in PDLM• NBI Enable in PEWS• Real-Time Experiment control in RTCC

– New PDV pulse schedule

R Felton 9 (22) Session Leader Training November 2010

Feedback Theory

• U = Line-Integrated Density (e18/m2)

• Uref = Reference waveform (e18/m2)

• E = Control Error• Yfb = Feedback• Yff = Feedforward

waveform (0..10V)• Y = Control output

0..10V == 0..100% opening

ReferenceWaveform -

PIDControl +

FeedforwardWaveform

Uref E Yfb Yff

GIMPSU and ValvePlasma

Measurement& Validation

YU

Control Error E = Uref – U

Feedback Yfb = KpE + Ki∫Edt + KddE/dt

Kp = G G gainKi = G / Ti Ti integral time constantKd = G * Td Td differential time const.

Control Output Y = Yfb + Yff

di

TT

Gey )1(1

1)()(

Feedback as Laplace transform

R Felton 10 (22) Session Leader Training November 2010

Feedback Practice

60960 Sensor: KG1/LID3PID: G=1, Ti=1000s, Td=0.7s (=1)Actuators: GIM8: FB + 20% FFP onlyDensity exceeds reference

60961 Sensor: KG1/LID3PID: G=1, Ti=2s, Td=0.7s (=1)Actuators: GIM8: FB + 20% FFPIDensity on target

Error

Density

GIM

Electron Flow

R Felton 11 (22) Session Leader Training November 2010

Feedback Practice

62733

Sensor: KG1/LID3

PID: G=1, Ti=3s, Td=0.7s (=1)

Actuators: GIM8: FB + 20% FF

X-point formation increases pumping

e(t) ≠ k v(t)

Gas flow

e(t) and v(t)

n(t) and nref(t)

t=61s t=62s

R Felton 12 (22) Session Leader Training November 2010

Gas Pulse Schedule

• Gas species• Reservoir

pressure• Control mode

– Puff– Dose– Feedback– RTC

• Control Times

• Density Reference for Feedback

R Felton 13 (22) Session Leader Training November 2010

GIM Set-up

Puff: 100% opening of GIM for a time

FBack: Feed-back in time windowwith feed-forward (Dosing) waveform

Dosing: time window in waveformV2 valve open increases volume

R Felton 14 (22) Session Leader Training November 2010

PDLM (aka PDF) Schedule

• Requested Density waveform• Control Mode = PID• Gain G• Integral Time Constant Ti• Differential Time Constant Td

and strength α

di

TT

Gey )1(1

1)()(

R Felton 15 (22) Session Leader Training November 2010

Plasma Density Feedback

• Input reference : target density waveform units = “Fringes” (1018 m-2) starts at GAS=39.5, allow for pump-out at X-point formation allow for heating phase

• Input signal : DENS1 (usually)• Output signal : Vopen (0.. 10 V) to any GIM

Vopen = VopenFF + VopenFB• Proportional Gain: Vopen/LID

A simple flat unit gain works well, add extra GIMs to increase effective gain at appropriate time (e.g. Xpoint). Select GIMs for appropriate speed and location.

R Felton 16 (22) Session Leader Training November 2010

Plasma Termination

• Normal– PDLM follows requested

density, usually a ramp down from experiment level to zero

• Soft Stop– When Plasma Termination

Network (PTN) triggers a Soft Stop, PDLM ramps down from the last-good density to zero, proportional to plasma current.

• GIM Stop– When Plasma Termination

Network (PTN) triggers a GIM Stop, PDLM switches off the GIMs designated to be switched off by the GIM Stop.

R Felton 17 (22) Session Leader Training November 2010

2nd Feedback Loop

• Experimental Control a Signal e.g. Frad using Gas– Better to use RTCC

• Target waveform– Normalised range -1 to +1– Starts at GAS = 39.5s

• Signal– Normalised range -1 to +1 – Selected and scaled in RTSS pages

• PID Feedback– P I D tuning needs several pulses – P I D can be estimated from a step response

• Step Gas, observe change in Density and Signal• G ~∆Signal / ∆Density, Ti ~ rise-time

R Felton 18 (22) Session Leader Training November 2010

Real-Time Control

• RTCC output PDF1– Sets the reference waveform for the Main Loop (Density)– Modify target density dynamically, according to events

• RTCC output PDF2– Sets the reference waveform for the Auxiliary Loop (Norm. Sig.)– Modify target signal dynamically, according to events

• RTCC output GIM groups A,B,C – Sets the GIM opening directly (0..100%)– Trigger puffs/doses on events, e.g. NTM, Pellet Centrifuge– Experimental Feedback Control, e.g. 3He, Frad

R Felton 19 (22) Session Leader Training November 2010

Real-Time Experiments

• 3He Control for Mode ConversionY = 100ΦHe / (100ΦHe + 18ΦDa)

U = 3He gim

• Detached Plasma Y = Φ Da/ Φ Br

U = D2 gim

• Guards for Tritium puff

Detached Plasmaserendipitidous gain

R Felton 20 (22) Session Leader Training November 2010

Gas Inventory

• Gas injected per pulse

• Gas loaded on cryo pumps

R Felton 21 (22) Session Leader Training November 2010

Programme Coordination

• The Gas Form– Programme

Intranet• SL requests

– Gas species– Reservoir

pressure• Approved at

Weekly C/O Coord

• Printed for – Shift Techs– Vac Group– EiC

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R Felton 22 (22) Session Leader Training November 2010

JET Operating Instructions

• JOI 4.2 – Pews Density Limits for Neutral Beam Injection Valid in any

Plasma Configuration

• JOI 4.7 – Alternative Pews Density Limits for Neutral Beam Injection, Valid

in Fat or Superfat Plasma Configuration Only

• JOI 7.2 – Vacuum Conditions for Tokamak Operation

• JOI 7.4 – Operation of Gas Introduction Matrix

• JOI 7.5 – Avoiding Potentially Explosive Mixtures