FTU: Experimental Results and...

C Gormezano ENEA Frascati Italy/ MIT presentation 9 February 2001

FTU: Experimental Results and Prospects

Frascati Tokamak Upgrade is a high field, high densityexperiment:

a=0.3m, R=0.93m, limiter machine

Bt up to 8T, Ip up to 1.6MA. Additional heating systemsinclude:

LHCD 8 GHz, 3 gyrotrons, tpulse=1s, one launcher,P=1 MW

ECRH 140 GHz, 3 gyrotrons, tpulse=0.5s,capability ofinjecting at oblique angle, P=1.1 MW

IBW 433MHz, 1 klystron, tpulse=1s, one launcher,P=0.35 MW

C Gormezano ENEA Frascati


FTU Experiment ctd

Multiple Fast pellet injector: up to 8 pellets at 1.3km/s

Titanisation system to reduce low Z impurity content

Diagnostics include:

Turbulence measurements: heterodyne correlationreflectometer( up to 3.4 1020m-3 in O-mode, up to~2 1020m-3 in X-mode)

FEB camera (on loan from CEA): Hard X-rayemmission20-200keV , 17 chords


FTU Experimental Programme

Programme is focused on transport and MHD studies:

Achieved improved confinement regimes

MHD stabilisation

Elements of programme

Quasi steady operation at high density(multiplepellets)

ECRH transport and MHD stabilisation

LHCD

LHCD and ECRH synergy

IBW

Preparation of upgrades


High Field, High Density Operation

Improved confinement with multiple pellet injectionalready reported at Bt=7T, Ip=0.8 MA (EPS 97)

m=1 mode/sawteeth suppressed by pellet injection

Peaking of density profile with line averaged density aboutconstant : inward pinch effect for particles

Disruption following third pellet injection

In 2000, operation at higher field/current + titanisation

At 8T / 1.2 MA improved confinement, m=1 mode/sawteeth not entirely suppressed.

Steady conditions achieved with 5 pellets injected at 0.1 sinterval( ~ τ E): max duration of current plateau

At 8T/1.6 MA (qa~2.7), improved confinement phase notyet achieved


Multiple Pellets at 7T/0.8 MA and 8T/1.2 MA

At 8T, improved confinement phase observed at 4thpellet: compared to 7T, 3-4 increase in neutron yield(record in FTU)

7T / 0.8 MA 8T / 1.2 MA


Multiple Pellets: m=1 Activity

In 7T/0.8 MA pulse, m=1 activity suppressed following pelletinjection, but disruption after 3nd pellet : m=2 tearing mode?Impurity accumulation?

In 8T/1.2 MA pulse, m=1 activity is only reduced but nodisruption observed: different current profile ? (titanisation?)

Start of improved confinement phase


Pellet Ablation Profiles

Pellet ablation takes place atabout mid-radius. Similarprofiles at 7T/0.8 MA than at8T/1.2 MA

A scan of ablation profiles versus target parameters has not yet been done

7T / 0.8 MA

8T / 1.2 MA

2ndpellet


Multiple Pellet : Energy Confinement Time

Much higher kinetic energy achieved at higher current

(probably) peaked pressure profiles and total storedenergy significantly higher than ITER89P (L-modewithout density dependence)

7T/0.8MA8T/1.2 MA


Multiple Pellet: Impurity Transport

Core total radiation reduces during high current pulse:remaining m=1 activity cleaning the core ?

Other observations indicate an outward pinch for impurities


Turbulence Spectra in Multiple Pellet Injection

In some cases when density increase is not too large(access of reflectometer to plasma core), strong changein core turbulence spectrum

Density fluctuations at the periphery have standardfeatures: Low Frequency, quasi-coherent and broadbandspectra

After pellet injection,simultaneous disappearance of LowFrequency and quasi-coherent feature

Indicates:

Link between LF and quasi-coherent spectra

Formation of improved confinement zone in plasmacore

VA Vershkov et al subm. to Phys.Rev.Lett.


Prospects for Multiple Pellet Injection

Understanding of inward pinch for deuterium andoutward pinch for impurities under study (M Romanelli)

Optimisation not yet done:

timing of pellets,

size of pellets,

operation at higher current,

use of LHCD to stabilise m=1 mode/change currentprofile

Heating with LH waves

High field side pellet launch

Real question: is it possible to use such a method withplasmas with central core temperatures > 7 keV ?


ECRH Transport and MHD Stabilisation

ECRH on current ramp with hollow current profile to producehigh Te and Te in plasma core

ECRH in post pellet to try to maintain good electronconfinement at high density

Vary deposition profiles to study relevance of profileresiliency (critical Te models)and role of inward energy pinch

Assess MHD tearing mode activity by fine adjustment ofresonant absorption layer and consequence on coreconfinement

Study mode coupling effects and role of neo-classical tearingmodes

Transport

MHD stabilisation


ECRH on current ramp-up: local analysis

The interpretative analysis indicates low χe values in the plasma core, similar to the one obtained in ohmic discharges, despite high Te and ∇ Te

#15020

r (m)

(keV)

(w/m3)

0.050 s0.060 s0.070 s0.080 s0.095 s

PECRH

oh

Prad

Poh

t=0.095 s

Te

Heat flux vs. ne∇ Te for r/a<0.15

χe=0.2 m2/s

χe=0.4 m2/s

G Bracco IAEA Sorrento


On-axis Heating of Hollow Current Profiles

When low Z impurities (C,O) content is low, hollow Te profilesare obtained in the startup phase. On-axis ECRH restores rapidly peaked Te profiles, in conditions of inverted magnetic shear.

A strong reconnection occurs at a time when qmin becomes lower than 2

Te

#17389

r (m)

(keV)0.108 s0.113 s0.118 s0.123 s

oh

t=0.098 s

#17389

Te

Ti

<nl>ne(0)

Ip

PECRH

(keV)

t (s)

(w)

(A)

(m-3)


Radial Scan of Power Deposition

The localization of the ECRH has been changed at fixed Bt by tilting the ECRH launchers; all pulses have hollow pre-ECRH Te profiles.

#17389

#17389

#17392

#17392

#17393

#17393

(w/m3)

Te

R (m)

(kev)

pECRH

t=0.116 s

Diffusive behaviour found: the experiment can be simulated using an ad-hoc χe in the range 0.3-0.5 m2/s in the plasma core

r (m)

(keV)

simulTe

oh

exp#17393


Off-axis ECRH in Post-pellet Phase

r (m)

Te

ne

(m-3) OH

PECRH

OH

0.610 s0.630 s0.650 s

t=0.595 s

0.650 st=0.595 s

(keV)

Electron density is near to the cut-off density 2.4x1020 m-3 at the ECRH deposition layer.

Electron temperature profiles show a diffusive behaviour in the core region of high density plasma.

Other similar experiments in FTU:

on-axis ECRH in post-pellet phase: results in a fast onset of sawtooth activity and a strong density pump out.

pellet injection on a ECR heated plasma: results in a broader ne(r), sawtooth is not supressed, no signs of enhanced energy confinement,


ECRH stabilisation of MHD Tearing Modes

Fine adjustment of ECRHresonance location withbeam steering (heating andcurrent drive can be adjustedseparately)

Mirnov oscillations verysensitive to ECRHabsorption radius andlocation of the island o-point.

Stabilisation in not obtained ifthe distance exceeds theisland width (From Cirant IAEA2000)

#18015

#18021

#18034

rabs≈rO-point

rabs≈rO-point+1 cm

rabs≈rO-point+2 cm

ECRH starts at 0.5 s.


Increase of Core Te when m=2 modes are stabilised

In #18004 (red traces)and #18015 (dark greentraces) the absorbedpower is the same, butthe absorption islocalised betweenplasma centre and theisland and stabilisationfails.

Core confinementimproves with TearingMode stabilisation

Needed off-axis power ~15% of total power


ECRH: Conclusions

Enhanced confinement phase achieved either withhollow current profiles(ramp up phase) or with peakeddensity profiles(post pellet phase). But limitation due toMHD activity: current profile control needed?

Profile resiliency issue still controversial

m=2 tearing mode stabilisation achieved with localisedheating

More detailed studies impaired by lack of reliability ofgyrotrons. More power needed to progress (4 gyrotronsin second half of 2001?)


Ion Bernstein Waves Experiments

IBW can produce sheared poloidal flow: possiblestabilisation turbulent fluctuations. No clearexperimental evidence so far

IBW experiment in FTU designed to minimise impurityproduction: phase waveguide array

P up to 0.35 MW at 433MHz: 15MW/m2

In order to have good absorption, Hydrogen plasma atBt=7.9T:

4th Ion Cyclotron Harmonic

Full absorption anticipated at a/r = 1/3


Ion Bernstein Waves Injection

IBW produces moderateedge parametric activity andsimultaneous increase ofcentral Te (4ms delay;nodirect Te heating) andpeaking of density

Injection of Neon (edgecooling) can also increasedensity and transient Teincrease, but cannotproduce a peaking of thedensity profile

Strong MHD activityterminates enhanced phase0.9 1.0 1.1 1.2

FTU SN 15788

4ΩH=ω0

0

2

4

6

8

1019

m-35

4

3

2

1

0

keV

Te

ne


Ion Bernstein Waves: Transport Analysis

0

2

1

3

4

5

6

Pa

10-1

10-2

100

101

χ e (m

2 /s)

b)4ΩH = ω0

0.9 1.0 1.1 1.2

Major radius (m)

FTU SN 15788

Simulation indicates that0.1MW are sufficient tosuppress turbulence

Pressure radial profilebefore and during IBWheating and thermaldiffusivity from JETTO code,show reduction of heatdiffusivity in the inner part ofthe discharge

Higher power andturbulence measurementswould allow a more definiteanswer


Lower Hybrid Current Drive

3 Gyrotrons at 8 GHz- 1 launcher made of 3 parts (classic)

Full current drive achieved at Ip=350 kA/ ne= 5 1019m-3

Main studies include

Establish LHCD dominated discharges MHD stable

Use LHCD at highest possible densities

Use LH waves at high density(multiple pellets) asheating method

Synergy with ECRH at EC resonant field

Synergy with ECRH with down-shifted EC resonance

LHCD physics making use of the FEB camera

LHCD active coupling with edge ECRH


LHCD/EC Synergy (down-shifted resonance)

at BT=7.2 T, cold ECresonance(5.3T)outside vacuum vessel

EC waves damped onLH induced fastelectrons

When EC wavesapplied, substantialincrease of electrontemperature anddecrease of loopvoltage

0

0.4

0.8

102

0 m-

3

ne

#18181

0

0.2

0.4

Volt

Vloop

0

40

80

keV T

rad @ f=270 GHz

(Michelson Interferometer)

00.20.4

0.6

MW

att

PLH

PECH

1

3

5

0.4 0.5 0.6 0.7 0.8 0.9

keV

Te0

time (sec)


LHCD/EC Synergy (down-shifted resonance) ctd

Comparison between experimental dataand simulation(solid lines) for:

(a) LHCD only(0.6 MW)

(b) LHCD + ECRH (0.35 MW)

(c ) LHCD + ECRH (0.7 MW)

0

1

2

3

4

5

6

-0.4 -0.2 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0

0.7 c)

0.0 a)

0.35 b)0.0 _th

0.7 _ th

T e (

keV

)

distance from magnetic axis (m)

Symb PECH

(MW)

simulations

a )

b )c )

#18181

X-rays profiles (from FEBcamera) do not substantiallychange from LHCD only(0.62s)to LHCD+EC(0.7s)

Te from Thomson scatteringFEB camera inverted profiles


LHCD/EC Synergy (down-shifted resonance) ctd

Large fraction of EC power absorbed when a large fast electron populationis present: so far up to 55% at relatively lower density( increase with higherLHCD power ?)

Substantial increase of fast electron tails during synergy phase

0

20

40

60

80

100

120

200 250 300 350 400

T rad (k

eV)

Frequency (GHz)

#18182

LH+ECH

LH only

red: exp.

blue: theory-------------

0 .2

0.3

0.4

0.5

0.6

4.5 5 5.5 6 6.5

Pa

bs/P

inc

ne(10^19 m-3)


LHCD / ECRH Synergy (cold resonance)

At BT=5.3 T, resonance ison plasma axis

electron temperature(from Thomson scattering)on the point closest to themagnetic axis (r=3 cm)

Ip= 0.35MA,loop voltageclose to zero

At this density and withPLH high enough, plasmais MHD stable


LHCD/ECRH Synergy (cold resonance)ctd

Fast electron indicatorsshow no change withEC: no directinteraction EC/fastelectrons

Te increase on a region(15 cm) much widerthan EC resonance (~2cm): ITB formed byLHCD ?

Simulation (Peysson)assumes Bohm-gyroBohm model

From Thomson scattering, r=3cm from axis


2001 FTU Experimental Campaign

New Hardware

ECRH 4 gyrotrons by end of 2001 (1.2 to 1.4 MW) (on-going discussions to have improved gyrotrons withdepressed collector)

IBW Installation of a second launcher (mid 2001) to doublelaunched power (0.7 MW)

LHCD 6 gyrotrons being commissioned. 2nd launcherrepaired and installed. Nominal LHCD FTU power to beachieved (2 to 2.5 MW)

Boronisation system being installed

Diagnostics Improved Thomson scattering,new MHDloops, pellet tracker (Padova)


2001 FTU Experimental Campaign ctd

High Field , High density Optimisation of multiple pelletinjection. Try to control MHD events, heat with LH waves(at full power) Develop RI modes

ECRH Heat transport (heat pumping effects) and profileresiliency,Tearing mode control (optimisation and role ofECCD)

LHCD Assess plasma heating and current drive at highdensity at full power. Improve coupling in ramp-up phase.Optimise scenarios with core improved confinement (ITB)

LHCD/EC Optimisation of down-shift schemes,Development of up-shift schemes, Optimisation ofsynergy at 5.3T (coupling of more than 4 MW additionalheating power to the plasma)


2001 FTU Experimental Campaign ctd

IBW Reproduce and optimise transport barriers with higherpower and turbulence measurements. Control of IBW ITBswith LHCD profile control

Topic groups on MHD, turbulence and data consistencywill support the experimental campaign

FTU upgrades are being considered and might have animpact on the experimental programme

FTU-D modification of poloidal coils to achieve X-pointconfiguration: achievement of high beta plasmas withadvanced scenarios


FTU-D SCIENTIFIC OBJECTIVES AND PECULIARITIES

Improved confinement regimes (H89>2) (H-mode,mode with ITB)

High bN (2, 3.5) physics in a bootstrap dominatedplasma with the associated MHD stability (NTM)and j and p-profile control issues at high aspect ratio (A=5-6) high magnetic field (B=5T, or higher) high density (n >1 1020m-3), in condition of dominant electron heating (LH+ECRH) wall close to the plasma (impact on MHD limits)


FTU-D SHAPING CAPABILITY

D.N. equilibrium: B= 5T Ip=350kA a=0.19m A=5.7 κ=1.58 δ=0.8 βp=2.1

Operations also in S.N. and different B (2.5T) IP=450kA in SN IP=500kA in limiter plasmas


FTU-D Extends FTU Operations to High βN and βP

βN (~ a factor 1.5XH)

FB (~ a factor 3XH)

Power density

B=5T

βR=β/(εS)


COMPARISON FTU/FTU-D τE=H x τE 89-P

B (T) 5 5q 3.5 2.4IP (MA) 0.36 1.P (MW) 4 4H 2 1n(1020m-3) 1.5 1.5tE (ms) 32 32β 0.8 0.5βp 2.1 0.3βN 2.1 0.8FB 0.57 0.1<T> 1.77 1.0τskin 0.43 0.53

FTU-D FTU 5T for ECRH

1MA for FTU361kA FTU-D

High density

Gain in β, βN, FB

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