Effet Multipactor - Présentation Nicolas Fil...theoretical modelling using state of the art MHD and...

EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK

1MIT PSFC, 175 Albany Street, Cambridge, Massachusetts 02139, USA2Ecole Polytechnique Fédérale de Lausanne (EPFL), SPC, CH-1015 Lausanne, Switzerland3CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK4Department of Physics and Astronomy, UCI, California 92697, USA5CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France6PPPL, Princeton University, Princeton, New Jersey 08543, USA7See the author list of X. Litaudon et al., Nucl. Fusion 57, 102001 (2017).

N. Fil1, M. Porkolab1, V. Aslanyan1, P. Puglia2, S. E. Sharapov3, S. Dowson3, H. K.

Sheikh3, S. Taimourzadeh4, L. Shi4, Z. Lin4, P. Blanchard2, A. Fasoli2, D. Testa2, J.

Mailloux3, M. Tsalas3, M. Maslov3, A. Whitehead3, R. Scannell3, S. Gerasimov3, S.

Dorling3, G. Jones3, A. Goodyear3, K. K. Kirov3, R. Dumont5, G. Dong6 and JET

Contributors7

Nicolas Fil, APS DPP 2018 – Poster | 05 November 2018

Abstract

The resonant detection and measurement of the damping rates of Alfvén Eigenmodes (AEs) is of

critical importance to the design of experiments and development of models of AE stability [1].

We performed experimental measurements on JET with the AE Active Diagnostic (AEAD), and

theoretical modelling using state of the art MHD and gyrokinetic codes.

The AEAD has undergone a major upgrade [2]. It can provide a state of the art excitation and

real-time detection system thanks to its new amplifiers, filters, digital control system and to the

newly installed magnetic probes. Weakly-damped AEs have been resonantly probed with

external antennas. With GTC [3] we have simulated both stable and unstable AEs by using

equilibria and diagnostic data from JET pulses dedicated to TAEs studies. Good agreement was

obtained between simulations and experiments which adds confidence to further predictions for

next-step burning plasma experiments, including JET and ITER.

[1] W. W. Heidbrink, Phys. Plasmas 15, 055501 (2008)

[2] P. Puglia et al., Nucl. Fusion 56, 112020 (2016)

[3] Z. Wang et al., Phys. Rev. Lett. 111, 145003 (2013)

*This work has been carried out within the framework of the EUROfusion Consortium and has received funding

from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views

and opinions expressed herein do not necessarily reflect those of the European Commission.

This work has been part-funded by the RCUK Energy Programme [grant number EP/P012450/1] Support for

MIT was provided by the US DOE / DE-FG02-99ER54563, for the Brazilian group the FAPESP Project

2011/50773-0, and for the Swiss group in part by the Swiss NSF.

| PAGE 2


Physics issues

| PAGE 3

For typical fusion plasma parameters, the phase velocity of the Alfvén wave is just lower than fusion-

born alpha particle speed.

⇒ Possibility of Alfvén wave-particle resonance: effective exchange of energy and momentum with the modes. The exchange time scale is much shorter than alpha particle relaxation time, then:

⇒ self-heating process can be compromised ⇒ damages to the first wall by the ejection of highly energetic alphas

Dispersion relation with (solid) and without

(dashed) toroidal coupling of the waves

Excite AEs with an external antenna of variable frequency

[W.W. Heidbrink, Phys. Plasmas 15, 055501 (2008)]

Frequency

gap

In toroidally confined plasmas:

⇒ the index of refraction is periodic

⇒ gaps appear in the continuous spectrum

Possibility of weakly-damped Alfvén Eigenmodes (AEs)

in these gaps.

Frequency (left) and mode structure (right).


Leadership of project

Analysis software and

physics

Engineering support

Part-time PostDoc

National Instrument units for

control/digitization

Engineering support

PostDoc time for operation, analysis

and modelling work

A collaborative international project at JET

| PAGE 4

Modelling activities

Use of Gyrokinetic Toroidal Code

(GTC)

Project management

Engineering and system

integration

Redesign of the whole system

Responsible for ongoing

maintenance

Engineering of amplifiers

Control system and

commissioning

Engineering support

Filters and test modules


Aims of the project

| PAGE 5

Wide range of experiments and theoretical studies of Alfvén Eigenmodes (AEs): structure,

frequencies and stability.

The AEAD can probe stable AEs of the plasma and measure its net damping rate.

Upgraded AEAD system covers the toroidal mode number (n) range that is anticipated to be

the most unstable in ITER, 4 ≤ n ≤ 15

The AEAD will be continuously operated in the full range of isotope experiments preceding a

full DT campaign.

Modelling work, gyrokinetic simulations of AEs in collaboration with UC Irvine to supplement

the ongoing use of ideal MHD codes, such as MISHKA.

Experimental and theoretical studies of Geodesic Acoustic Modes (GAMs), Beta (Acoustic)

AEs (BAE/BAAEs) and Reverse Shear AEs (RSAEs) which are predicted to be of

importance by drift-kinetic and gyrokinetic theory.

Precise validation of the quantitative model of each of the individual damping mechanisms,

and their significance in different ITER and DEMO relevant scenarios.


Aims of the AEAD upgrade

The AEAD has gone through a major upgrade to allow for the probing of AEs in plasma

configurations which were not possible in the past due to insufficient power. Use of

independently controlled amplifiers to allow arbitrary phasing which is crucial to couple to

modes with high toroidal number n.

Individual 4kW RF amplifiers

Separate excitation & real time control of relative phase between antenna currents

Increased RF current (15A limit).

Increased frequency range of operation, 10kHz – 1000 kHz

Selection of the antennas’ toroidal spectrum for Alfvén Eigenmodes of interest with

toroidal mode number 4 ≤ n ≤ 15

New Digital Control System (named Master Driver)

To control the system and drive the currents in the individual antenna coils with the

desired frequency, amplitude and relative phase.

New set of filters for lower frequencies have been procured and commissioned last year

Study of GAMs, BAE/BAAEs and RSAEs

| PAGE 6


JET upgraded Alfvén Eigenmode Active

Diagnostic (AEAD)

| PAGE 7

8 in-vessel antennas – 18 turns

4 on each side of the torus (180o)

(2 antennas on Octant 8 unavailable)

Photo: Antennas 1 – 4 Octant 4

Matching units:

Filters available:

125 – 250 kHz

75 – 150 kHz

25 – 50 kHz

19th low pass filters

-70 dB of the 3rd harmonic

200 kHz/s frequency sweeps


Matching Unit – Low Pass Filters

19th order low pass Chebyshev filters

70 dB attenuation for third harmonic

200 kHz/s frequency sweeps

Limited at 15A and 1.1kV at feedthrough (protected

from overvoltage and overcurrent in real time)

| PAGE 8

Frequency (kHz)

Goal: -70dB

250 kHz filter50 kHz filter

Cut-off frequency


Master Driver (MD)

To control the system and drive the currents in the individual antenna

coils with the desired frequency, amplitude and relative phase.

Essentially a National Instruments (NI) PXI express chassis.

LabView Real Time (RT) and Field Programmable Gate Array (FPGA)

software performing the various amplifier control functions

Creates reference signal for up to 8 antennas

Real time control < 1𝑚𝑠Frequency control ∆𝑓 < 0.1%Phase control ∅ ± 3°

| PAGE 9


AEAD capabilities, improvement ongoing

6 switching amplifiers: power of 4kW with pulse times of 15 seconds

Operation close to feed-through limits of 1.1kV and current limit of 15A

Phase errors depends on the frequency range, target of few degrees

| PAGE 10

Operation with ASYNC shots (vacuum):

50 100 150 200 250

Freq (kHz)

I (A

)

12

10

8

6

4

50 100 150 200 250

Freq (kHz)

Z(O

hm

s)


JET restart discharges

SYNC shots (plasmas) with the 125 – 250 kHz filters:

Operation of the AEAD at the end (63s to 66s) of repeated 1.85MA/2.2T shorter He4

pulses with BreakDown in He.

Reproductive observation on these pulses, JPN ∈ 93066, 93072 (on 24/10/2018)

| PAGE 11

Similar

observations

with:

T001

T002

T007

T008

T009

H302

H303

H304

H305

Phase control disable


Antennas

Damping rate measurement

Modes appear as resonances on magnetic sensors at the

mode frequency

Damping rate proportional to “quality factor” (q) – width of

the resonance

Equivalent to damped harmonic oscillator

Transfer function:

| PAGE 12


TAEs damping rate measurements in M15-24

experiments

Damping rates of

marginally stable TAEs

have been measured in

M15-24 experiments

(Real) Frequencies

consistent with nearby

unstable modes (at

51.05s compared to

50.6s)

Too few coils to perform

reasonable mode

number analysis.

For next campaigns:

25 magnetic coils

installed during last

shutdown

| PAGE 13


Modelling activities, GTC

New collaboration with UC Irvine to work with the Gyrokinetic Toroidal Code (GTC)

Gyrokinetic simulations of low frequency AEs.

To supplement the ongoing use of ideal MHD codes, such as MISHKA.

Used to determine the structure, frequencies and stability of AEs in JET plasmas.

GTC electromagnetic simulations have bulk and “fast” ions, treated gyrokinetically or with a

reduced MHD model

Electrons are treated with a fluid-kinetic model [1] or with a fluid (adiabatic) response

| PAGE 14

[1] Z. Wang, Z. Lin et al. Phys. Plasmas 22, 022509 (2015).

Gyrokinetic ions/fast ions Fluid-kinetic electrons

- Electrostatic potential


Mode structure and growth in GTC

Equilibrium chosen when many unstable

modes are observed; n=5 chosen

Fully gyrokinetic ions, kinetic electrons and

fast ions at ~747 keV (500 keV at mode

location)

Net growth rate and frequency deduced from

the oscillations of highest amplitude poloidal

harmonic (m=11)

Antenna structure is composed of two

poloidal harmonics to resemble fast ion

mode structure (m=11, m=12)

Radial profile of each is Gaussian

Amplitude response has beat pattern which

peaks (time indicated by red cross)

| PAGE 15


Synthetic antenna (I)

Maximum amplitudes of beat pattern

(as denoted by red cross) similar to a

driven damped harmonic oscillator

Spectral response is similarly given

by the following:

Damping rates of marginally stable

modes can therefore be determined

| PAGE 16


Synthetic antenna (II)

Changing the physics mode allows the effect of different damping mechanisms to be

determined

Electron Landau damping: kinetic/adiabatic electrons

Ion Landau damping: gyrokinetic/MHD-like ions

| PAGE 17

Mechanism Drive/damping

Continuum 0%

Electron Landau -0.09%

Ion Landau -1.64%

Radiative -1.18%

Energetic

particle

+4.29%


Modes relative to Alfvén Continuum

ALCON routine calculates

continuum structure

including acoustic couplings

Modes plotted relative to

observed plasma-frame

frequency

Error bars equivalent to

FWHM

| PAGE 18


Three measurements of marginally stable TAEs made in two discharges by the Alfvén

Eigenmode Active Diagnostic (AEAD)

Frequency and damping rate deduced from transfer function:

Small number of remaining magnetic probes leads to large uncertainty in damping rate

and mode number (the latter cannot be consistently identified)

GTC comparison to AEAD, antenna measurements

| PAGE 19


GTC comparison to AEAD (I)

Multiple candidate modes must be

simulated

Labels refer to the dominant poloidal

harmonics driven by the synthetic

antenna

This signal is peaked between the two

harmonics’ rational surfaces

Closest match to the observed modes

(red lines) in plasma-frame frequency

and damping rate (n=6, m=5,6) is the

best candidate to describe the

observed mode

| PAGE 20


GTC comparison to AEAD (II)

Closest candidate modes from

GTC have been matched to the

AEAD measurements

Uncertainty in damping rate

arises from variability of the

different coils

Uncertainty in the frequency

arises from uncertainty in plasma

rotation

Error bars denote rotation

uncertainty based on

spectroscopic plasma rotation

measurements

| PAGE 21

[V. Aslanyan et al. submitted

to Nuclear Fusion]


Ongoing work on the AEAD

Improve the control of the antennas currents/voltages

Improve the phase control between the antennas

Enable the real-time tracking mode of the AEs

Target specific antennas’ toroidal spectrum for AEs of interest.

Measurements of damping rates of TAEs in plasmas with a variety of q-profiles and with

the effects of fast ions from neutral beams and ICRH.

Detection and measurements of low frequency modes, BAEs, BAAEs, GAMs and RSAEs

| PAGE 22

Magnetic signal from JPN 54895

showing GAMs.

Use of the 25 – 50 kHz filters

to observe these modes on JET

AELM can track and differentiate

modes in real time


Future work, Modelling

Extensive use will be made of the “antenna” version of GTC to determine the stability of

predicted modes.

Damping mechanisms will be identified by GTC simulations

Extensive comparisons between GTC simulations and the existing codes CSCAS, MISHKA

and CASTOR-K will be made.

Simulations will be extended to lower frequency modes, such as BAEs BAAEs, and GAMs to

form an important synergy with AEAD measurements.

| PAGE 23

Purely theoretical, not observed

experimentally: EP-driven mode

observed by changing the FI

temperature

Characteristic of BAE:

• Single m harmonic and high

growth rate, 𝜸/𝝎~𝟐𝟎%• f~40 kHz

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Effet Multipactor - Présentation Nicolas Fil...theoretical modelling using state of the art MHD and...

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