Research Thrust for Reliable Plasma Heating and Current ...€¦ · Confinement Experiments and...

Research Thrust forReliable PlasmaHeating and CurrentDrive using ICRF

J.B.O. Caughman, D.A. Rasmussen,L.A. Berry, R.H. Goulding, D.L. Hillis,P.M. Ryan, and L. Snead (ORNL), R.I.Pinsker (General Atomics), J.C. Hoseaand J.R. Wilson (PPPL)

2 Managed by UT-Battellefor the U.S. Department of Energy

• Research Thrust Summary

– Address issues needed to ensure reliable ICRF H&CD

– Integrated modeling effort to understand RF

interactions from the antenna to the core

– Validation of the models using dedicated test stands

and facilities

• near-field RF plasma interactions

• RF breakdown

• PMI issues

• existing/upgraded confinement devices.

• Issues and Gaps with ICRF Antennas

• Scientific and Technical Requirements to

Address the Issues

• Summary

Research Thrust for Reliable PlasmaHeating and Current Drive using ICRF


Issues and gaps with ICRF Antennas

• Plasma heating and current drive using the Ion Cyclotron Range ofFrequencies (ICRF) are important elements for the success of fusion

– Most DEMO concepts identify ICRF as the main heating system

– Launching structures for ICRH or LHCD must operate in a high radiation,high heat-flux environment

– The exposed antenna surfaces must be resistive to high heat (1-10MW/m2) and neutron fluxes with acceptable levels of impurity production

– DEMO will require operating at ~700 °C

• The issues and gaps cut across the disciplines of plasma theory andsimulation, RF technology, materials, diagnostics, and reactorengineering (reliability and maintenance)


Issues and gaps with ICRF Antennas• Many of the issues concern the interaction of the near field of the antenna

with the plasma in the scrape of layer (SOL)

– The RF plasma sheath depends on the antenna structure/phasing

– The resulting hot spot formation and enhanced local erosion serves as a localimpurity source and can degrade core confinement

– The parasitic RF losses in this region include edge modes, parametricinstabilities, and non-linear wave-particle interactions.

• RF breakdown/arcing is one of the main power limiting issues withoperating the antenna in the plasma environment and is poorly understood

– The anticipated large outer gap on DEMO (and ITER) and the resulting impacton loading will likely push operating voltages to at least as high as the ITERdesign limit of 45 kV

– The interactions of the plasma with the antenna surfaces (including ELMs)result in particles and local gas load that affect breakdown and loading

• The antenna structure and Faraday shield will likely be constructed fromlayered or coated materials and the behavior of these structures in anuclear environment is a concern


Issues with coupling through the SOL

• Many of the issues concern theinteraction of the near field of theantenna with the plasma in thescrape of layer (SOL)

• The RF plasma sheath dependson the antenna structure/phasing

• The resulting hot spot formationand enhanced local erosionserves as a local impurity sourceand can degrade coreconfinement

• The parasitic RF losses in thisregion include edge modesparametric instabilities, and non-linear wave-particle interactions.

C-Mod

NSTX


RF Breakdown and powerhandling Issues

• RF breakdown/arcing is one ofthe main power limiting issueswith operating the antenna inthe plasma environment and ispoorly understood

• The anticipated large outer gapon DEMO (and ITER) and theresulting impact on loading willlikely push operating voltages toat least as high as the ITERdesign limit of 45 kV

• The interaction of the plasmawith the antenna surfaces(including ELMs) and the role ofthe resulting particles and localgas load on breakdown andloading


Uncertainty in plasma density profiles

• The plasma density profile in the scrape-off region between the separatrixand the antenna determines the loading (how easy it is to couple power tothe plasma)

• It is probably the biggest uncertainty in the ICH system design

– Can cause factor > 3 uncertainty in power-handling ability

Plasma density profiles for 3 antenna-separatrix gaps for ITER Scenario 2 (burning, Q = 10) plasma

0.01

0.1

1

10

0 5 10 15 20

8 cm13 cm17 cm

Density (

10

19 m

-3)

Distance from antenna (cm)

Propagation begins for kz 4 m-1


Short-decay density profile

0.01

0.1

1

10

0 5 10 15 20

8 cm13 cm17 cm

Density (

10

19 m

-3)

Distance from antenna (cm)



Long-decay density profile


Layered materials in a nuclear environment

• The antenna structure and Faraday shield

will likely be constructed from layered or

coated materials, and the behavior of these

structures in a high temperature nuclear

environment is a concern

• Production of radiation-induced defects

(vacancies, interstitials, traps,…)

• Changes of microstructure – including

surface

• Change of chemical composition

(transmutation, He production)

• Degradation of IC material properties:

ductility (hardening)

He embrittlement

Thermal conductivity

Increased swellingSee Theme IV talks by Kurtz, Stoller, etc.

Grain boundary


ITER- ICRH antenna – FS element designCross section through the bar

Be

CuCrZr

Stainless steel

10/8 SS tubes

30 mm

8

4

10

mm

Manufacture : HIP or brazing

Protection bars

Left/Right

Protection

bars

Top/bottom Faraday

Screen Bars

G. Agarici – 12/12/2007 Cadarache


Scientific and Technical Requirementsto Address the Issues

• There are fundamental gaps in our understanding of the RF/plasmainterface

• Reliable and predictable antenna operation will not be possibleunless these gaps are closed

• The approach should include:

– Integration of fully 3-D heating codes with realistic antenna andconfinement device geometry is needed to understand the coupling ofRF power from the antenna to the core

– Fundamental understanding of factors that will limit antenna operation,such as ELMs, parasitic losses, and breakdown

– Nuclear materials issues

• An integrated effort will need fundamental improvements in modelingcoupled with experimental validation of the models on both dedicatedtest stands and on confinement devices.


-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

z (m)

Re(E//) (V/m for 1V @ feeder), x=5mm

-1 -0.5 0 0.5 1

1.5

-1

0.5

0

0.5

1

1.5 Upper box corner zone

Lower box corner zone

Modeling Effort Requirements

• The modeling effort requires integration of many existingcodes as well as improvement in or construction of newcodes

• Sheath and near field modeling need 3D antennastructures with self-consistent current distribution in thepresence of anisotropic magnetized plasma in the scrapeoff region

• Models need to be able to couple wave interactions fromthe antenna surfaces to full wave propagation/absorptionin the core plasma

• The models must also be able to explain why the plasmaoperation degrades vacuum voltage standoff, includingELM effects

• Material modeling is needed, including the effects of highneutron fluxes, tritium retention, erosion expectations,joining/bonding of materials, and heat transfer at elevatedtemperatures

• Modeling components need to be validated onConfinement Experiments and Test Stands (RF and PMI)

TOPICA Model


RF Test Stand Validation

• An RF test stand will allow for the detailed study andunderstanding of antenna/plasma interactions withoutimpacting valuable confinement machine time until themajority of the model inputs are verified, including:

– SOL interactions, including RF sheath dynamics, antenna phasingeffects, hot spot formation, localized erosion, transport along andacross magnetic field lines, and wave-particle interactions

– Detailed/accurate measurements of plasma density, electrontemperature, and potentials in the vicinity of the antenna, togetherwith IR measurements to determine power fluxes

– Arcing/breakdown issues can also be addressed where multipleparameters can be controlled and tested

– New diagnostics, control models, and antenna concepts can bedeveloped and operationally verified before implementation on aconfinement experiment

– A dedicated RF test stand will allow for easy access and rapidchanges in antenna geometry or test conditions


RF Test Stand Requirements

• The plasma can be created by a variety ofmethods, including high-field launchedmicrowaves at 28 GHz (or 53.2 GHz) or by usinga helicon-based plasma source

• Long pulse operation (several minutes) isdesirable for testing realistic antenna conditions

• A secondary plasma source, such as a plasmawasher gun, could be located on a flux tubeattached to the test antenna and pulsed tosimulate ELMs

• Plasma volume on the order of a cubic meter at a density of ~1018/m3

with a magnetic field strength of ~1 Tesla

• Test region large enough to insert a moderate size two-strap variablephase antenna (30 cm wide, 60 cm high)

• Magnetic connection lengths of ~1 meter, and numerous plasmadiagnostics


• PMI Test Stand

– Validate materials models to ensure qualified solutions.

– Main issues include

• thermal stresses at the joints subjected to high heat and neutron fluxes

• cracking due to voids in brazes

• erosion of coatings, and similar issues

• Requirements [e.g. as in Hillis talk]

• High incident heat fluxes (1-10 MW/m2)

• Ability to handle neutron irradiated materials at elevated temperatures

PMI Test Stand


Neutron Irradiation Alters Bulk PFC Properties

• Need to test neutron irradiated and toxic plasma facing and internal

components under high heat loads and plasma exposure to prepare

for DEMO

– PMI processes (Erosion, fuel retention and dust formation)

– Thermo-mechanical properties of PFCs (fatigue, shock resistance)

– Modification of bulk heat transfer properties

– Testing of the integrity of coatings, brazes, welds for cooling lines, etc. for

internal components

• Need to perform tests under simultaneous high heat loads ~ 10

MW/m2 and high surface temperature ~ 700 0C


• Validation on Confinement Experiments

– Requires significant operational time on confinement experiments

– Allows study of wave propagation, power absorption, and wave interactions with

closed flux surfaces

– Begin with existing antennas in these machines and do targeted experiments

with full set of relevant diagnostics

– Add new or modified antenna structures DESIGNED to test and validate

predictions of plasma-wave interactions, plasma heating/current drive, parasitic

losses, large-gap coupling issues, impurity generation, elevated temperature

operations, and diagnostic/control strategies.

– Build new antennas for operation and validation that include all aspects in a

fully nuclear environment at elevated temperatures and long pulse lengths

• The neutron damage and tritium retention in layered/coated materials in the antenna

structure may cause blistering, swelling, and potential delamination of these materials.

• The potential for RF breakdown and coolant contamination would be greatly increased

and needs to be explored.

Scientific and Technical Requirements toAddress the Issues on Confinement Experiments


The ICRF Thrust crosses the Matrix

• Test RF Antennas in a nuclear

environment• Validate nuclear capable RF Antennas

on CTF

• Integrated long pulse tests of innovative

approaches

* Validate edge loss processes in existing

devices and determine power flow in the

SOL

Existing/Upgraded/New

Non-DT Confinement FacilitiesNew DT Confinement Facilities

Existing/Upgraded/New

Test Stands• Validate techniques for computing

heating performance and self-consistent

heat and particle fluxes to high-power,

energized RF Antenna, which interact

with and alter the edge plasma, including

RF breakdown in the antenna structure

• Test nuclear-capable RF Antennas on

non-nuclear facilities

• In-situ tests of material surface dynamic

response

RF Antennas (ICRF, LH) Reliable and verified techniques for: •Develop/extrapolate/innovate new

concepts (SOL work needed for this step)

• Compatibility with cw high heat

• Compatibility in nuclear

• Low impurity generation

• improved models of RF wave

• RAMI

• Innovation

* Computing all wave processes that can

lead to edge losses in SOL

Gaps & Issues by Topic Area

• Sufficient EM-plasma coupling

without arcing

Technology DevelopmentPhysics. Theory & Modeling

• computing self-consistent heat and

particle fluxes to high-power, energized

components which interact with and alter

the edge plasma.

• Qualify structural, shield and coating

material with which to construct RF

Antennas, including joining/bonding

technologies.• Modeling and development of new

materials

• Tests of coating techniques and

advanced refractory alloys and metal

doping

* Computing process that contribute to RF

Breakdown in the antenna structure


Summary: Needs for ICRF on DEMO tofill the gap

• Benchmarked predictions of SOL parameters and their impact onloading, RF-edge interactions, and materials

– Temperature, density, and neutral profiles

– Perp and parallel heat fluxes to Faraday Shield

– Verified sheath models

• Validated power coupling model from antenna to core plasma

• RF breakdown tolerance with an arc protection and arc/ELMdiscrimination system

• Load tolerant matching with advanced feedback control

• Demonstrated methods for large gap coupling (gas puffing, etc.)

• High temperature gas cooled FS, strap, and structural componentsqualified for operating in a steady state, fully nuclear environment

Research Thrust for Reliable Plasma Heating and Current ...€¦ · Confinement Experiments and...

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