Experimental Pathways to Understand and Avoid High-Z Impurity … · 2016-11-08 · 1 M.L. Reinke, APS-DPP 2016 Experimental Pathways to Understand and Avoid High-Z Impurity Contamination

1 M.L. Reinke, APS-DPP 2016

Experimental Pathways to Understand and Avoid High-Z Impurity Contamination from

ICRF Heating in Tokamaks

M.L. Reinke 58th Annual Meeting of the American Physical Society Division of Plasma Physics

San Jose, CA October 31st – November 4th

S.J. Wukitch, J.L. Terry, B. LaBombard, Y. Lin, J. Wright, R. Mumgaard, A. Kuang, M. Chilenski, V. Bobkov, P. Jacquet, J. Hobirk, C. Giroud, S. Menmuir and teams from


ICRF Heating Proven, but Integration Challenge Remains • heating via Ion Cyclotron Range of Frequency

(ICRF) waves has reactor-relevant features – low-cost ($/MW), commercially available sources – existing technology spans frequency range for

high-field concepts X2,T ~ 120 MHz @ 12 T – no core density cutoff, increasing absorption

efficiency with βi

– demonstrated use for core MHD control (sawteeth) and for avoiding on-axis impurity accumulation

• decades of experience show ICRF heating linked to increased impurity contamination – critical issue H-mode tokamaks with high-Z PFCs

B.C Stratton – NF (1984) PLT

demonstrate RF antennas that can survive the plasma, and plasmas that can survive the RF


Recent Experimental Work Demonstrates Progress In Resolving Core ICRF Impurity Contamination Problem

• examples of the ICRF impurity problem and leading explanation of the proposed mechanism(s) – sputtering via rectified sheath-induced voltages and E×B convective cells

• Alcator C-Mod and JET experimental results demonstrating impurity sources are primary concern relative to E×B convective cells

• improvements in ICRF antenna design and operation that can reduce, but do not eliminate the impurity contamination

• scoping research on SOL impurity screening that could further reduce core contamination through use of high field side antennas – sensitivity of core high-Z build-up to enhanced confinement regime

emphasis on simple experiments that demonstrate concepts and progress


Experience with ICRF on Metallic Divertor Tokamaks M.-L. Mayoral et al 2014 Nucl. Fusion 54 033002 • ICRF impurity story on JET

– in JET-C, ICRF linked to increased Ni, and JET-ILW, ICRF linked to W despite having Be main/RF limiters

– H-mode scenarios are not dominantly RF-heated, and JET ‘lives’ with this and uses ICRF for sawtooth/impurity control


Experience with ICRF on Metallic Divertor Tokamaks • ICRF impurity story on JET

– in JET-C, ICRF linked to increased Ni, and JET-ILW, ICRF linked to W despite having Be main/RF limiters

– H-mode scenarios are not dominantly RF-heated, and JET ‘lives’ with this and uses ICRF for sawtooth/impurity control

• ICRF impurity story on Alcator C-Mod – all high-Z (molybdenum) PFCs including

main/RF limiters + divertor – boronization required sustain H-modes at

high stored energy, keep PRAD low – decay in performance correlated with

integrated RF joules through antennas – boronization not required for I-mode (low 𝜏𝑍)

B. Lipschultz Phys. Plasmas 13,

056117 (2006)

removal of boronization


Leading Explanation for Mechanism(s) Responsible INCREASED SOURCE

(sputtering via sheath rectified voltages) • on field lines seeing ICRF AC potential, plasma surfaces have in imbalance between e- and i+ losses due to the I-V curve

see: F. W. Perkins, Nucl. Fusion 29, 583 (1989).


Leading Explanation for Mechanism(s) Responsible INCREASED SOURCE

(sputtering via sheath rectified voltages) • on field lines seeing ICRF AC potential, plasma surfaces have in imbalance between e- and i+ losses due to the I-V curve

• a DC potential develops to balance the losses, ~ 100 V – moves to dominant D+ sputtering

• picture can also hold off of antenna linked field lines if single pass absorption is low (far field sources)

see: F. W. Perkins, Nucl. Fusion 29, 583 (1989).


Leading Explanation for Mechanism(s) Responsible

– DC potential to balance AC losses

INCREASED SOURCE (sputtering via sheath rectified voltages)

INCREASED TRANSPORT (radial flux via E×B convective cells)

– potential structure from antenna gives rise Eθ and Er, crossed w/ BT

– partially observed on multiple tokamaks

see: D. A. D’Ippolito, Phys. Fluids B 5 3603 (1993).


Proposed Experiment to Decouple Sources and Transport • avoid direct measurement of complex, 3D

boundary physics phenomenon – develop a simple, repeatable, transferable test

• inject non-recycling impurities (N2) toroidally and poloidally localized to an active ICRF antenna – move puff location relative to antenna or change

active antenna (if you have more than one) – perform in L-mode, avoid confusion w/ ETB & ELMs

• measure relative change in core using single chord VUV or charge-exchange spectroscopy

• results could be followed up with validation work – ex: D2 injection on loading [Zhang – NF (2016)]


• D(H) heating at 80 MHz in q95 ~ 4.4 plasmas at Bt=5.4 T and n/nGW ~ 0.2

• a single dipole antenna was powered and puff rotated around antenna

• N2 injected via slow capillary gas feeds and H-like nitrogen (N VII) observed by radially viewing XEUS spectrometer

C-Mod Experimental Setup





• puffed on RF shots and Ohmic references – comparison shows active component and an RF

induced intrinsic source, demonstrated by O VIII – observed far SOL change in N II at gas puff

C-Mod Experimental Methods








• use a scaled RF trace to derive correction








• use a scaled RF trace to derive correction • corrected RF & Ohmic are nearly the same


C-Mod Experiments Show Insensitivity to Puff Location

field line mapping of puff locations to active antenna


Similar Experiments Conducted on JET in April 2016

• D(H) heating at 42 MHz in L-mode plasmas at q95 =4.0, Bt=2.6 T and n/nGW = 0.34

• single gas puff (outboard midplane) powering different dipole antennas, NEAR (A+B) and FAR (D) – +/- 90o phasing also checked

against standard, 180o phasing

• fully stripped nitrogen measured with charge-exchange @ r/a < 0.88 (modulating a single PINI)

results summarized in V. Bobkov, NME (2016)


Results Show Weak Dependence on Active Antenna and Phase

(both heating phase 180o) • subsequent shots changed active RF antenna, small differences observed

• compare 𝑃𝑃 ≡ Δ𝑁𝐶𝐶𝐶𝐶/Γ𝑁𝑁before and after puff and normalize – PFNEAR/PFFAR = 0.88 +/- 0.12

NEAR PUFF

FAR FROM PUFF


Results Show Weak Dependence on Active Antenna and Phase

• subsequent shots changed active RF antenna, small differences observed

• compare 𝑃𝑃 ≡ Δ𝑁𝐶𝐶𝐶𝐶/Γ𝑁𝑁before and after puff and normalize – PFNEAR/PFFAR = 0.88 +/- 0.12

• comparing different phasing also shows changes for antenna far from puff – PF+90/PF180 = 0.90 +/- 0.15 – PF-90/PF180 = 0.66 +/- 0.11

• weak, but measurable effect at -90 deg. phase, overall results similar to C-Mod

180 deg -90 deg +90 deg ALL FAR FROM PUFF


Solutions to Core Impurity Problem Linked to Reducing Impurity Sources: The C-Mod Field Aligned Antenna • does not eliminate role of convective cells in setting antenna survivability • use novel antenna engineering and operation to reduce impurity sources

– similar focus on antenna design for impurity source reduction @ AUG [Bobkov – NF 2016]

‘Field Aligned (FA) Antenna’ 78 MHz quadrupole

straps ⊥ to magnetic field

𝑩

‘Toroidally Aligned (TA) Antenna’ two 80MHz dipoles

straps ⊥ to toroidal field

𝝓

For more information: S.J. Wukitch, PoP 20 05611 (2013) NO4.00008 (Wed. AM)

http://www-internal.psfc.mit.edu/research/alcator/pubs/index.htm

http://www-internal.psfc.mit.edu/research/alcator/pubs/index.htm


Field Aligned Antenna Reduces But Does Not Eliminate Core High-Z Contamination

• ‘fiducial’ shots run interleaved with other experiments as integrated energy through both ICRF antennas increased after boronization – 0.7 MA, EDA H-mode which could be sustained with power from single antenna (1.7 MW) – alternated shots with long FA phase and long TA phase (8 total plasmas)

• H-modes using field-aligned antenna show consistently lower Mo than those using toroidally aligned antenna

• Mo level continues to rise in H-modes using the FA antenna – H-modes still require wall

conditioning for high performance


Difference in Core Mo Content Linked to TA Antenna Limiter

• Mo I source at TA Limiter is higher when TA is powered and increases with integrated usage – supported by comparison with B II emission


Difference in Core Mo Content Linked to TA Antenna Limiter

• Mo I and B II at FA limiter are similar when either TA or FA antenna is powered – rotating appears to mitigate local limiter source


Divertor and Main Limiter Mo Sources Grow Similarly for Both Antennas

• main-chamber limiter Mo I continues to increase as boronization is removed – does not appear to

saturate, but similar source rate for each antenna

• outer divertor Mo I emission saturates, while core Mo continues to rise – good screening, consistent

w/ history [Lipschultz, 2001]

MAIN LIMITER

OUTER DIVERTOR


Further Reduction in Core Contamination May Be Possible by Moving ICRF Antenna to High-Field Side

is the low-field side the best place for ICRF antennas? • placement on the high field side where prior work

suggests better impurity screening [McCracken, PoP (1997)]

– quiescent SOL, no ELMs or energetic ion losses – low neutral pressure allowing increased RF voltages

• reactor relevant D-T-(3He) minority heating schemes – nHe-3/ne ~ 1-2% and fICRF = 2ΩcT = ΩcHe-3

– D-T- (3He) has high single pass absorption independent of temperature helping to mitigate far field RF impurity sources

engineering challenge, motivated by physics B. LaBombard et al 2015 Nucl. Fusion 55 053020 G.M. Wallace, AIP Conf. Proc. 1689, 030017 (2015) http://www-internal.psfc.mit.edu/research/alcator/pubs/APS/APS2014/Bonoli_APS-invited_2014.pdf

put R

F an

tenn

as h

ere


Experiments Measure Relative Screening of HFS vs. LFS Impurity Sources as Topology and Regime are Varied

1150730027

0.6 0.8 1.0 1.2 1.4 1.6 Time (s)

-0.002 0.000

0.002

0.004

0.006

0.008

A U

NVII

Model * 0.00478, Tau => 0 ms

1160615014

0.6 0.8 1.0 1.2 1.4 1.6 Time (s)

-0.002 0.000

0.002

0.004

0.006

0.008

A U

NVII Model * 0.101, Tau = 70 ms

• puff trace N2 from low-field side (LFS) and high field side (HFS) midplane on separate shots, measure core nitrogen evolution using VUV spectroscopy – known influx rate (𝛤𝑍) from calibrated puffs into empty vacuum vessel – comparing the same plasma, compare penetration factors (𝑃𝑃) from VUV spec.

𝝏𝝏𝝏𝝏𝝏

= 𝑷𝑷 ∙ 𝚪𝝏 −𝝏𝝏𝝉𝝏

𝝏𝝏 𝝏 = 𝑷𝑷 ∙ 𝒎𝒎𝒎𝒎𝒎 𝝏

𝒎𝒎𝒎𝒎𝒎 𝝏 = 𝒎−𝝏/𝝉𝝏 𝚪𝝏𝒎𝒔/𝝉𝒛𝝏𝒔𝝏

𝟎

𝝏𝑽𝑽𝑽 𝝏 = 𝑷𝑷𝑽𝑽𝑽 ∙ 𝒎𝒎𝒎𝒎𝒎(𝝏)

N VII from XEUS

EDA H-mode

Ohmic

N2 GAS PUFF

N2 GAS PUFF 𝜏𝑍 from CaF2 laser ablation


Ohmic Screening Experiments Demonstrate Important Balance Between Parallel Flows and E×B Flows in SOL • HFS impurities up to ~x5

better screened than LFS • poorest HFS screening does

not occur in DN • results show effects, which

can compete or add – strong LFS→HFS flows

(measured with probes) – dispersal via E×B drift

(from N V emission patterns)

• new Rev. B discharges confirm hypothesis, showing screening at SSEP=-5 mm

LFS puff

HFS puff

more details see: B. LaBombard – NME (2016) http://dx.doi.org/10.1016/j.nme.2016.10.006

http://dx.doi.org/10.1016/j.nme.2016.10.006


Screening Strongly Depends on Confinement Regime • in I-mode, factor of ~2 better

screening on HFS vs. LFS • E×B works w/ parallel flows

I-mode EDA H-mode

• EDA H-mode, larger E×B drift • in favorable topology, HFS

screening is much worse?

see: B. LaBombard IAEA (2016)


Conclusions and Implications • N2 gas puff experiments demonstrate RF-induced sources are more important than RF-

driven E×B convective cells for high-Z contamination – empirical approach on JET and Alcator C-Mod, transferable to other devices

• innovative design of ICRF antennas, such as field aligning, helps reduce antenna limiter sources, but does eliminate issue of core high-Z build-up – in C-Mod EDA H-modes boronization still required to constrain core Mo levels

• further reduction may be possible by locating ICRF antennas on the high field side, gas puff experiments allow impurity screening to be explored – balance of parallel and ExB flows controls penetration factors, which depends on

topology and confinement regime

• results combine to form design criteria for the use of ICRF in devices which will be unable to use wall conditioning techniques and will rely more heavily on ICRF (WEST, ITER, high-field tokamak concepts)

• also demonstrate progress in solving a long-standing problem linked to ICRF using simple experimental tests to gain further insight into important physics mechanisms

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Experimental Pathways to Understand and Avoid High-Z Impurity … · 2016-11-08 · 1 M.L. Reinke, APS-DPP 2016 Experimental Pathways to Understand and Avoid High-Z Impurity Contamination