TOWARDS ICRF ANTENNAS COMPATIBLE WITH HIGH …

Wukitch – 22nd RFTC 1

TOWARDS ICRF ANTENNAS COMPATIBLE WITH HIGH PERFORMANCE PLASMAS:

CHARACTERIZATION AND MITIGATION OF ICRF ANTENNA – PLASMA EDGE INTERACTION

S.J. Wukitch, Y. Lin, J. Terry, M.A. Chilenski, T. Golfinopoulos, R. Hong, A. Hubbard, R.T. Mumgaard, R.T. Perkins, M.L. Reinke,

G. Tynan, and the Alcator C-Mod Team22nd Top. Conf. on Radiofrequency Power in Plasmas

May 31–June 2 2017; Aix en Provence, France


RF Actuators Recognized as Essential Tools for Steady State Tokamak

Efficient, steady state current drive is required to make a tokamak a viable concept for fusion electricity.

• Power required for current sustainment is a major constraint upon plant efficiency.

Ion Cyclotron Range of Frequency (ICRF) power is attractive for central current drive :

Najmabadi et al., Fusion Eng. Design 80, 2006.

• ICRF waves readily penetrates with no core density cutoff• Utilize reliable, efficient and economical sources, including high field

concepts.

For utilization, impurity contamination and low plasma-wall interaction are required.


ICRF Impurity Contamination has a Long Been a Challenge to ICRF Utilization

In tokamaks with metallic plasma facing components, various mitigation techniques have been used: low Z coatings or armor.

• Solutions do not scales to expected reactor environment.

Contamination is thought to be related to:

• Rectified RF fields that increase impurity sources.

• Increased transport via convective cells.

0.04

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0.12

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951205026&960116010

PRAD [MW]

PICRF [MW]

WMHD [MJ]

1.0

H ITER89

Time (s)

bare high Z PFC

boronized high Z PFC

Greenwald et al., Nucl. Fusion 37, 793 (1997).


Key Results

Experimental evidence suggests that minimizing RF image currents results in lower RF enhanced plasma potentials and impurity source.

• Extend ASDEX-U 3-strap antenna results to 4-strap antenna.• Suggest antenna design path forward to be compatible with high

performance plasmas.

Impurity contamination is dominated by increased impurity source rather than a change in transport or impurity penetration.

Scrape off layer plasma characteristics have significant influence on RF enhanced potentials and radial electric field structure.


Experimental Setup: Brief Overview of C-Mod ICRF Antennas

Field aligned antenna = current straps normal to the total B-field, and is helical to conform to plasma shape.

Classic antenna = straps and side protection tiles normal to the toroidal B-field and is cylindrical.

B-field line

B-field line

Field Aligned Antenna Classic Antenna

View of C-Mod Invessel Outer Wall

Define two terms:Antenna impurity source = local impurity source at the antennaFar field impurity source = impurity source away from antenna


Minimize RF Image Currents in Antenna Limiters to Reduce Plasma Potential Enhancement

ASDEX-Upgrade implemented a 3-strap antenna that enables operation without image currents.*

• Vary the ratio of center strap power to outer strap power, Pcent/Pout to minimize image currents.

• Power ratio between 1.5:1 and 2:1 minimizes the antenna impurity source.

Extend results to a 4-strap antenna.• Is this a simple and effective method

to optimize antenna performance?

Strap Currents

Imag

e Cu

rren

ts

Imag

e Cu

rren

ts

*Bobkov NF 56 (2016)


Field Aligned Antenna Configured to Allow Scan of Current Strap Ratio

Operate Field Aligned Antenna with the center two straps as one pair and the outer two straps as second pair.

• >-35 dB decoupling allows for varying Pcent/PT from 0->1.

• Monitor antenna impurity source and scrape off layer (SOL) radial electric field.

Strap 1

Strap 4

Strap 2

Strap 3

TransmitterPouter

TransmitterPcent

Decoupling stub

Center straps

Outer straps

1 2 3 4


Investigate Antenna Performance Dependence on Pcent/Ptotal Ratio

Utilize identical L-mode plasmas where:

• Power scan with constant Pcent/Ptotalratio.

• Constant power with Pcent/Ptotal ratio scanned.

• Reversing Pcent/Ptoal scan produces mirror antenna impurity source history.


Antenna Impurity Source is Dependent on Pcent/Ptotal Ratio

Antenna impurity source is minimized for 0.5<Pcent/Ptotal < 0.8.

• Small amount of power in outer straps cancels image current from center straps.

Antenna impurity source is highest for Pcent/Ptotal < 0.3.

• Antenna is dipole but all the power is excited by straps closest to antenna box. Pcent/Ptotal Ratio

Antenna impurity vs. Pcent/Ptotal

• Expect highest image current because there is no cancelation image currents.

Antenna impurity source for center dipole phase, Pcent/Ptotal > 0.8,is higher than antenna operated as four strap antenna.

• Suggests minimum in impurity source is a function of cancellation of image currents rather than reducing RF fields at the limiter.


Far Field Impurity Source Has Similar Dependence

Unexpected result – minimizing RF near field interaction with scrape off layer reduced far field impurity source.

Far field impurity source is minimized for 0.5<Pcent/Ptotal < 0.8.

At both Pcent/Ptotal < 0.3 and Pcent/Ptotal> 0.8, impurity generation is increased.

RF power fraction Pcent/Ptotal

Far-field impurity vs. Pcent/Ptotal


Heating Effectiveness has Optimum Pcent/Ptotal

Scanned Ptotal at fixed Pcent/Ptotalratio.

• Pcent/Ptotal = 0.8 leads to higher stored energy than Pcent/Ptotal = 0.2.

At fixed Ptotal and scanning Pcent/Ptotal ratio, heating effectiveness has an optimum Pcent/Ptotal ratio.

Plasma stored energy

Pcent/Ptotal Ratio

Total power Ptotal

80%20%




• Pcent/Ptotal = 0.8 leads to higher stored energy than Pcent/Ptotal = 20%.


• 0.5< Pcent/Ptotal <0.8 has the most effective heating


Pcent/Ptotal Ratio



At Ptotal ≥ 1 MW, heating strongly depends on Pinner/Ptotal• At Ptotal ≤ 0.5 MW, stored energy is independent of Pinner/Ptotal; • At higher Ptotal ≥ 1 MW, Pcent/Ptotal has significant effect.


Pcent/Ptotal Ratio


• Pcent/Ptotal = 0.8 leads to higher stored energy than Pcent/Ptotal = 20%.


• 0.5< Pcent/Ptotal <0.8 has the most effective heating


RF Enhanced Plasma Potential Monitored by Gas Puff Imaging

GPI diagnostic measures the poloidal velocity, V of the SOL turbulence.

• Monitors the radial region between ~1 cm behind the antenna tile radius to the last closed flux surface with 0.4 cm resolution.

• Maps to the corners of both the Field Aligned and Classic antennas.

In the far SOL, turbulence is convectedat the local ExB velocity, VEr/B.• V and corresponding Er are small in ohmic discharge.• Dramatic change V with application of RF.• Use V as proxy for RF enhanced plasma potential.


Ohmic

1160

7060

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0.6 0.7 0.8 0.9 1.00.20.61.0

Time (s)

4

2

PRF [MW]

E r Pro

xy

0

1

0

0.80.60.40.2

Pcent/Pcent+Pout

1

Ant

. Im

p. S

ourc

e B

II [a

.u.]

RF Enhanced Plasma Potential has Strong Dependence on Pcent/PTotal Ratio

For optimum Pcent/Ptotal ratio, V is unchanged from ohmic phase.

• Suggests no RF enhancement of plasma potential despite P=1 MW.


RF Enhanced Plasma Potential has Strong Dependence on Pcent/Pout Ratio

For optimum Pcent/Ptotal ratio, V is unchanged from ohmic phase.

• Suggests no RF enhancement of plasma potential despite P=1 MW.

For Pcent/Ptotal~ 0.5, is still present as found previously.

For Pcent/Ptotal < 0.3, RF enhanced is highest.

• Dipole antenna with straps closest to antenna box. Little image current cancellation.

RF power fraction Pcent/Ptotal

Plasma potential vs. Pcent/Ptotal

For Pcent/Ptotal > 0.9, center dipole has higher than 4-strap, dipole antenna.

• Without cancellation of image currents enhanced potential returns.


Quick Summary of Key Results

Experimental evidence suggests that minimizing RF image currents results in lower RF enhanced plasma potentials and impurity source.

• Antenna impurity and Far field impurity sources are minimized for an 0.5<Pcent/Ptotal<0.8.

• In L-mode, heating effectiveness is higher when antenna is operated for 0.5<Pcent/Ptotal<0.8.

• For optimum Pcent/Ptotal ratio, V is unchanged from ohmicphase.


Is Core Impurity Contamination a Result of Increased Sources, Transport Modification or Both

For a Classic antenna, measured molybdenum source at the antenna scales with antenna power.

• Both local antenna source rate and the core impurity concentration scale with RF power.

• Strong source when RF fields are present.

Possible causes:• Increased impurity sources due to RF-

enhanced sputtering,• Modified impurity penetration/transport,

or • Combination of these effects.

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Mo

Sour

ce ra

te

NM

O (1

014)

NMOΓMo, Antenna(1016/sec)

990805

B. Lipschultz et al., NF 2001


Impurity Penetration Factor is defined as Impurity Brightness Relative to Impurity Injection Rate

A proxy nitrogen penetration factor, PF, is defined as

PF is found by finding the scale factor between N2 flow rate and N brightness.


Inject non-recycling impurity (N2) toroidallyand poloidally localized to an active antenna in L-mode, n/nGW~0.2 discharge.

• puff location#1: maps to upper corner of antenna• puff location#2: maps to middle of antenna• puff location#3: maps to lower corner of antenna• puff location#4: HFS midplane.

Measure relative change in core trace impurity.

active antenna

J antennaor

FA-J ant

E antenna

pufflocation

1

pufflocation

2

pufflocation

3

pufflocation

4

Impurity Penetration Experimental Setup


Emission from H-like N (N VII) is taken as measure of N in core plasma.

N2 gas puff arrives at the plasma ~0.75 s.

• Colored area shows background level prior to puff.

Use a N emission scaled to RF power to derive correction for H-like N emission.

w RF

w/o RF

Increase Core N Proportional to RF Power


Emission from H-like N (N VII) is taken as measure of N in core plasma.

N2 gas puff arrives at the plasma ~0.75 s.

• Colored area shows background level prior to puff.

Use a scaled RF trace to derive correction for H-like N emission.

w RF

w/o RF

Penetration of Puffed Impurity is NOT Affected by the Presence of RF-induced Potentials

Corrected RF and ohmic heated discharges are nearly identical.• Independent of puff location.• Similar response also shown by O VIII.


Core Impurity is also NOT Affected by the Presence of the ICRF in L-modes Discharges

Be-like Ca emission from laser blow-off of CaF2 is monitored.

• Discreet injection ~1.3 s and confinement time is derived from decay of Ca intensity.

No difference between ohmic and RF discharges.Conclusion: impurity contamination is dominated by increased impurity

source.


Investigate Scaling of RF Enhanced Plasma Potentials

How does RF enhanced plasma potentials vary with RF power, magnetic field, and impurity seeding.

L-mode plasmas are investigated to avoid complications from H-mode, ELMs etc.

Focus on toroidally aligned antennas.

• FA has same behavior.

As expected, maximum RF enhanced plasma potential is proportional to PRF

1/2.



Compare RF heated discharges with constant q95.

• 2.7 T - 2nd harmonic H scenario.• 5.4 T - fundamental H minority

scenario.• 7.9 T -fundamental 3He minority

scenario.• Both 2nd harmonic H and 3He

minority are weak single pass absorption compared to H minority.

Expect poloidal velocity measured at 2.7 T to be 3 times 7.9 T case.

• Confirmed in ohmic phase of discharge.



Compare RF heated discharges with constant q95.

• 2.7 T - 2nd harmonic H scenario.• 5.4 T - fundamental H minority

scenario.• 7.9 T -fundamental 3He minority

scenario.• Both 2nd harmonic H and 3He

minority are weak single pass absorption compared to H minority.

Find poloidal velocity at 2.7 T is ~twice the measured poloidal velocity at 7.9 T.

• Suggests that Er for 2.7 T is lower than that at 7.9 T for same injected RF power.


Higher RF Induced fmax Observed at High Field, High Current

Induced peak potential is significantly larger for 7.9 T discharge.

• Difference in single pass is unlikely to be explanation: both D(3He) and H 2nd harmonic are weak single pass.

• Antenna loading was higher for higher currents – not an effect of antenna Q.






Plasma potential response to RF power is ordered by Greenwald fraction.






Plasma potential response to RF power is ordered by Greenwald fraction.

• Increasing n/nGW leads to higher density in far scrape off.

• Open question: does the impurity source decrease with increasing n/nGW?

0.1

1.0

1020

m- 3

-20 -10 0 10 20(mm)

FarSOL 0.17

0.230.280.370.43

ne /nGL-modeprofiles

Nea

r SO

L

Distance into SOL

[LaBombard, Phys. Plas. 2008]


Radial Electric Field Profile is Larger than Local Skin Depth

Er penetration length, , is 10x width calculated by slow wave penetration.

• ~1-2 cm compared to local skin depth ~1-3 mm

Model based on self-consistent exchange of transverse RF current between flux tubes predicts (Faudot PoP (2010)).

• ~(L||ci/2)1/2~BT-1/2 where L|| is connection length and ci is Larmor

radius.• For BT=5.4 T, L|| ~200 cm and ci~0.01 cm resulting in ~1 cm.

Penetration length increases with B instead of decreasing.


Radial Electric Field Profile is Larger than Local Skin Depth

Er penetration length, , is 10x width calculated by slow wave penetration.

• ~1-2 cm compared to local skin depth ~1-3 mm

Model based on self-consistent exchange of transverse RF current between flux tubes predicts (Faudot PoP (2010)).

• ~(L||ci/2)1/2~BT-1/2 where L|| is connection length and ci is Larmor

radius.• For BT=5.4 T, L|| ~200 cm and ci~0.01 cm resulting in ~1 cm.

Penetration length increases with B instead of decreasing.

Penetration length increases with injected power.


Reduce RF Induced max with Impurity Seeding

Low Z seeding has a strong influence on measured potentials with same injected ICRF power.

• Peak values of RF-induced plasma potentials decrease by about 50%.

• Scaling with RF power deviates from the PRF

1/2.


Reduce RF Induced max with Impurity Seeding

Low Z seeding has a strong influence on measured potentials with same injected ICRF power.

• Peak values of RF-induced plasma potentials decrease by about 50% at high power.

• Scaling with RF power deviates from the PRF

1/2.

Observed for a variety of seeding gases.

• Largest reduction observed at highest power and power density.

This suggests that the SOL parameters have a strong impact RF enhanced potentials for given RF power.


Conclusions

Emerging experimental evidence suggests that minimizing RF image currents results in lower RF enhanced plasma potentials.

• Antenna and Far field impurity sources, heating effectiveness, and Vhave a minimum for an optimum Pcent/Pout.

• Corresponds to range where RF image currents would be minimized.

Impurity penetration and transport experiments indicate impurity contamination is dominated by increased impurity source.

Scrape off layer has significant influence on RF enhanced potentials and radial electric field structure.

• Increasing n/nGW fraction leads to lower maximum RF enhanced plasma potential.

• Low Z seeding resulted in lower maximum RF enhanced plasma potential and deviation from PRF

1/2 scaling.

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