Field Aligned ICRF Antenna Design for EAST *

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1 Field Aligned ICRF Antenna Design for EAST * S.J. Wukitch 1, Y. Lin 1, C. Qin 2, X. Zhang 2, W. Beck 1, P. Koert 1, and L. Zhou 1 1) MIT Plasma Science and Fusion Center, Cambridge, MA USA. 2) Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui P.R. China Key Results: EAST has a goal to reliably couple 70% of source power into plasma for >100 s. Increasing coupling efficiency is critical. Proposed EAST field aligned antenna is based on successful C-Mod field aligned antenna design. Utilize end fed center grounded antenna straps similar in length as existing I antenna. Antenna strap spacing is 350 mm on center k =9 m -1 and coupling efficiency ~45%. Field aligned antenna provides path towards reliable, high power antenna operation. Inherently load tolerant enables high coupled power. Achieves near elimination of RF enhanced heat flux and antenna impurity no need to use low Z coatings. *Work supported by US DoE award DC-SC and Chinese National Magnetic Confinement Fusion Science Program No.2015GB st RF Top. Conf. 1

2 Successful ICRF Utilization Faces Several Challenges EAST has a goal to reliably couple 70% of source power into plasma for >100 s. Antenna performance has strong influence on reliable power delivery and heating effectiveness. Coupling high coupling efficiency through SOL. Load tolerance critical to handling ELMs and confinement transitions Impurity production seek to minimize. Robust long-distant coupling gas puffing can be useful tool. Voltage and power handling 15 kv/cm for E B Antenna conditioning minimize time required to obtain high power For optimum performance, good core absorption scenario should be identified Seek strong single pass absorption. Avoid cavity modes. And antenna coupling efficiency should be maximized. 21st RF Top. Conf. 2

3 Antenna General Specifications ICRF antenna power capability: 4.2 MW into ELMy H-mode CW bandwidth: MHz transmitter bandwidth 2.5 T operation H minority is 38 MHz, 2nd harmonic is 76 MHz 3 T operation H minority cyclotron frequency = 45.6 MHz Maximum average electric field is not to exceed 15 kv/cm where E B. Maximum voltage 45 kv. Mechanical stress based upon disruption: 0.2 MA/s Bake out temperature: 300 C Preferred antenna strap is end fed, center grounded RF antenna limiter: m Follow 5 mm behind plasma limiter and LCFS Antenna to operate CW. Current straps, Faraday shield and protection tiles to be actively cooled. Coolant: water Coolant pressure: 3 atm Coolant flow velocity: 2 m/s 21st RF Top. Conf. 3

4 Core Absorption: High Single Pass Absorption for H Minority Estimate minority H heating scenario single pass absorption with analytic formula and include impact of energetic ions. 1 kev and 5 kev curves represent thermal plasma single pass absorption. 10 kev tail shows how quickly the minority energy impacts single pass aborption. Single pass absorption (%) kev is typical minority ion energy C-Mod k =10 m -1 EAST k =4.8 m -1 EAST k =9 m kev 5.0 kev kev 10.0 kev 5.0 kev 1.0 kev n H /n e (%) For reference, we show C-Mod values which has demonstrated high heating efficiency. For k =4.8 m -1, the single pass absorption is similar to C-Mod conditions and expect good absorption. For k =9 m -1, single pass absorption is approaching 40% for 1 kev plasma. 21st RF Top. Conf. 4

5 Edge Density Profile Determines Antenna Loading Edge plasma density profile determines the antenna resistive loading. Sets the distance to propagation and Determines the transmission impedance. Antenna geometry determines antenna reactance. Modified by plasma - breaks symmetry of the off diagonal terms in the impedance matrix Cut-off density Plasma load variations are encountered R major [cm] during confinement transitions and edge localized mode (ELMs) activity. Antenna coupling efficiency is estimated by. 21st RF Top. Conf. 5 n e [m -3 ] LCFS Near SOL Scrape-off Layer Main Plasma Limiter Far SOL Limiter Shadow For C-Mod both Classic, 11 m -1, and FA antenna, 14 m -1, have similar coupling efficiency. Distance to cutoff is ~2.2 cm (cutoff density is 5x10 18 m -3 ). Classic =0.77 and FA =0.71. R. Bilato, Nucl. Fusion 45, L5 (2005). Antenna Limiter

6 For EAST, Improving Coupling is Critical EAST antenna coupling has low coupling efficiency. I port antenna: k =14.4 m -1 for [0,,0, ] and has cutoff density, n ecut ~9x10 18 m -3. =0.28 B port antenna: k =12.6 m -1 for [0, and has n ecut ~6.4x10 18 m -3. =0.33 Electron Density [m -3 x10 19 ] Cutoff Density ~9x10 18 m T, Ip= 373 ka, r/a Operate 4 strap antenna in current drive [0, ] and /3 phase [0, ] to improve coupling efficiency. CD phase k =0.072 cm -1, n ecut ~2.2x10 18 m -3, and =0.53. /3 phase k =0.048 cm -1, n ecut ~1x10 18 m -3, and =0.65. For new antenna, attempt to minimize k while maintaining good SPA. 21st RF Top. Conf. 6

7 Current Strap Layout for Conceptual EAST FA Antenna Antenna straps are to normal to total magnetic field, field line pitch is 7. Current strap is end fed-center grounded. Strap length 700 mm I and B antenna straps are ~640 mm. Folded strap is contingent design. Current straps are 350 mm on center. Corresponds to k ~9 m -1. Coupling efficiency ~45% assuming distance to cutoff is unchanged. n ecut ~4x10 18 m -3 bottom of pedestal Antenna is to replace I port antenna. Antenna structure will occupy wall space between H and J port without interfering with H or J port. Limits strap spacing; thus coupling efficiency. 21st RF Top. Conf. 7

8 Vacuum Transmission Line Behind Back Plate For end fed/center grounded, coaxial to strip line feeds are a challenge. Standard 6 coax may fit if current EAST feedthrus are used. If the feedthrus are moved closer to the back plate, new feedthrus will be required. Design will be similar to the feedthrus used on the C-Mod FA antenna. Position coax feeds to maximize clearance between neighbors. 21st RF Top. Conf. 8

Justification for Field Aligned Antenna Concept Field Aligned Antenna B-field line View of C-Mod Invessel Outer Wall Classic Antenna B-field line Field aligned antenna utilized symmetry along B-field

9 Justification for Field Aligned Antenna Concept Field Aligned Antenna B-field line View of C-Mod Invessel Outer Wall Classic Antenna B-field line Field aligned antenna utilized symmetry along B-field line to reduced unwanted parallel RF electric fields. Field aligned antenna is distinguished by current straps that are normal to the total B-field. And is helical to conform to plasma shape. Classic antenna has straps and side protection tiles normal to the toroidal B- field and is cylindrical. 21st RF Top. Conf. 9

10 Field Alignment Antenna is Inherently Load Tolerant Reflection coefficient is the square root of reflected power to forward power. Field Aligned antenna reflection coefficient occupies less area than Classic antenna. Impedance variation is reduced. Impedance variation depends primarily on the real part of the antenna load. Reflection coefficient from Classic antenna occupies a large phase and amplitude range. 21st RF Top. Conf. 10

11 Speculate Antenna Impedance Matrix Becomes Symmetric Generic antenna impedance matrix: a 11 and a 22 are the plasma resistive load L 11 and L 22 are the strap self inductance M 12 and M 21 are mutual coupling b 12 and b 21 represent the coupling through the plasma particularly E. Field alignment significantly reduces asymmetry in impedance matrix. b 12 and b 21 become negligible. Density profile changes should only result in changes in resistive load. 21st RF Top. Conf. 11

12 ELMs Loading Perturbation is Largely Resistive for Field Aligned Antenna Edge density profile evolution with ELMs provides opportunity to test hypothesis. Changes distance to cutoff and propagation in SOL. Expect field aligned antenna to have only resistive change in reflection coefficient. Field aligned antenna impedance change is due resistive change. Classic antenna reflection coefficient phase and amplitude both vary. Allows for fixed pre-matching to reduce region of high VSWR. 21st RF Top. Conf. 12

13 Load Tolerance and Pre-Matching Improves Coupled Power Mechanical Stub Tuner and Phase Shifter Fixed Stub Antenna 11 no pre-match Field Aligned Antenna VSWR 2 MW Amplifier 9 ~ Decoupling Stub VSWR 7 5 with pre-match 2 MW Amplifier Discharges On C-Mod, fixed matching stub was installed to lower reflected power in unmatched line. Prior to installation of stub average reflected power is 65%. Average reflected power after installation is ~17%. Maximum achieved power increased to 3.7 MW (4 MW source). Can utilize active matching to maintain reflected power below 5%. 21st RF Top. Conf. 13

RF Enhanced Heat Flux Leads to Localized Peak Heat Flux RF enhanced heat flux is the heat flux that appears on the energized ICRF antenna. Heat flux appears when antenna is energized.

14 RF Enhanced Heat Flux Leads to Localized Peak Heat Flux RF enhanced heat flux is the heat flux that appears on the energized ICRF antenna. Heat flux appears when antenna is energized. Universally observed but antenna design dependent. For ITER, the ICRF antenna design specification has specified a RF enhanced heat flux of 1.5 MW/m 2 and 0.156% injected power deposited onto the energized antenna. JET has measured between 2-10% injected power on the antenna. Tore Supra has found ~3.5% for their classical antennas. Visible light image of Classic C-Mod antenna shows interaction on the top and side tiles. 21st RF Top. Conf. 14

15 Eliminated RF Enhanced Heat Flux to FA Antenna Analyzed the thermocouple data over a three month operational period. Define reference discharges as discharges where the Classic antenna power >90% of total injected RF joules (blue squares). Compare with discharges where the FA antenna power injects >70% of injected RF joules (red squares). Discharges heated with the FA antenna have lower total energy deposited on FA antenna. Removes uncertainty in designing cooling capability and potential failure due to thermal overload. Estimated Energy [kj] Energy Deposited onto FA Antenna Classic Antenna Field Aligned Antenna Injected RF Energy [MJ] 21st RF Top. Conf. 15

16 ICRF Impurity Contamination is Ameliorated with Low Z Armor Antenna impurity source has been correlated with core contamination. Strong impurity source from the antenna is observed when the antenna is energized. 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. From design perspective, use low Z material armor to ameliorate. Typically a coating that is difficult to verify thermomechanical properties. Flaking is problematic. Mo Source rate Antenna Molybdenum Source and Contamination in L-Mode Plasma with RF Heating N MO Γ Mo, Antenna (10 16 /sec) Antenna Power (MW) N MO (10 14 ) B. Lipschultz et al., NF st RF Top. Conf. 16

17 Local Antenna Impurity Source is Eliminated for FA Antenna Strong molybdenum source at the Classic antenna when the Classic antenna is powered. Mo source at the Classic antenna increases with each power step. Molybdenum source at the Field aligned antenna when the Field aligned antenna is energized is no higher than the reference. FA antenna impurity source is low despite strong local RF fields. Important advance: antenna can be made from reactor compatible plasma facing materials Mo I Source [a.u.] Field Aligned antenna P ICRF [MW] Classic antenna reference Time (s). No coating is required of Faraday screen or protection tiles. For EAST Tungsten or molybdenum rods would tolerate thermal loads, edge fast electrons, better than present B4C-copper-stainless steel rods. 21st RF Top. Conf. 17

18 Antenna Source is Eliminated due to Alignment E r xb B T B Tor B A E r E r xb E r xb E θ E θ xb B Convective cell is driven by antenna near fields. RF enhanced plasma potential is largest near the antenna face. And is highest near top and bottom of antenna box. Speculate antenna is aligned to convective cell. FA antenna does not intercept convective cell structure. Classic antenna intercepts convective cell. 21st RF Top. Conf. 18

19 Antenna Source is Eliminated due to Alignment Test alignment hypothesis by intentionally misaligning antenna to plasma. Lowered plasma position Blue trace plasma response to Classic antenna with misalignment. Green trace plasma response to Field Aligned antenna with misalignment. Increased impurity contamination needs local RF fields. Red trace plasma response with plasma centered - recover low impurity contamination. W MHD [kj] P RAD [MW] Mo core [a.u.] P ICRF [MW] Classic Antenna un-fa Antenna Time (s) FA Antenna 1.4 Suggest local impurity source is tied to convective cell. Cell strength goes as Er/B ITER will be more like C-Mod than JET or ASDEX-U due to magnetic field. Suggest field scan both at C-Mod and cross machine could be illuminating. 21st RF Top. Conf. 19

20 Impurity Contamination is Reduced But In C-Mod, boronization is still required for high performance H-mode. Residual impurity contamination associated with FA antenna. Radiation is integrating during H-Mode. Since local source being eliminated, we will need to examine far field sheath or transport changes due to the RF. If far field sheath, EAST has potential to test since single pass absorption is greater than for C-Mod. P RAD [MW] W MHD [kj] P ICRF [MW] Plasma Contamination in H-Mode Classic Antenna Field Aligned Time (s) 21st RF Top. Conf. 20

21 Summary EAST has a goal to reliably couple 70% of source power into plasma for >100 s. Increasing coupling efficiency is critical. Proposed EAST field aligned antenna is based on successful C-Mod field aligned antenna design. Utilize end fed center grounded antenna straps similar in length as existing I antenna. Antenna strap spacing is 350 mm on center k =9 m -1 and coupling efficiency ~45%. Field aligned antenna provides path towards reliable, high power antenna operation. Inherently load tolerant enables high coupled power. Achieves near elimination of RF enhanced heat flux and antenna impurity no need to use low Z coatings. 21st RF Top. Conf. 21

Evaluation of a Field Aligned ICRF Antenna in Alcator C-Mod

Evaluation of a Field Aligned ICRF Antenna in Alcator C-Mod 24th IAEA Fusion Energy Conference San Diego, USA October 8-13 2012 S.J. Wukitch, D. Brunner, M.L. Garrett, B. Labombard, C. Lau, Y. Lin, B.