Current Drive and Plasma Rotation Considerations for ARIES-AT T.K. Mau

1. Current Drive and Plasma Rotation Considerations for ARIES-AT T.K. Mau University of California, San Diego Contributors: R.L. Miller (UCSD), C.E. Kessel (PPPL), L.L. Lao, M.S. Chu (GA) ARIES Project Meeting March 20-21, 2000 University of California, San Diego

2. OUTLINE • Seed Current Drive Efficiency Using RF Waves • for N = 5.6, 6.0, 6.8 Equilibria • - Assess penalty for 10% backoff from  limit • Current Drive Efficiency Using Tangential • Neutral Beam Injection • Rotation Generation Using NBCD Power • NBI System Consideration (preliminary) • Conclusions, Recommendations & Future Work

3. Seed CD Requirements for Typical ARIES-AT Equilibrium • ARIES-AT equilibrium profiles are optimized to give high N and • excellent bootstrap alignment (Ibs/Ip > 0.9). • Seed current jseed () = jeq () - jbs () - jdia () in -direction. • Two regions of seed CD: (1) On axis (2) Off axis j profiles ne Te N = 6.0 Ibs/Ip = 0.944 EQ BS off-axis seed 1.02 MA on-axis seed 0.22MA Dia n, T profiles

4. Current Drive Techniques Consideration • In ARIES-RS, three RFCD systems are used: (1) ICRF/FW, (2) HHFW, • and (3) LHW. Total CD power = 80 MW. • We re-consider the selection of CD techniques for ARIES-AT, and • determine: • - For on-axis drive, • (i) ICRF/FW is baseline driver • (ii) ECCD is viable alternative in view of recent • advances in experimental database, window and • gyrotron technologies. • - For off-axis drive, • (i) LHW is baseline driver for CD only. • (ii) NBI is the choice for both CD and rotation drive .

5. RF Current Drive on “AT Plasma” • Current drive is required in two locations : • - On-axis: provides bootstrap seed and controls q(0) • - Off-axis: controls qmin location and enhances  limit. • Radio frequency systems are used for integrability to fusion power core. • RF power launch location • and spectra are selected for • maximum CD efficiency • and profile alignment. • For an AT plasma with • R=5.5 m, A=4, I=19 MA, • Bo=8 T, N=6.0, the CD • requirements are: • - On-axis: ICRF @ 95 MHz, • 12 MW, I/P = 0.02 A/W • - Off-axis: LHW @ 3.6 GHz, • 50 MW, I/P = 0.02 A/W AT Plasma: N = 6.0, IBS/I = 0.94 <Te> = 16 keV, Zeff = 1.8 B = 6.32 Off-axis CD: LHW On-axis CD: ICRF/FW

6. On-Axis Seed CD with ICRF Fast Wave Power • CURRAY ray tracing code is used. • Wave frequency is chosen to place • 2fcD resonance at R > Ro+a, and • 2fcT resonance at R << Raxis, to avoid • ion and alpha absorption. • Power is launched 20o above OB • midplane with spectrum peak • for best current profile alignment. • Plasma & wave parameters : • R = 5.52 m, A = 4,  = 2.2,  =0.8, • Bo = 8 T, Ip = 19 MA, N = 6.0, • Teo = 27.8 keV, neo,20 = 5.1, • Zeff = 1.8 • f = 95 MHz, N|| = -2.0. OB FWCD f = 95 MHz N|| = -2 Pe/P = 0.99 electron ion

7. Off-Axis Seed CD with Lower Hybrid Power • CURRAY ray tracing code is used for analysis. • Six waveguide modules, each launching a different N|| spectrum, are required • to drive the required off-axis seed current profile. These are located at the • OB midplane, although results are not sensitive to waveguide location. • Alpha absorption is not an issue for off-axis drive at a high enough frequency. • For the same plasma, frequency • is 3.6 GHz, and the launched • spectra are: • N||P(MW) Icd/Isd • 1.6 9.1 0.2 • 1.8 3.1 0.1 • 2.0 6.8 0.2 • 2.5 8.4 0.2 • 3.0 5.3 0.1 • 4.0 17.0 0.2 LHCD 2.5 f = 3.6 GHz 4.0 total 2.0 3.0 N|| = 1.6 1.8

8. RFCD Efficiency Scaling w.r.t. Te and Zeff • Using the same equilibrium, normalized RFCD efficiency, B = <n>IpR/Pcd, • is calculated as <Te> and Zeff are varied. • - n,T profiles are adjusted to give maximum bootstrap alignment without • overdrive. So, profile peakedness and Ibs/Ip vary, but within a narrow • range. • Under these conditions, good CD • efficiency is obtained at higher Zeff • and <Te> > 17 keV, where there is • less seed current to drive. • Current profile matching can be • reasonably achieved by adjusting • RF spectra, except at low <Te> • and high Zeff, where the calculated • CD efficiency is less reliable.

9. Distribution of CD Power between LHW and ICRF • Because of the low on-axis seed current, the bulk of CD power is • in the LHW system driving off-axis seed current. • The fraction of power in • LHW system is decreased • at higher <Te> because • of higher local CD • efficiency in the off-axis • region.

10. RFCD Power Requirements on ARIES-AT • Power requirements were calculated for on-axis CD with ICRF/FW • and off-axis CD with LHW, for three ARIES-AT design points. • R = 5.2 m, A = 4,  = 2.2,  = 0.8, Ip ~ 13 MA, Bo ~ 6 T, Pnet = 1000 MW. • Full N (%) <Te>(keV) Ibs/IpPIC(MW) PLH(MW) • 5.6 8.4 15.8 0.925 3.021.7 • 6.0 9.2 15.9 0.943 3.921.2 • 6.8 10.6 17.8 0.915 4.265.1 • The total CD power (25 MW for N = 5.6, 6.0) is significantly lower • than for ARIES-RS (~80 MW), due to higher bootstrap fraction and • better alignment. • Number of RFCD systems is reduced to two. • On-axis seed current is small, requiring only ~4 MW of ICRF power. • ECCD may be an attractive alternative.

11. Is there a Penalty in Backing Off 10% from Full Beta Limit ? • All CD efficiencies have been evaluated for equilibria at full beta limit. • At 90% beta limit,  0.9 x N,limit ( Ip / a Bo ), one anticipates a drop • in BS fraction, which may lead to higher CD power and lower B. • To assess this possible penalty, multiply p() by 0.9, adjust profiles to obtain • maximum BS alignment, calculate CD power and compare with 100% p() case. • Results for one design point are: • N,limit = 6.0, <Te> = 16 keV, Zeff = 2.0, Ip = 19 MA, Bo = 8 T • N/N,limit To/<T> Ibs/Ip Pic (MW) PLH (MW) B • 1.0 1.764 0.944 7.5 59.6 5.80 • 0.9 1.632 0.905 20.4 66.4 4.02 • Backing off from -limit by 10% results in 30% reduction in B for this point, • and a higher proportion of ICRF power for on-axis CD. • There is a penalty in the form of higher CD power.

12. Stabilizing Kinks for ARIES-AT • The high beta achieved in ARIES-AT is mainly based on the premise that external kinks • can be stabilized with close fitting conducting walls. • - When conductivity is finite, resistive wall modes need to be stabilized by • (1) Toroidal plasma rotation, or • (2) Active feedback coils. • Toroidal rotation can be driven by • - Neutral beam injection: Ample experimental database; physics relatively • well understood; analysis tools exist. • - RF techniques : Observed rotation in RF heating experiments (e.g., TFTR, • JET, C-Mod); many proposed theories, all invoking wave-ion interactions, but none • at present can provide a self-consistent picture in explaining all observations. • NBI has stronger basis as rotation driver for ARIES-AT • Innovative RF rotation drive techniques need to be identified. • So, there are two approaches for CD and kink stabilization: • - Off-axis CD with LH waves, and RWM stabilization with feedback coils. • - Off-axis CD and rotation drive using NBI

13. Analysis Approach for NBI CD and Rotation Drive • In ARIES-AT studies, we have considered using NBI both for off-axis • current drive and rotation generation. • Our analysis approach is: • (1) Determine off-axis CD power requirement (using NFREYA code); • (2) Assess rotation speed induced by CD power; • (3) Compare with required rotation for RWM stability.

14. Determining NB Parameters for Off-Axis Current Drive • Three main criteria : (1) current profile alignment, (2) rotation generation • efficiency, and (3) CD efficiency. • Beam parameter variables: (1) beam injection angle, ; (2) beam energy, Eb. • The beam injection angle  can be adjusted to provide a driven • current profile that matches very well with the off-axis seed profile. • Lower  results in deeper penetration, broader profile, but lower • CD efficiency. Typically, 45o <  < 75o. Top View of Tokamak AT Plasma N = 6.8 seed Beamline Eb = 120 keV  = 70o 60o 50o  NBCD

15. Neutral Beam Current Drive in an AT Plasma • Beam energy is chosen at Eb = 120 keV, because (1) deep penetration not • required, (2) high rotation generation efficiency, and (3) present-day technology. • - Appears sufficient for penetration and alignment in regimes of interest except • when <Te> < 15 keV. • An AT plasma with R=5.5 m, • A=4, Ip=19 MA, Bo=8 T, • N = 6.0, <Te>=16 keV • will require: • - On-axis: ICRF/FW @ • 95 MHz & 12 MW • - Off-axis: NBI @ 120 keV •  = 65o, & 86 MW seed AT Plasma: N = 6.0, Ibs/I = 0.94 <Te> = 16 keV, Zeff = 1.8 B = 4.0 NBCD ICRF/FW

16. Comparison of CD Efficiency between RFCD and RF/NBCD • Considerably more power is needed when off-axis NBCD is used. • Rotation drive with NBI results in higher Pcd. • Dependencies of CD efficiency on <Te> and Zeff have similar • trends for both schemes. B = <ne>IpR/Pcd

17. Determining Required Rotation Speed • Calculation is done by M.Chu (GA) using the MARS code, invoking the sound wave • damping model, for a N = 5.6 AT equilibrium. • At a bulk toroidal rotation speed v, there is a window in wall location, rW/a, • where both resistive wall and ideal plasma modes are stable. Stability • window is larger for higher v v/vA(0)= • At rW/a = 1.2, the critical rotation • speed is • vcrit = 0.065 vAlfven(0). • Rigid-body rotation is assumed. • According to model, vcrit should be • lower at higher  and with an H- • mode edge. • - Calculations on strawman • equilibria are needed. RWM Normalized Growth Rate N = 5.6 n = 1 mode Courtesy of General Atomics Wall Location, rW/a

18. Assessment of Rotation Drive by NBI • Moderate energy beams are efficient in driving rotation because of • their high momentum content per unit power. • The physics of momentum transfer from beams and its radial transport • is a topic of present research. Measured rotation speeds are much • lower than neoclassical predictions, implying momentum confinement • is anomalous, and characterized by energy confinement time, E. • An estimate of beam induced rotation using simple momentum • rate balance, and assuming plasma to be rigid. • Momentum input rate per ion: ~ Pb (2mb/Eb)1/2 / Vpl <ni> • Momentum loss rate per ion: mi v / E • where v is bulk rotation speed, and Vpl is plasma volume. • Beam-induced rotation profiles will be calculated using ONETWO • transport code.

19. Rotation Driven by NBCD Power on ARIES-AT • Power requirements were assessed for on-axis CD with ICRF/FW and • off-axis CD with NBI, for three ARIES-AT design points. • R = 5.2 m, A = 4,  = 2.2,  = 0.8, Ip ~ 13 MA, Bo ~ 6 T, Pnet = 1000 MW. • N(%) <Te>(keV) Ibs/Ip PIC(MW) Pb(MW) v/vAlf(0) • 5.03 8.4 15.8 0.925 3.0 47.60.058 • 5.43 9.2 15.9 0.943 3.9 36.40.045 • 6.13 10.6 17.8 0.915 4.2 91.50.091 • NBCD power induces rotation speed that is within the range needed • for kink stabilization with wall at rw ~ 1.2a. • - In overdrive case, can replace part of Pb with lower PLH. • - In under-drive case, increase Pb, and operate at lower Ibs/Ip and • possibly higher N.

20. NBI System Design Considerations (Prelim.) • At Eb = 120 keV, the neutralization efficiency for D+ ions is 0.53, • which is quite adequate to allow for a positive-ion based system. • The ion source and accelerator can be based on the CLPS (Common • Long Pulse Source), developed at LBNL, which was installed on • DIII-D and TFTR NB injectors. Based on the TFTR design, the • 120 keV source has : • Source current = 70 A • Beam Perveance = 1.7 Perv. • Aperture size = 12 cm x 44 cm. • Projected beam efficiency b = Pinj/Psource = 0.48 • (incl. neutralization, collimation, beam reionization, etc.) • A typical 32-MW beam module will have a 2x2 array of sources with • beams combined and focused near first wall aperture. Drift duct has • 50 cm x 100 cm cross section. • - Aperture first wall area per module ~ 0.7 m2

21. ICRF Launcher Ideas (Prelim.) • Frequency = 68 MHz, Power = 5 MW • Folded waveguide: • - Large size; large radial thickness • - Consider raising frequency: f=(3,4)fcD @ R > Ro + a • Loop Antenna: • - Toroidal wavelength = 2 m. ~ antenna toroidal width • - Power flux limit = 10 MW/m2 • 1st wall aperture area > 0.5 m2 • - Use ITER antenna design ( current straps and Faraday shields ) • - Material choice: • * Structural : SiC with Cu surface layer ( < 1 mm) • W surface problematic due to high surface heat • dissipation • * Coolant : LiPb or other ?

22. Conclusions and Recommendations • For the ARIES-AT equilibria with higher N and better bootstrap • alignment, the RFCD power requirements are drastically reduced • to ~25 MW from ~80 MW in ARIES-RS. • - Only two RF systems are required (ICRF/FW and LHW). • - In this scenario, need active feedback coils to stabilize RWM. • Low-energy NBI was considered for both off-axis CD and rotation drive • to stabilize resistive wall mode. • - More NBCD power is required than for LHCD. • - Induced rotation speed is within range for RWM stabilization. • - Off-the shelf NB technology appears sufficient. • RECOMMENDATIONS for CD and kink stabilization, to preserve • attractiveness of ARIES-AT: • - Baseline scenario : LH off-axis CD, and active feedback coils and/or • innovative RF rotation drive for RWM stabilization • - Backup scenario : NBI off-axis CD and rotation drive

23. Discussions and Future Work • Comments: • A detailed calculation of beam-induced rotation profile and its • stabilizing properties will be useful. • A critical area of research: Understanding RF-induced rotation, and • physics extrapolation to reactor regime. • Future Work: • Calculate critical rotation speed for ARIES-AT N = 6.0 equilibrium • (work with GA). • Calculate B scaling for RFCD, including 10% backoff in beta. • Design feedback control coils, and configuration in fusion power core. • Design ICRF wave launchers : waveguides or loops? structural • materials choice? Cooling? • Identify possible RF techniques for rotatin drive and assess potential