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 1

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

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

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 .

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

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

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

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.

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.

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/Ip PIC(MW) PLH(MW) 5.6 8.4 15.8 0.925 3.0 21.7 6.0 9.2 15.9 0.943 3.9 21.2 6.8 10.6 17.8 0.915 4.2 65.1 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.

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.

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

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.

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

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

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

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

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.

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 47.6 0.058 5.43 9.2 15.9 0.943 3.9 36.4 0.045 6.13 10.6 17.8 0.915 4.2 91.5 0.091 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.

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

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 ?

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

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