Heating and Current Drive Systems for ARIES-AT T.K. Mau University of California, San Diego ARIES Project Meeting September 18-20, 2000 Princeton Plasma Physics Laboratory Princeton, NJ

OUTLINE CD Analysis for ARIES-AT equilibria at  = 9.1% (90% of limit) and with R = 5.2 m, I p ~ 13 MA, and B o = 5.9 T Power requirement, profile alignment and number of CD systems Normalized CD efficiency scaling vs T e and Z eff. RFCD launcher system definitions Conclusions and Discussions

Seed CD Requirements for Latest ARIES-AT Equilibria Latest series of ARIES-AT equilibria have profiles optimized to give high  N ( 90% of  limit ), and maximum bootstrap alignment ( I bs /I p > 0.9 ) at Z eff = 1.7, T e0 = 24, 26, 28 and 30 keV. Seed current is defined as: J sd = j eq - j bs - j dia - j ps in  - direction. Bootstrap alignment: 2 regions of seed CD : (1) On axis; (2) Off axis CD power and system requirements determined by driving seed current profile using RF techniques. n, T profiles RS core, L-mode edge TeTe nene T e0 = 26 keV  N = 5.4 f bs = EQ BS Dia+PS On-axis Seed: 0.04 MA Off-axis Seed: 1.05 MA

Seed CD Requirements at Z eff = 1.8 Modified n, T profiles RS core, L-mode edge TeTe nene T e0 = 26 keV  N = 5.4 f bs = BS EQ On-axis Seed: 0.03 MA edge +mid-radius Seed: 1.32 MA Dia Bootstrap current is sensitive to changes in Z eff. To extrapolate from Z eff = 1.7, adjust n and T profiles to obtain bootstrap alignment without overdrive. Three regions of seed current: (1) on-axis seed :  < 0.2, (2) mid-radius seed : 0.5 <  < 0.8 (3) edge seed :  > 0.8.

Current Drive at Z eff = 1.7 Needs two CD systems: 1.ICRF/FW for on-axis drive :  < 0.2; P fw ~ 1-2 MW 2.LHW for off-axis drive :  > 0.8; P lh ~ MW Very good current alignment can be obtained. T eo = 26 keV f bs = P fw = 1.4 MW P lh = 32 MW FW LH BS EQ RF Dia T eo = 30 keV f bs = P fw = 2.2 MW P lh = 36 MW EQ BS RF FWDia LH

Current Drive at Z eff = 1.8 Three CD systems are required: 1.ICRF/FW for on-axis drive :  < 0.2;P fw ~ 1 MW 2. LHW for off-axis drive :  > 0.8; P lh ~ MW 3.HHFW for mid-radius drive : 0.5 <  < 0.8 ; P hh ~ MW Fair current profile alignment T eo = 24 keV f bs = P fw = 1.1 MW P lh = 40 MW P hh = 16 MW T eo = 28 keV f bs = P fw = 0.8 MW P lh = 32 MW P hh = 16 MW FW Dia HH LH BS EQRF FW Dia HH LH BS EQRF

CD Efficiency Scaling vs T e0 and Z eff  B = I p R o /P CD Based on four equilibria optimized at Z eff = 1.7 and T e0 = 24, 26, 28, 30 keV. Thus, Z eff = 1.7 case has the highest CD efficiency. For Z eff = 1.7 and 1.6, only 2 RF systems are required (FW+LH). For Z eff = 1.8, 3 RF systems are required (ICRF/FW+LH+HHFW). Alignment not as good: results are less reliable.

T D 2T 2D,3T 4T 3D 5T4D,6T 96 MHz 68 MHz 135 MHz 22 MHz Frequency Options for Fast Wave On-Axis CD Criteria : Avoid ion and  absorption no resonance on OB side Reasonable antenna size higher frequency 68 MHz, 96 MHz, and 135 MHz appear feasible; similar power requirements 68 MHz is used in most calculations. R-a R+a Axis

ICRF Fast Wave Drives On-axis Seed Current Wave frequency is chosen to place 4f cT resonance at R > R o +a, and 2f cD resonance at R << R axis, to minimize ion and alpha absorption. Launcher is located on outboard midplane with N || = 2 spectrum for best current profile alignment. Plasma & wave parameters : R = 5. 2 m, A = 4,  = 2.2,  =0.8, B o = 5.9 T, I p = 13 MA,  N = 5.4, T eo = 26.8 keV, n eo,20 = 2.83, Z eff = 1.8 f = 96 MHz, N || = Z (m) R (m) X (m) Y (m) Axis  ARIES-AT P e /P = 0.90 P T /P = 0.02 P  /P = 0.08 I / P = A/W Driven Current

Off-Axis/Edge Seed CD with LH Waves Frequency = 3.6 GHz [ > 2 * f LH (  =0.8) ] - Less than 1% alpha absorption Usually five waveguide modules, each launching a different N ||, are required. - Located ~2 m. below OB midplane to give maximum penetration. Penetration to  < 0.8 is not possible for this class of AT equilibria. Low N || rays encounter mode conversion to fast wave at  >0.8 and propagates back to edge; higher N || rays get totally damped before reaching  = 0.8. start end N || = -1.6 e-damping limit MC limit Inaccessible Accessible

Mid-Radius CD Using High Harmonic Fast Waves (HHFW) At f ~ 20f ci, HHFW can penetrate deeper than LH waves. CD efficiency is found to be acceptable. Issues: - Strong absorption by energetic  ’s - Experimental database being developed on NSTX at 30 MHz. - No credible FW launcher design at f ~ 0.9 GHz. F = 0.9 GHz N || = -2 T e0 = 26 keV Z eff = 1.8 P  /P = 0.41 Absorption Current Drive T e0 = 26 keV Z eff = 1.8 I/P = A/W e 

Current Drive System Definition for ARIES-AT Reference Option : ICRF/FW + LHW - Requires two RF systems and highly compatible with core configuration - Requires lowest CD power (30-40 MW) - Likely narrow range of operation - Issues : (1) LH wave penetration limited to  > 0.8. Second Option : ICRF/FW + HHFW + LHW - Requires three RF systems; should be compatible with core design - Requires more CD power (40-60 MW) - Broader range of operation - Issues:(1) alpha absorption of HHFW power (2) HHFW antenna concept remains to be developed. Comments: - Because of small on-axis seed current, ECCD can be a viable alternative to ICRF/FW. - Should extra ICRF power be set aside for auxiliary heating? Can existing CD systems heat plasma to design point?

Definition of the ICRF Fast Wave Launcher System Assumed requirements for Z eff = 1.8, T e0 = 26 keV (strawman): - 1 MW of 96 MHz and N || = 1.5 for on-axis CD. At 96 MHz, similar j fw profile and I/P are obtained. Higher frequency is used to reduce size of launcher. Base launcher module is similar to ARIES-RS folded waveguide design : - Has 8 waveguides in a toroidal array, with 45 o phase shift - Each waveguide has 10 folds - Located at outboard midplane - Radial thickness with diaphragm = 0.97 m - Module dimensions are : 2.08 m (width) x 0.51m (height) with total aperture area = 0.99 m 2 Taking a maximum power density of ~40 MW/m 2, prudence requires us to set the power limit at ~20 MW. Extra power can be used for auxiliary heating and/or rotation drive. Structural material is SiC with W coating (as in divertors); high surface resistive dissipation [TBD]; structures (Faraday shields, straps and support) to be cooled with LiPb. Other choices will be explored.

Isometric View of Folded Waveguide Unit Design and dimensions are similar to ARIES-RS (f = 95 MHz)

Definition of the LH Wave Launcher System Calculated lower hybrid system requirements for Z eff = 1.8, T e0 = 26 keV: - 5 waveguide modules delivering a total power of 35 MW. Modulefrequency (GHz)N || Power (MW) Base unit is the passive/active multijunction grille, modeled after ITER-EDA design, and used in ARIES-RS. The grilles are located at ~2 m from the outboard midplane. Using ITER guideline for power flux capability: P (MW/m 2 ) < 20 f 2/3 (GHz), total required port area = 1.34 m 2.

Front View of LH Launcher Modules Shown are the designs for ARIES-RS, for illustration purpose only.

Consideration of HHFW Launcher System Calculated HHFW system requirements for Z eff = 1.8, T e0 = 26 keV: - Launched wave spectrum at 0.9 GHz and N || = Launch location : outboard midplane. - Power = 16 MW. At present, there is no proven design of FW launcher in 0.9 GHz range. Possibilities include: - Combline structure : data at 200 MHz (GA/JFT-2M) - Folded waveguide : no data close to 0.9 GHz Assume similar power scaling as ITER guideline for LH waves: - At 0.9 GHz, power density limit = 18.6 MW/m 2 (conservative!) - First wall penetration area = 1.16 m 2.

ICRF/FW LHW HHFW Blanket Sector Special Blanket Sector with RF Launchers There are 16 blanket sectors. One sector has a width of ~2.6 m at midplane. Sketch of locations of RF launchers in the sector is based on Z eff = 1.8, T e0 = 26 keV (strawman). Aperture area for the launchers: - ICRF/FW :0.99 m 2 - LHW :1.34 m 2 - HHFW :1.16 m 2 Total aperture area = 3.49 m 2 = 1% of first-wall area.

Conclusions and Discussions A series of ARIES-AT equilibria with  N = 5.4 and f BS = 0.91 at Z eff = 1.7 and T e0 = 24, 26, 28 and 30 keV have been analyzed for CD power and launch requirements. Extrapolations to Z eff = 1.6 and 1.8 are made. CD efficiency scalings were calculated vs T e0 and Z eff ; 2 RF systems are required for Z eff = 1.6, 1.7, while 3 systems are required for Z eff = 1.8, resulting in lower fBS and higher CD power requirements. Based on the present strawman with Z eff =1.8 and T e0 = 26 keV, 3 RF systems are required: LHW for edge CD, ICRF/FW for on-axis CD and HHFW for mid-radius CD. Power requirement is reasonable at ~ 52 MW level. Extra ICRF power for auxiliary heating and/or rotation drive should be provided. Launcher designs for both LH and ICRF systems have been on-going. Initial design results in launcher penetration equal to 1% of first wall area. It appears feasible to place all RF modules in one blanket sector.

Suggested Remaining Tasks CD power may be lowered, and number of RF systems may be reduced to two by looking at equilibria optimized at Z eff = 1.8 or higher, and with no mid-radius seed current drive ( 0.5 <  < 0.8 ). Complete detailed design of ICRF/FW and LHW launchers. - Dimensions of various modules - Wall dissipation with W coating on structures, and compare to Cu. Address the issue of auxiliary heating during start-up with existing CD systems: - How much extra ICRF power is required? At what frequency? - What are the implications for using LHW to heat the plasma?

Issues and Areas for Future Research Heating and Current Drive: - LHW penetration is limited in high-  plasma; HHFW is a possibility, but needs innovative antenna concept; - Investigate the dynamics of RF current profile control --- modeling, and physics and technological constraints - Refine modeling capability to self-consistently determine MHD stable equilibrium with bootstrap and externally driven currents; - Use wave spectrum calculated for RF launcher in ray tracing analysis; - Study roles of RF in rotation generation and transport barrier control RF Launcher: - EM field analysis inside folded waveguide in realistic geometry, and experiments in a tokamak environment - Detailed launcher cooling and thermal stress analysis - Structural material choice in SiC environment : SiC with metal coating - Wave coupling and loading during plasma transients