Current Drive and Plasma Rotation Considerationsfor ARIES-AT

T.K. MauUniversity 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, 2000University of California, San Diego

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

Te

ne

n, T profiles

j profiles

EQBS

Diaon-axis seed 0.22MA

off-axis seed1.02 MA

N = 6.0Ibs/Ip = 0.944

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

On-axisCD:ICRF/FW

Off-axisCD:LHW

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.

electron

ion

f = 95 MHzN|| = -2Pe/P = 0.99

OB

FWCD

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

total

N|| = 1.6

1.8

2.0

2.5

4.0

3.0

f = 3.6 GHz

LHCD

2

3

4

5

6

7

8

13 14 15 16 17 18 19 20 21

, <Average Temperature Te> ( )keV

Zeff = 2.0

1.8

1.6

-ARIES AT = 2.2

N= 6.0

ne( )/a n

e(0) = 0.2

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.

0.5

0.6

0.7

0.8

0.9

1

13 14 15 16 17 18 19 20 21

Average Temperature, <Te> (keV)

Zeff

= 2.0

1.8

1.6

ARIES-AT = 2.2

N= 6.0

ne( )/a n

e(0) = 0.2

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

• 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.

Analysis Approach for NBI CD and Rotation Drive

Beamline

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 ofTokamak AT Plasma

N = 6.8

Eb = 120 keV

seed

= 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

AT Plasma:N = 6.0, Ibs/I = 0.94<Te> = 16 keV, Zeff = 1.8

B = 4.0

ICRF/FWNBCD

seed

2

3

4

5

6

7

8

13 14 15 16 17 18 19 20 21

, <Average Temperature Te> ( )keV

-ARIES AT

N= 6.0 Z

eff = 2.0

1.8

1.6

Zeff = 2.0

1.8

1.6

+NBCD FW

+LHW 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

Wall Location, rW/a

Nor

mal

ized

Gro

wth

Rat

e

N = 5.6n = 1 mode

v/vA(0)=

Courtesy ofGeneral Atomics

• At rW/a = 1.2, the critical rotation speed is vcrit = 0.065 vAlfven(0).

Rigid-body rotation is assumed.

• According to model, vcrit should be lower at higher and with an H- mode edge. - Calculations on strawman equilibria are needed.

RWM

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 ABeam 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

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

Discussions and Future Work