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 1

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: 95 MHz,
12 MW, I/P = 0.02 A/W
- Off-axis: 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
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/Ip PIC(MW) PLH(MW)
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,  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
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:
95 MHz & 12 MW
- Off-axis: 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 = 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)
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: 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