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SUMMARY:Off-axis Neutral Beam Current Drive for Advanced Tokamak
DTSTART;VALUE=DATE-TIME:20210511T063000Z
DTEND;VALUE=DATE-TIME:20210511T065000Z
DTSTAMP;VALUE=DATE-TIME:20211016T204005Z
UID:indico-contribution-17126@conferences.iaea.org
DESCRIPTION:Speakers: Jin Myung Park (Oak Ridge National Laboratory)\nOff-
axis Neutral Beam Current Drive (NBCD) physics has been tested on DIII-D f
or Advanced Tokamak (AT) operation with increased off-axis injection power
($P_{OANB}\\simeq7$ MW) by using the newly available\, toroidally steerab
le co/counter off-axis neutral beam (CCOANB) injection capability. DIII-D
experiments confirm that the new CCOANB drives current as predicted by the
classical model NUBEAM for MHD quiescent plasmas (Fig. 1). Compared to on
-axis injection\, substantial broadening of the current and pressure profi
les has been achieved with dominant OANB heating by injecting both the new
CCOANB and the previous vertically steerable OANB. This is consistent wit
h predictions of the theory-based IPS-FASTRAN integrated modeling that has
guided the DIII-D beam system upgrade for the development of reactor rele
vant $\\beta_N>4$ steady-state scenarios. Projecting to the Compact Advanc
ed Tokamak (CAT) fusion pilot plant shows that off-axis NBCD aligns well w
ith the high bootstrap current $f_{BS}>0.8$ operation\, maintaining a broa
d current profile with $q_{min}>2$ and excellent off-axis NBCD efficiency.
\n![Measured NBCD for CCOANB (red) vs NUBEAM modeling (blue)][1]\nThe NBCD
profile driven by the new CCOANB was measured in H-mode plasmas and compa
red with modeling. The NBCD measurement$^1$ is based on the local measurem
ent of the magnetic field pitch angles from Motional Stark Effect (MSE) di
agnostics. These pitch angles are converted to the flux-surface-average cu
rrent density ($J_\\parallel$) and parallel electric field ($E_\\parallel$
)\, either using kinetic EFIT equilibrium reconstruction or a more direct
MSE analysis. This allows the beam driven current to be determined by $J_{
NB} = J_\\parallel – \\sigma_{NEO}E_\\parallel -J_{BS}$\, where $\\sigma
_{NEO}$ and $J_{BS}$ are the neoclassical conductivity and bootstrap curre
nt density calculated by the Sauter model using the measured kinetic profi
les as input. Differential NBCD measurement reduces model dependencies ($\
\sigma_{NEO}$ \, $J_{BS}$) and systematic uncertainties of measurements. I
n this study\, we compare two discharges\, either i) on- and off-axis or i
i) “Left” (more tangent) and “Right” (more perpendicular) off-axis
NBCD in otherwise similar discharge conditions. The measured NBCD profile
s driven by the new CCOANB in the co-current direction agree reasonably we
ll with classical model of Monte Carlo beam ion slowing down calculation N
UBEAM (Fig. 1). The NUBEAM modeling employs an accurate beam injection mod
el validated against fast visible image and neutron measurement$^2$. The t
oroidal field direction (+BT) was chosen for better alignment of NBI to th
e local B\, leading to good off-axis NBCD efficiency. The minimum value of
q was maintained at $q_{min}>1$ without any significant core MHD activiti
es. The measured $J_{NB}(\\rho)$ in Fig. 1 shows a clear hollow NBCD profi
le with the peak at about half the minor radius $\\rho\\sim0.5$. The net d
riven current normalized to the total injection power is $I_{NB}/P_{NB} =
14.9$ kA/MW\, which is as good as on-axis NBCD since the increased fractio
n of trapped electrons reduces the electron shielding in the outer radius
region. The measured NBCD ($I_NB$) increases with the off-axis NB power. H
igher NB power operation produced mild low-frequency MHD modes (n=2 and 3)
\, resulting in the reduced NBCD compared with the classical model predict
ion. The measured NBCD profiles at the highest inject power matches the NU
BEAM modeling with a modest anomalous beam ion diffusion $D_b = 0.3 ~{\\rm
m^2/s}$. However\, even including the effect of finite $D_b$\, the measur
ed NBCD efficiency ($I_{NB}/P_{NB}$) does not decrease with $P_{NB}$.\n\nA
low pressure peaking factor has been obtained with dominant off-axis NB h
eating. Figure 2 compares two discharges with different ratio of the OANB
power to the total NB power ($f_{OANB} = P_{OANB}/P_{NB}$) at ~same inject
ion power for elevated $q_{min} > {\\sim1.5}$ discharges. The pressure pea
king\, $f_p = p_0/\\langle p \\rangle$ decreases substantially when the on
-axis NB power (blue traces) is replaced by the new CCOANB (red traces)\,
while maintaining $\\beta_N\\sim3$ with high total ($H_{89}\\sim2.3$) and
thermal ($H_{98}\\sim1.25$) energy confinement. The lower pressure peaking
also results in the increased low-n ideal $\\beta_N$ stability limit$^3$.
Both discharges inject full available power from the vertically steerable
OANB at the maximum tilt angle. This profile broadening has been reproduc
ed by the IPS-FASTRAN modeling$^4$ that integrates theory-based models of
core transport (TGLF)\, edge pedestal (EPED1)\, equilibrium (EFIT)\, stab
ility (DCON)\, heating and current drive (NUBEAM\, TORAY) self-consistentl
y to find steady-state ($d/dt = 0$) solutions. Figure 3 compares the p
ressure profile between the measurement (kinetic EFIT equilibrium reconstr
uction) and the IPS-FASTRAN modeling\, where all transport channels ($n_e$
\, $T_e$\, $T_i$\, rotation\, and current) are predicted without any signi
ficant free input parameters except the density and rotation values at the
pedestal top. \n\n![Pressure broadening with dominant off-axis NB heating
at $\\beta_N>3$][2]\n\nThe measured off-axis NBCD does not lose CD effici
ency by going to a larger radius\, which is beneficial for future AT react
ors. The IPS-FASTRAN modeling predicts significant improvement of energy c
onfinement time for broad current profile with flat or weak negative magne
tic shear\, compared with monotonic $q_0\\sim1$\, for the AT reactors. Pro
jecting to the CAT $^5$ Fusion Pilot Plant with R = 4 m\, B = 7 T shows an
excellent off-axis NBCD efficiency for high Greenwald density fraction op
eration leading to high fusion performance. Off-axis NBCD aligns well with
the high bootstrap current $f_{BS}>0.8$ operation maintaining broad curre
nt profile with $q_{min}$>2. \n\n![Comparison between measurement (solid)
and IPS-FASTRAN modeling (dashed) for the discharges shown in Fig 2.][3]\n
\nThis material is based upon work supported by the U.S. Department of Ene
rgy\, Office of Science\, Office of Fusion Energy Sciences\, using the DII
I-D National Fusion Facility\, a DOE Office of Science user facility\, und
er Awards DE-AC05-00OR22725\, DE-FC02-04ER54698\, DE-AC02-09CH11466\, DE-F
G02-07ER54917\, DE-SC0012656\n\n$^1$ J.M. Park\, et al.\, Phys. Plasmas **
16**\, 092508 (2009). \n$^2$ B.A. Grierson\, et al.\, submitted to this co
nference.\n$^3$ B.S. Victor\, et al.\, submitted to this conference.\n$^4$
J.M. Park\, et al.\, Phys. Plasmas **25**\, 012506 (2018).\n$^5$ R.J. But
tery\, et.al.\, IAEA FEC\, FIP-P3/26 (2018).\n\n\n\n [1]: https://fusion.
gat.com/conference/event/104/attachments/161/1648/Park.Jin.Myung.IAEA2020.
Fig1.png\n [2]: https://fusion.gat.com/conference/event/104/attachments/1
61/1649/Park.Jin.Myung.IAEA2020.Fig2.png\n [3]: https://fusion.gat.com/co
nference/event/104/attachments/161/1650/Park.Jin.Myung.IAEA2020.Fig3.png\n
\nhttps://conferences.iaea.org/event/214/contributions/17126/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17126/
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