Report on IOS-JA1.0: Modeling of ITERlike experiments
C. Kessel, PPPL
ITPA-IOS Meeting, October 18-21, 2011
Kyoto, JapanJA1.0: Modeling of ITER-like experiments•! April 2011: Organization of existing results on current rampup phase
The present part of this IOS JA will be on the current rampup phase of ITER-likeexperiments. It should be noted that we can not assess V-s aspects without going
through the flattop phase, so this will need to be included as necessary. The current
rampup phase of the baseline 15 MA discharge in ITER is approximately 100 s, with afastest rampup of about 65 s. There does not appear any technical reason to go
slower than 100 s. The current diffusion time for this phase of the discharge (using<Te> = 2.5 keV, n = 4.5e19, Zeff = 2.0, κ = 1.8, and q = 2) is about 13 s, so there areabout 7-8 current diffusion times in the ramp phase. This is variable depending on thetransport, but the ramp time is clearly several current diffusion times. The current
ramp phase consumes most of the volt-seconds (V-s) available from the solenoid, andthe time available for the flattop burn phase is determined by the leftover V-s. This V-slimit is really a limit on the central solenoid (CS) coil’s current and field, in particular,
the CS1 coil. It is desirable to have flexibility in the rampup phase V-s consumption inorder to preserve access to ~500 s flattop burn. In addition, the control of the current
profile in the rampup phase is of interest to avoid vertical instabilities associated withhigh li, or peaked current profiles. The Ip ramp rate, heating, and H-mode onset time,and density are the primary tools for manipulating the V-s consumption and current
profile evolution. The various heating sources are expected to have different effects
on the rampup characteristics through their species heating and current drive.

JA1.0: Modeling of ITER-like experiments•!

Experimentally the four tokamaks (D3D, JET, AUG, C-Mod) above have already generated
discharges and many of these have been simulated in one form or another.

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Need to gather simulations performed to date on the present experiments
Examine primary similarities and differences (heating sources, divert times, Ip rampup
trajectories, and V-s and li evolutions).
Need to gather simulations that have been done to reproduce these experimental
discharges, and organize how these were done (models used, codes used, etc.)
We may want to do further simulations to utilize similar approaches
Cross-modeling of various devices with various energy transport models
Develop some conclusions about what these experiments and simulations are telling us,

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Are we reducing V-s consumption over ohmic rampup, by how much, with which source
Are we modifying the current profile with the heating/CD source
Is there a energy transport model that is sufficiently accurate to recommend for ITER
Can density reduction be an effective way to save V-s
How well are the simulations reproducing these discharge features
Is there physics outside our simulations that is important , should we pursue it

I am requesting that modelers/experimentalists who have participated in these ITER-like
expts or simulations to gather what they have done. I have included an example for myself
as a guide.

JA1.0: Modeling of ITER-like experiments
Example for myself:
Alcator C-Mod / TSC modeling:
Early discharges (2008, 1080201010 – ohmic, 1080201031 - ICRF)
demonstrated V-s savings with ICRF heating, but the li evolution was unreliable
to assess due to variations in the rampup phase – using TSC (ICRF from
TRANSP) modeling with CT energy transport, modified to agree with theexperiment, confirmed the V-s savings – comparison of transport models (CT,
BgB, and CDBM) showed some agreement that varied among the models.
Original CT model had excessively peaked profiles and did not match Te(rho)profiles. Adjustments were made to thermal diffusivity to provide broader Te
and adjust the central Te(0) in order to agree better with experiments.
Newer discharges (2009, 1090911006 – ohmic, 1090911012 – ICRF,
1090911026 - ICRF) developed early divert and reproducible rampup which
again showed that V-s were saved with ICRF heating, but li was unchanged
from ohmic when 1 and 1-2 MW of ICRF were injected – TSC modeling
reproduced both the V-s savings and the lack of li variation, however, the CT
model can not reproduce the details of the Te profile even when adjusted tomatch Te(0) and profile overall.

Cont’d
The ICRF is found to make very specific changes to Te profile in the plasmacore, but otherwise make little change in the outer half of the plasma minor
radius. Slightly different values were necessary with these discharges as forearlier ones. Need to examine BgB and CDBM for these cases. These
discharges are being made available for modeling.
Rampup experiments were done to examine density variations for ohmic, ICRF
heated and LH heated discharges. Only the lowest densities with heating
showed V-s savings over ohmic discharges, however rampups were not as wellcontrolled, and flattop phases were not controlled so complete assessment of
V-s savings is not clear. Will likely redo these expts.
Requested similar input from others to start to gather information…..got one
response from Irina Voitsekhovitch
Culham meeting; presentations by Litaudon, JM Park
Seoul meeting: presentations by JM Park, Litaudon
Princeton meeting: presentations by Kessel, Budny, Parail, …

What are we after?
We are trying to provide information to ITER on rampup phase of the discharge
based onExperimental results
Modeling of experiments, to test models against experiments, match
multiple parameters simultaneously
Extrapolability to ITER
Spending lots of time on transport models….because they influence V-s
and current profile evolution…..database setup
What are we looking at? Te(ρ), Ti(ρ), li(1), q(0)/sawtooth onset, Vsurf,
Te(0), Ti(0), T(0)/<T>, sawtooth radius, τE, …..
Have we shown that we can save V-s with heating? By how much and withwhat source.
Have we shown that we modify li significantly or not with heating, from agiven source?
Is there a limit to ramping up Ip fast and heating…MHD?
Other methods for rampup control; density, early H-mode, Ip ramp rate

Recent C-Mod analysis of transport models
Expt
TSC simCentral Te, ECE, keV

1090911012
ICRF heated, 1 MW
Ip rampup to 0.54 s

Coppi-Tang (χmult = 3.15, Cq= -2.5)

CDBM

GLF+Coppi-Tang

Bohm gyro-Bohm

Profile comparisons at 2 time slices for CMod 1090911012
This observation is typical, models are not BAD, they are just not accurate,
and their level of accuracy varies between points in the ramp, or between
discharges

Voitsekhovitch

VoitsekhovichImpact of transport models on various
quantities, C-Mod
TSC simulations

Sawtooth onset

What will we tell ITER about energy
transport in rampup and its consequences?
It appears there are no sufficiently accurate transport models for the L-mode
rampup phase, therefore ITER should1)! Use any model they prefer and force it to match a global energy
confinement time (89P or 97L)2)! Same as # 1, but also provide a temperature profile width or shapethat reflected experimental discharges (this probably requires
different shapes for different regimes like ohmic, ICRF heated, NB
heated, EC heated…)
3)! Introduce a fix to GLF23 and use it4)! ?????
Then we model our experiments with the “model” we recommend for ITER
and quantify the differences
Subsequent modeling of ITER with the “model”, and demonstrate that theramp rate, heating power, or other technique can compensate possible
differences between the “model” and the actual values, and avoid excessive
V-s consumption or vertical instability

Discussion
For JA 1.0 focused on the rampup phase, what are the tasks to do….
1)! Begin collecting rampup data from experiments (database cases first) to
focus modeling activities
1)! compare V-s, li, q(0), Te(ρ),etc. VS ohmic, heated, varying sources,
dIp/dt, density, H-mode onset time2)! Continue transport model simulations of experiments, and establish ability
to reproduce the experimental parameters
1)! Exercise with database2)! How do we measure agreement between expt and simulation?
3)! What parameters are compared with expt, besides Te(rho) and Ti(rho)3)! Examine energy confinement times of plasmas during rampup, checkagainst L-mode scalings4)! Examine experimental Te profiles from various machines and ohmic/heateddischarges for shapes5)! What else???