1. Impact of a Mock-up Ferromagnetic TBM on Plasma Operations in C-Mod
J A Snipes, L Giancarli, A Loarte
This is a proposal for a series of experiments to investigate the impact of a mock-up ITER ferromagnetic test blanket module on plasma operations in C-Mod.
ITER Organization, St Paul-lez-Durance, France

2. Helium Cooled Lithium-Lead Test Blanket Module
To give you all an idea of what a Test Blanket Module is, here I show as an example the design of the EU Helium Cooled Lithium-Lead TBM. The purpose of this TBM is to test tritium breeding methods with flowing liquid lithium-lead that will be bombarded by neutrons from the fusion reactions in ITER. Despite the complexity, it is basically a large piece of ferromagnetic material 1.66 m tall by nearly half a meter wide containing 1300 kg of ferromagnetic steel. There are two modules mounted next to one another in a horizontal port and there will be a total of six modules in 3 ITER ports (#16, #18 AND #2) at 0 and ±40° toroidally. Since these designs will soon be finalized for construction, ITER urgently needs to know what impact such a mass of ferromagnetic material will have on plasma operation.
Mass of ferromagnetic steel = 1300 kg
Six TBMs to be installed with two in each horizontal port at  = 0° and ±40° 2

3. TBM Effects on Plasma Operations
30 G
60 G
TBM induced stray fields can:
Affect the plasma breakdown
Modify the plasma equilibrium
Enhance magnetic braking of v
Produce locked modes through increased 2/1 error field
Possibly affect the H-mode threshold
Reduce thermal confinement in H-mode and L-mode
Increase energetic particle losses
The TBMs when immersed in the magnetic field of ITER will produce a stray field that will penetrate into the plasma. Vacuum field calculations such as shown here indicate a stray field from two TBMs of order 0.07 T will be induced at the last closed flux surface with of order 30 G at the plasma center. Such stray fields can affect plasma breakdown and will need to be compensated to maintain a proper null. They can also modify the plasma equilibrium and calculations indicate as much as a 12 mm radial bulge will be created by the TBMs. Stray fields also increase the m=2, n=1 error field and can lead to locked modes. Stray fields may also affect the H-mode threshold. Experiments on JET show that enhanced TF ripple reduces H-mode confinement noticeably even with as low as 0.5% ripple. Since the localized ripple induced by the TBMs has predominantly low toroidal mode numbers, it may penetrate deeper into the plasma and have effects not only on H-mode but also on L-mode confinement. Calculations and experiments also show that enhanced TF ripple leads to increased energetic particle losses, which could lead to first wall damage in ITER. 3 3

4. Proposed TBM Physics Experiments
Motivation: ITER urgent need for final TBM design, may affect TBM size, proximity to plasma, & TBM correction coils
Goals: Quantify effects of a mock-up ferromagnetic TBM on breakdown, equilibrium, plasma rotation, locked modes, H-mode threshold and confinement, and energetic particle losses in C-Mod as a function of collisionality to attempt to extrapolate the results to ITER conditions
Plan of execution: Design, build, and install a retractable mock-up TBM coil in a horizontal port to provide an additional ~1% local TF ripple when close to the plasma. Measure TBM mock-up effects on plasma operation at low, intermediate, and high collisionality. Where the effects are largest, repeat experiments retracting the TBM to reduce local additional TF ripple by a factor of 2 and compare the effects on plasma operation. Determine TBM mock-up location where plasma effects are only just measurable. Perhaps, one week of dedicated run time would be required to get all of these results. If the effects are large, it may be difficult to piggy-back on other experiments.
Now, let me discuss the proposed TBM-mock-up experiments on C-Mod. The motivation for these experiments is that this is an urgent ITER need to finalize the design of the TBMs and the results of such experiments may affect the TBM size, their proximity to the plasma, and the design of possible TBM correction coils that would be mounted directly to the TBMs. The goals of the experiments are to quantify the effects of a mock-up ferromagnetic TBM on breakdown, equilibrium, plasma rotation, locked modes, the H-mode threshold and confinement, and on energetic particle losses in C-Mod as a function of collisionality so that the results may be extrapolated to ITER. JET ripple experiments show that the effects of ripple on H-mode confinement disappear at high collisionality, so it would be important to start with low collisionality but also scan the collisionality to determine how the TBM effects on the plasma scale with collisionality. The plan of execution would first be to design a retractable mock-up TBM for C-Mod that would be installed on rails into a horizontal port and be large enough to increase the local ripple in front of the TBM mock-up by an additional 1% when the TBM mock-up is close to the plasma. Experiments would first be done at low collisionality and then repeated at intermediate and high collisionality to determine how the effects scale. Where the effects are largest, the experiments would be repeated retracting the TBM mock-up by an amount such as to reduce the local additional TF ripple by a factor of 2 and compare the effects on plasma operation. We would like to determine what is the radial location of the TBM mock-up where plasma effects are only just measureable to attempt to decide where the TBMs should be placed relative to the plasma in ITER. 4 4