3.7 Plant Control - General Atomics Fusion Group

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ITER G A0 FDR 1 01-07-13 R1.0Figure 3.7.3-4 illustrates the key features of plasma operation scenarios. The parameters ofthe scenario – current, fusion power, auxiliary heating and/or current drive input and burnduration – can vary, but the sequencing requirements and general waveforms remain invariant.Figure 3.7.3-4Waveforms for Standard Driven-burn Operation Scenario3.7.3.2.1 Plasma Discharge SequencePlasma InitiationA pulse proceeds from pulse initiation with attainment at t = 0 of PF coil system premagnetisationwith a plasma-axis flux linkage of φ SOP (start of pulse), as shown in Figure 3.7.3-4. This is followed by a 2 s plasma initiation phase that begins with the application of predeterminedvoltages to the PF coils. These voltages and the initial PF currents are chosen toproduce a dynamic multipole field null with a low poloidal field (< 2 mT within an 0.8 mradius) that is positioned near the port-mounted start-up limiters at t ≈ 1 s. The in-vesselloop voltage is ~ 12 V (E ≈ 0.3 Vm -1 ) at this time. With a gas fill pressure of about 1.3 mPa,Townsend avalanche breakdown occurs within 50 ms.Plant Description Document Chapter 3 .7 Page 14
ITER G A0 FDR 1 01-07-13 R1.0During start-up, 2 MW of EC power at 120 GHz will be provided for 2 s to facilitate initialbreakdown over a range of pre-fill pressures and error-field conditions, and will also providesupplemental heating after breakdown to ensure rapid ionisation (burn-through) of impuritiesin the initial low-current startup plasma.Current Ramp-upThe current ramp-up with average dI/dt ≈ 0.15 MAs -1 is coordinated with a minor radiusexpansion and elongation increase to maintain a nearly constant edge safety factor 5 ≤ q ≤ 6.The plasma remains limited until I SOD (start of divertor) (≈ 0.5 I P0 ) is reached, when a singlenull (SN) divertor configuration is formed (q 95 ≈ 4). Continuation of the current ramp-upresults in an I P0 current flat-top (q 95 ≈ 3) at t ≈ 100 s.Shape and Configuration ControlDynamic control of the plasma shape and position is required throughout the scenario. Theclearance gap between the first wall and the separatrix are controlled, in particular near theoutboard equatorial plane to permit and efficient coupling to the plasma from IC and LHlaunchers.Heating to Driven BurnFollowing SOF (start-of-flat top), auxiliary heating is applied. This, coordinated with plasmafuelling, results in the attainment of H-mode confinement and sustained fusion burn with~ 500 MW of fusion power within ~ 50 s. Attainment of the rated fusion power is denotedas the start-of-burn (SOB). The most demanding requirement in this phase is the transitionfrom L to H mode confinement, which requires that a certain threshold heating power, linkedwith plasma density, be provided. It is assumed that the required plasma shape and currentare achieved at SOF and that plasma heating and β increases are carried out at constant shapeand current. Scenarios, which modify plasma shape and current during the plasma heatingbefore SOB, can also be considered.In extended-pulse-duration/steady-state operation, the scenario is more complicated.Following SOF, auxiliary power heats the plasma and generates a toroidal current which,together with the bootstrap current, replace the Ohmic current. Generally, the current profilebefore the addition of auxiliary power is somewhat different from the final profile. Thecurrent diffusion time across the whole plasma volume is large, e.g. ~ 200 s for T e ≈ 5 keV and~ 800 s for T e ≈ 10 keV. So during the sustained burn, the current profile will evolve to thefinal state over several 100 s.Sustained BurnOnce SOB is attained, accumulation of thermal helium results in the need to further increasethe plasma density to maintain a fixed fusion power: the approach to sustained burn withstationary helium level requires a few tens of seconds. The burn proceeds until the end-of-Plant Description Document Chapter 3 .7 Page 15
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3.7 Plant Control - General Atomics Fusion Group

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