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					Pilot Plant: Volt-seconds Needed
      for Flexible Operation.
               John Sheffield,
   ISSE – University of Tennessee – Knoxville
              October 25, 2010.
        Should we allow for Pulsed Operation?

• How much should one rely on non-inductive start-up?
• Non-inductive start-up in ARIES-AT is estimated to take 1.5 hours.
• On the other hand, ITER takes about 3-5 minutes to raise the
  current and fusion power.
• The likelihood is that a Pilot Plant would have an intermediate start-
  up time e.g. 10 to 20 minutes, and need to provide some of the initial
  volt-seconds (~ 1.5 LI) inductively.

• Is fully non-inductive operation possible with acceptable current
  drive power?
• Rob Goldston has raised the point about German studies.
• Issue of operation of a Pilot Plant at lower bN where the bootstrap
  current and non-inductive current drive do not provide all the current.
• This suggests that there need to be volt-seconds available for a
  partially inductively-driven flat-top—of a few hours?
              Some Key Points

• The Pilot Plant should be able to meet its
  mission under a range of operating conditions –
  bN, n, T, Zeff.
• Current drive will be needed to ensure long
  pulse lengths (SS), and also for profile control.
• In pulsed operation, shut down/start-up times
  must be long enough to minimize stresses in the
• Pulse length must be long enough to meet the
  limit on number of pulses to meet the mission.
             What pulse length and down-time?

•   What pulse lengths should be available when bootstrap current and non-
    inductive current drive cannot provide all the current, e.g. at intermediate bN
•   A study at Argonne, Ehst, D.A., Jardin, S., and Kessel, C. “Starlite,”
    ANL/FPP/TM-284, 1995 looked at the consequences of repetitively pulsed
    operation for a tokamak reactor. They concluded that pulsed operation
    would be acceptable if there were a massive heat storage system available
    to maintain acceptable temperature gradients in the blanket during the down
    time (I recollect that their study assumed that it would be desirable to restrict
    gradients to ~ 50OC across the blanket.

•   How many cycles the plant should experience in its lifetime.
•   For example,15- 30 minutes down might have 5-10 minutes to shut down
    and reload the transformer and 10-20 minutes to start-up.
•   Incidentally, if the system has a double null divertor it might be simpler to
    reverse the current on successive shots.
•   Pulse length > 2 hours might be a good idea ~ 10 pulses/day, ~ 1000/year.
          Reference Pilot Plant-1

• I have used a combination of information from
  Tom Brown and Chuck Kessel.

• R = 4 m, a = 1 m, k = 2, q = 3.6.
• B = 5.4 T, I = 8.2 MA.
• fG = 0.8, bN = 3, PF = 510 MW.

• Jon Menard’s recent paper has similar values.
                       V.s required

Required V.s is freq = LpI + 0tr RpIdt + Rp I tF

• tF = the flat-top time, Lp and Rp are the plasma
  inductance and resistance respectively.
• I is in Amps. 0tr RpIdt ≈ 0.5 LpI (V.s).
• Lp ≈m0R[ln(ac/a) + 0.25} (H), where (ac/a) is the ratio
  of the average poloidal coil radius divided by the average
  plasma radius.
• Rp ≈ {2.6 x 10-4Rgr Zeff ln(Le)}/a2kTe01.5 (Ohms).
Parameter               ITER             ARIES-       Pilot Plant-1    Pilot Plant-1
                                         AT           2.0 x 107 A/m2   2.7 x 107 A/m2
R (m)                   6.2              5.2          4.0              4.0
Rcsmax (m)              2.0              2.25?        0.882            0.882

Rcsmin (m)              1.3              2.05?        1.172            1.172

Rcsmean (m)             1.65             2.15?        1.027            1.027

DRcs winding pack (m)   0.63              0.20 (a)   0.29             0.29
Bcsmax (T)               13.5             5?           7.3              10
Bcsmin (T)              -12.0            -5?          -7.3             - 10
jcs A/m2 winding pack   1.7 x 107        2 x 107 ?    2 x 107          2.7 x 107
1.3fcs V.s              284              142          63               86
1.5 LI V.s.             204              172          84               84
1.3fcs/1.5LI            1.40             0.83         0.75             1.02
Pulse length (hrs)      0.11 – SS (b)    SS           SS (c)           0.02 – 1.3 (d)
   (a) Note that ARIES-AT has plenty of room to provide more V.s.
    (b) ITER operating at reduced Q.
    (c)Assumes that full non-inductive SS is possible.
    (d) Range no to full non-inductive start-up.
         Questions for Pilot Plant-1

• What is acceptable maximum for solenoid
  current density and field?
• At fG = 0.8, the peak electron temperature
  is ~ 16 keV, but 31 keV in ARIES-AT.
• Lower fG would lead to more inductive
  current drive at fixed beta—longer pulses.
• What range of density and temperature
  should be explored in meeting mission?
Parameter                 Pilot Plant-1   Pilot Plant-2

R/a                       4/1             4.4/1.1
B (T)                     5.4             5.1
I (MA)                    8.2             8.5
PF (MW)                   510             540
Rcsmax m                  0.882           1.082
Rcsmin m                  1.172           1.472
Rcsmean m                 1.027           1.277
DRcs winding pack m       0.29            0.39
Bcsmax T                   10              10
Bcsmin T                  - 10            - 10
jcs A/m2 winding pack     2.7 x 107       2.0 x 107

1.3fcs V.s                86              133
1.5 LI V.s.               84               95
1.3fcs/1.5LI              1.02            1.40
Pulse length (hours)      0.02 – 1.3      0.8 – 2.8
fBS/fCD                   0.58/0.24       0.58/0.27
CD scaled from ARIES-AT
  Pilot Plant-2 is more flexible than Pilot plant-1

• An example Pilot plant-2 is 10% larger, with the
  radial space used mainly to provide more V.s.
• For the same solenoid field, the current density
  is lower than in case 1. Higher field and even
  more V.s. should be possible.
• Compared to Pilot Plant-1, it has longer flat top
  times particularly when non-inductive start-up is
  limited –greater flexibility to explore a range of
  plasma conditions at high duty factor.

• The following areas should be explored:
 - Maximum permissible current density/field in the
 - Extent one can rely on non-inductive start-up.
 - Issue of whether non-inductive flat top is
   possible or too restrictive on operating space.
 - Operation with less than full non-inductive
   current drive.
 - Necessary down time and limit on number of
   cycles to keep stresses at acceptable level.

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