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Plasma Geometry | PlasmaGeom

The control of the plasma shaping is done by the PlasmaGeom class in plasma_geometry.py

The plasma geometric major radius R_0 (rmajor) and aspect ratio A (aspect) define the size of the plasma torus. The plasma minor radius a (rminor) is calculated from these values. The inverse aspect ratio is given by, \epsilon (eps) = 1/A .

The shape of the plasma cross-section is given by the elongation of the last closed flux surface (LCFS) \kappa (kappa) and the triangularity of the LCFS \delta (triang), which can be scaled automatically with the aspect ratio if required using certain switch conditions of ishape:

Plasma Geometry Parameters | geomty()

This subroutine calculates the plasma geometry parameters based on the given input values. The plasma geometry parameters include the shaping terms, plasma aspect ratio, elongation, and triangularity. The function uses various scaling laws and formulas to calculate these parameters based on the specified shape type.

Elongation & Triangularity

  • ishape = 0 -- kappa and triang must be input. The elongation and triangularity of the 95% flux surface are calculated as follows, based on the 1989 ITER guidelines 1: $$ \kappa_{95} = \kappa / 1.12 $$ $$ \delta_{95} = \delta / 1.5 $$
  • ishape = 1 -- kappa and triang must not be input. They are calculated by the following equations, which estimate the largest elongation and triangularity achievable for low aspect ratio machines based on the STAR code2:

$$ \kappa = 2.05 \, \left(1 + 0.44 \, \epsilon^{2.1}\right) $$

$$ \delta = 0.53 \, \left(1 + 0.77 \, \epsilon^3\right) $$

The lower limit for the edge safety factor qlim is also set here $$ q_{\text{lim}} = 3 \, \left(1 + 2.6 \, \epsilon^{2.8}\right) $$

The values for the plasma shaping parameters at the 95% flux surface are calculated using a fit to a family of equilibria calculated using the FIESTA code, equivalent to that used in ishape = 8.

$$ \kappa_{95} = \frac{(\kappa - 0.39467)}{0.90698} $$

$$ \delta_{95} = \frac{(\delta - 0.048306)}{1.3799} $$

  • ishape = 2 -- The Zohm ITER scaling 3 is used to calculate the elongation, where input variable fkzohm = F_{kz} may be used to adjust the scaling, while the input value of the triangularity is used unchanged

$$ \kappa = F_{kz} \, \times \, \mathrm{minimum} \left( 2.0, \, \, 1.5 + \frac{0.5}{A-1} \right) $$

The elongation and triangularity of the 95% flux surface are calculated as follows, based on the 1989 ITER guidelines 1:

$$ \kappa_{95} = \kappa / 1.12 $$ $$ \delta_{95} = \delta / 1.5 $$

  • ishape = 3 -- The Zohm ITER scaling3 is used to calculate the elongation (as for ishape = 2 above), but the triangularity at the 95% flux surface is input via variable triang95, and the LCFS triangularity triang is calculated from it, rather than the other way round.
  • ishape = 4 -- The 95% flux surface values kappa95 and triang95 are both used as inputs, and the LCFS values are calculated from them by inverting the equations given above for ishape = 0.
  • ishape = 5 -- The 95% flux surface values kappa95 and triang95 are both used as inputs and the LCFS values are calculated from a fit to MAST data:

$$ \kappa = 0.913 \, \kappa_{95} + 0.38654 $$

$$ \delta = 0.77394 \, \delta_{95} + 0.18515 $$

  • ishape = 6 -- The input values for kappa and triang are used directly and the 95% flux surface values are calculated using the MAST scaling from ishape = 5.
  • ishape = 7 -- The 95% flux surface values kappa95 and triang95 are both used as inputs and the LCFS values are calculated from a fit to FIESTA runs:

$$ \kappa = 0.90698 \, \kappa_{95} + 0.39467 $$

$$ \delta = 1.3799 \, \delta_{95} + 0.048306 $$

  • ishape = 8 -- The input values for kappa and triang are used directly and the 95% flux surface values are calculated using the FIESTA fit from ishape = 7.
  • ishape = 9 -- The input values for triang and rli are used, kappa and the 95% flux surface values are calculated.

$$ \kappa = \left(\left(1.09+\frac{0.26}{l_i}\right)\left(\frac{1.5}{A}\right)^{0.4}\right) $$

Where l_i is the plasma normalised internal inductance.

The elongation and triangularity of the 95% flux surface are calculated as follows, based on the 1989 ITER guidelines 1:

$$ \kappa_{95} = \kappa / 1.12 $$ $$ \delta_{95} = \delta / 1.5 $$

  • ishape = 10 -- The input values for triang are used directly to calculate 95% flux surface values. kappa is calculated to a fit from CREATE data for a EU-DEMO type machine with aspect ratios of (2.6\le A \le 3.6). Coefficient values are rounded to 2 decimal places

$$ \kappa_{95} = \frac{(18.84 -(0.87 \times A)) - \sqrt{4.84A^2 -28.77 A+52.52+14.74 \times m_{\text{s,limit}}}}{2a} $$ Where m_{\text{s,limit}} is the inputted vertical stablity margin limit (default = 0.3)

If \kappa_{95}>1.77 then its value is adjusted:

\kappa_{95} = \kappa_{95}^{\frac{1.77}{\kappa_{95}}} + \frac{0.3 \ (\kappa_{95}-1.77)}{\frac{1.77}{\kappa_{95}}}

The elongation and the triangularity of the 95% flux surface is calculated as follows, based on the 1989 ITER guidelines 1:

\kappa = 1.12 \ \kappa_{95}
\delta_{95} = \delta / 1.5

  • ishape = 11 -- The elongation is calculated directly dependant on the aspect ratio for spherical tokamak aspect ratios.4

    \kappa = 0.95 \left(1.9+\frac{1.9}{A^{1.4}}\right)

The elongation and triangularity of the 95% flux surface are calculated as follows, based on the 1989 ITER guidelines 1:

$$ \kappa_{95} = \kappa / 1.12 $$ $$ \delta_{95} = \delta / 1.5 $$

An explicit constraint relating to the plasma's vertical stability may be turned on if required. In principle, the inner surface of the outboard shield could be used as the location of a conducting shell to mitigate the vertical displacement growth rate of plasmas with significant elongation 5. The maximum permissible distance r_{\text{shell, max}} of this shell from the geometric centre of the plasma may be set using input parameter cwrmax, such that r_{\text{shell, max}} = cwrmax*rminor. Constraint equation no. 23 should be turned on with iteration variable no. 104 (fcwr) to enforce this

The plasma surface area, cross-sectional area and volume are calculated using formulations that approximate the LCFS as a revolution of two arcs which intersect the plasma X-points and the plasma midplane outer and inner radii. (This is a reasonable assumption for double-null diverted plasmas, but will be inaccurate for single-null plasmas, snull = 1)

Plasma-Wall Gap

The region directly outside the last closed flux surface of the core plasma is known as the scrape-off layer, and contains no structural material. Plasma entering this region is not confined and is removed by the divertor. PROCESS treats the scrape-off layer merely as a gap.

The plasma and first wall clearance can be calculated or input by setting the iscrp switch.

  • iscrp == 0, then the inboard and outboard plasma wall gaps are set to be 10% of the plasma minor radius (a).
  • iscrp == 1, then the inboard and outboard plasma wall gaps are set by defining scrapli and scraplo respectively.

Geometrical properties | xparam()

This method calculates the radius and half angle of the arc describing the inboard and outboard plasma surfaces. This calculation is appropriate for plasmas with a separatrix. It requires the plasma minor radius (a, rminor), elongation (\kappa, kappa) and triangularity (\delta, triang).

Input Variable Description
rminor, a Plasma minor radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
xi Radius of arc describing inboard surface [\text{m}]
thetai Half-angle of arc describing inboard surface
xo Radius of arc describing outboard surface [\text{m}]
thetao Half-angle of arc describing outboard surface

Geometrical properties

Figure 1: Geometrical dimensions used to determine plasma shape parameters
x_i^2 = a^2\kappa^2 + (ay + a +\delta a)^2 \\ = a^2\kappa^2 + a^2L^2, \ \ \ \text{where} \ \ L= 1+\delta+y
x_i = (a+\delta a +ay)+a-\delta a \\ = aL+a(1-\delta) \\ = a(L+T), \ \ \ \text{where}\ \ T=1-\delta
\therefore a^2\kappa^2 + a^2L^2 = a^2(L+T)^2 \\ \kappa^2+L^2 = L^2 +2LT +T^2 \\ \therefore L = \frac{\kappa^2-T^2}{2T}
\fbox{$ \mathtt{thetai} = \theta_i = \arctan \left({\frac{\kappa}{L}}\right) \\ \mathtt{xi} = x_i = a(L+1-\delta)$}

Similarly for the inboard side:

x_o^2 = a^2\kappa^2 + (aw + a -\delta a)^2 \\ = a^2\kappa^2 + a^2M^2, \ \ \ \text{where} \ \ M= 1-\delta+w
x_o = aM +\delta a + a \\ = aM+a(1+\delta) \\ = a(M+N), \ \ \ \text{where}\ \ N=1+\delta
\therefore a^2\kappa^2 + a^2M^2 = a^2(M+N)^2 \\ \kappa^2+M^2 = M^2 +2MN +N^2 \\ \therefore M = \frac{\kappa^2-N^2}{2N}
\fbox{$\mathtt{thetao}= \theta_o = \arctan \left({\frac{\kappa}{M}}\right) \\ \mathtt{xo}=x_o = a(M+1+\delta)$}

Surface Area | xsurf()

This function finds the plasma surface area, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix. It uses the geometrical properties derived in xparam()

Input Variable Description
rmajor, R Plasma major radius [\text{m}]
rminor, a Plasma minor radius [\text{m}]
xi Radius of arc describing inboard surface [\text{m}]
thetai Half-angle of arc describing inboard surface
xo Radius of arc describing outboard surface [\text{m}]
thetao Half-angle of arc describing outboard surface
Output Variable Description
xsi Inboard surface area [\text{m}^2]
xso Outboard surface area [\text{m}^2]

Outboard surface area

Figure 2: Inboard surface area calculation parameters
\mathtt{rc} = R_0-a + \mathtt{xi} \\ \mathtt{xsi} = 4\pi \times \mathtt{xi} (\mathtt{rc} \times \mathtt{thetai} -(\mathtt{xi} \times \sin({\mathtt{thetai}))})

For the outboard side:

Outboard surface area

Figure 3: Outboard surface area calculation parameters
\mathtt{rc} = R_0+a - \mathtt{xo} \\ \mathtt{xso} = 4\pi \times \mathtt{xo} (\mathtt{rc} \times \mathtt{thetao}+ (\mathtt{xo}\times \sin({\mathtt{thetao}))})

Sauter geoemtry | sauter_geometry()

Plasma geometry based on equations (36) in O. Sauter, Fusion Engineering and Design 112 (2016) 633–645 'Geometric formulas for system codes including the effect of negative triangularity'

Input Variable Description
rminor, a Plasma minor radius [\text{m}]
rmajor, R Plasma major radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
pperim Plasma Poloidal perimeter length [\text{m}]
sarea Plasma surface area [\text{m}^2]
xarea Plasma cross-sectional area [\text{m}^2]
vol Plasma volume [\text{m}^3]
\mathtt{w07} = 1
\epsilon = \frac{a}{R}

Poloidal perimeter (named Lp in Sauter)

\mathtt{pperim} = 2.0\pi a (1 + 0.55 (\kappa - 1))(1 + 0.08 \delta^2)(1 + 0.2 (\mathtt{w07} - 1))

A geometric factor

\mathtt{sf} = \frac{\mathtt{pperim}}{2.0\pi a}

Surface area (named Ap in Sauter)

\mathtt{sarea} = 2.0\pi R (1 - 0.32 \delta \epsilon) \mathtt{pperim}

Cross-section area (named S_phi in Sauter) $$ \mathtt{xarea} = \pi a^2 \kappa (1 + 0.52 (\mathtt{w07} - 1)) $$

Volume $$ \mathtt{vol} = 2.0\pi R (1 - 0.25 \delta \epsilon) \mathtt{xarea} $$

Poloidal perimeter

The poloidal plasma perimtere length pperim is calculated as follows: $$ \mathtt{pperim} = 2.0 \times (\mathtt{xo} \times \mathtt{thetao} + \mathtt{xi} \times \mathtt{thetai}) $$

The shaping factor for i_plasma_current = 1 is also calculated here: $$ \mathtt{sf} = \frac{\mathtt{pperim}}{ 2.0\pi a} $$

Plasma Volume | xvol()

The plasma volume is calculated using the xvol method with the inputted R_0 & a along with the outputs of xparam. The cvol iteration variable can be used to scale this output

Input Variable Description
rmajor, R Plasma major radius [\text{m}]
rminor, a Plasma minor radius [\text{m}]
xi Radius of arc describing inboard surface [\text{m}]
thetai Half-angle of arc describing inboard surface
xo Radius of arc describing outboard surface [\text{m}]
thetao Half-angle of arc describing outboard surface
Output Variable Description
xvol Plasma volume [\text{m}^3]

Calculate the volume for the inboard plasma side:

\mathtt{rc} = R_0 - a + \mathtt{xi} \\ \mathtt{vin} = (2\pi \times \mathtt{xi}) \times(\mathtt{rc}^2 \times \sin{(\mathtt{thetai})} - (\mathtt{rc}\times \mathtt{xi} \times \mathtt{thetai})-(0.5\times\mathtt{rc} \mathtt{xi} \times \sin{(2\times\mathtt{thetai})})+(\mathtt{xi}^2\times \sin{(\mathtt{thetai})})-\left(\frac{1}{3}\times \mathtt{xi}^2 \times (\sin{(\mathtt{thetai})})^3\right)

Calculate the volume for the outboard plasma side:

\mathtt{rc} = R_0 + a - \mathtt{xo} \\ \mathtt{vout} = (2\pi \times \mathtt{xo}) \times(\mathtt{rc}^2 \times \sin{(\mathtt{thetao})} + (\mathtt{rc}\times \mathtt{xo} \times \mathtt{thetao})+(0.5\times\mathtt{rc} \times\mathtt{xo} \times \sin{(2\times\mathtt{thetao})})+(\mathtt{xo}^2\times \sin{(\mathtt{thetao})})-\left(\frac{1}{3}\times \mathtt{xo}^2 \times (\sin{(\mathtt{thetao})})^3\right)

The volume is then the difference between the two volumes

\mathtt{xvol} = \mathtt{vout}-\mathtt{vin}

Plasma cross-sectional area | xsecta()

This function finds the plasma cross-sectional area, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix.

Input Variable Description
xi Radius of arc describing inboard surface [\text{m}]
thetai Half-angle of arc describing inboard surface
xo Radius of arc describing outboard surface [\text{m}]
thetao Half-angle of arc describing outboard surface
Output Variable Description
xsecta Plasma cross-sectional area [\text{m}^2]
\mathtt{xsecta} = \mathtt{xo}^2 \times ( \mathtt{thetao} - \cos{(\mathtt{thetao})} \times \sin({\mathtt{thetao}}) ) + \mathtt{xi}^2 \times (\mathtt{thetai} - \cos{(\mathtt{thetai})} \times \sin{(\mathtt{thetai}))}

Legacy claculations

STAR Code plasma surface area | surfa()

This function finds the plasma surface area, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix. It was the original method in PROCESS6.

Input Variable Description
rminor, a Plasma minor radius [\text{m}]
rmajor, R Plasma major radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
sa Plasma total surface area [\text{m}^2]
so Plasma outboard surface area [\text{m}^2]
\mathtt{radco} = a \frac{(1.0 + (\kappa^2 + \delta^2 - 1.0)}{(2.0 \times (1.0 + \delta))}
\mathtt{b} = \kappa \times a
\mathtt{thto} = \arcsin{(\mathtt{b}/\mathtt{radco})}
\mathtt{so} = 4.0\pi \times \mathtt{radco} \times ((R + a - \mathtt{radco}) \times \mathtt{thto} + \mathtt{b})

Inboard side

\mathtt{radci} = a \frac{(1.0 + (\kappa^2 + \delta^2 - 1.0)}{(2.0 \times (1.0 - \delta))}
\mathtt{b} = \kappa \times a
\mathtt{thti} = \arcsin{(\mathtt{b}/\mathtt{radci})}
\mathtt{si} = 4.0\pi \times \mathtt{radci} \times ((R - a + \mathtt{radci}) \times \mathtt{thti} - \mathtt{b})
\mathtt{sa} = \mathtt{so} + \mathtt{si}

Plasma poloidal perimeter calculation | perim()

This function finds the plasma poloidal perimeter, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix.

Input Variable Description
rminor, a Plasma minor radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
perim Plasma poloidal perimeter length [\text{m}]

Inboard arc

\mathtt{denomi} = \frac{(\delta^2 + \kappa^2 - 1.0)}{(2.0 \times (1.0 - \delta))}+ \delta
\mathtt{thetai} = \arctan\left(\frac{\kappa}{\mathtt{denomi}}\right)
\mathtt{xli} = a \times (\mathtt{denomi} + 1.0 - \delta)

Outboard arc

\mathtt{denomo} = \frac{(\delta^2 + \kappa^2 - 1.0)}{(2.0 \times (1.0 + \delta))} -\delta \\
\mathtt{thetao} = \arctan\left(\frac{\kappa}{\mathtt{denomo}}\right) \\
\mathtt{xlo} = a \times (\mathtt{denomo} + 1.0 + \delta) \\
\mathtt{perim} = 2.0 \times (\mathtt{xlo} \times \mathtt{thetao} + \mathtt{xli} \times \mathtt{thetai})

Plasma volume calculation | fvol()

This function finds the plasma volume, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix.

Input Variable Description
rmajor, R Plasma major radius [\text{m}]
rminor, a Plasma minor radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
fvol Plasma volume [\text{m}^3]
\mathtt{zn} = \kappa \times a
\mathtt{c1} = \frac{{(R + a)^2 - (R - \delta \times a)^2 - \mathtt{zn}^2}}{{2 \times (1 + \delta) \times a}}
\mathtt{rc1} = R + a - \mathtt{c1}
\mathtt{{vout}} = -\frac{2}{3} \pi \times \mathtt{zn}^3 + 2 \pi \times \mathtt{zn} \times (\mathtt{c1}^2 + \mathtt{rc1}^2) + 2 \pi \times \mathtt{c1} \times \left(\mathtt{zn} \times \sqrt{\mathtt{rc1}^2 - \mathtt{zn}^2} + \mathtt{rc1}^2 \times \arcsin{\left(\frac{\mathtt{zn}}{\mathtt{rc1}}\right)}\right)
\mathtt{c2} = \frac{-((R - a)^2) + (R - \delta \times a)^2 + \mathtt{zn}^2)}{(2 \times (1 - \delta) \times a)}
\mathtt{rc2} = \mathtt{c2} - R + a
\mathtt{vin} = -\frac{2}{3} \pi \times \mathtt{zn}^3 + 2 \pi \times \mathtt{zn} \times (\mathtt{rc2}^2 + \mathtt{c2}^2) - 2 \pi \times \mathtt{c2} \times \left(\mathtt{zn} \times \sqrt{\mathtt{rc2}^2 - \mathtt{zn}^2} + \mathtt{rc2}^2 \times \arcsin{\left(\frac{\mathtt{zn}}{\mathtt{rc2}}\right)}\right)
\mathtt{fvol} = \mathtt{vout} - \mathtt{vin}

Plasma cross sectional area calculation | xsecto()

This function finds the plasma cross-sectional area, using the revolution of two intersecting arcs around the device centreline. This calculation is appropriate for plasmas with a separatrix.

Input Variable Description
rminor, a Plasma minor radius [\text{m}]
kappa, \kappa Plasma separatrix elongation
triang, \delta Plasma separatrix triangularity
Output Variable Description
xsect0 Plasma cross-sectional area [\text{m}^2]
\mathtt{denomi} = \frac{(\delta^2 + \kappa^2 - 1.0)}{(2.0 \times (1.0 - \delta))}+ \delta
\mathtt{thetai} = \arctan\left(\frac{\kappa}{\mathtt{denomi}}\right)
\mathtt{xli} = a \times (\mathtt{denomi} + 1.0 - \delta)
\mathtt{cti} = \cos(\mathtt{thetai})
\mathtt{sti} = \sin(\mathtt{thetai})
\mathtt{denomo} = \frac{(\delta^2 + \kappa^2 - 1.0)}{(2.0 \times (1.0 + \delta))}- \delta
\mathtt{thetao} = \arctan\left(\frac{\kappa}{\mathtt{denomo}}\right)
\mathtt{xlo} = a \times (\mathtt{denomo} + 1.0 + \delta)
\mathtt{cto} = \cos(\mathtt{thetao})
\mathtt{sto} = \sin(\mathtt{thetao})
\mathtt{xsect0} = \mathtt{xlo}^2 \times (\mathtt{thetao} - \mathtt{cto} \times \mathtt{sto}) + \mathtt{xli}^2 \times (\mathtt{thetai} - \mathtt{cti} \times \mathtt{sti})

  1. N.A. Uckan and ITER Physics Group, ITER Physics Design Guidelines: 1989, ITER Documentation Series, No. 10, IAEA/ITER/DS/10 (1990) ↩↩↩↩↩

  2. J.D. Galambos, 'STAR Code : Spherical Tokamak Analysis and Reactor Code', Unpublished internal Oak Ridge document. ↩

  3. H. Zohm et al, 'On the Physics Guidelines for a Tokamak DEMO', FTP/3-3, Proc. IAEA Fusion Energy Conference, October 2012, San Diego ↩↩

  4. Menard, J.E. & Brown, T. & El-Guebaly, L. & Boyer, M. & Canik, J. & Colling, Bethany & Raman, Roger & Wang, Z. & Zhai, Yunbo & Buxton, Peter & Covele, B. & D’Angelo, C. & Davis, Andrew & Gerhardt, S. & Gryaznevich, M. & Harb, Moataz & Hender, T.C. & Kaye, S. & Kingham, David & Woolley, R.. (2016). Fusion nuclear science facilities and pilot plants based on the spherical tokamak. Nuclear Fusion. 56. 106023. 10.1088/0029-5515/56/10/106023. ↩

  5. H.S. Bosch and G.M. Hale, Improved Formulas for Fusion Cross-sections and Thermal Reactivities', Nuclear Fusion 32 (1992) 611 ↩

  6. J D Galambos, STAR Code : Spherical Tokamak Analysis and Reactor Code, unpublished internal Oak Ridge document ↩