October 13, 2022

Transition-driven transport in stellarators

Elizabeth Paul

Princeton University/PPPL

NCSX

ITER w/ ripple

ARIES-CS

Collisionless guiding center tracing in QA configurations

1

Quasisymmetry error

Loss fraction

Collisionless guiding center tracing in QH configurations

Wistell-A

HSX

Ku-5

2

Quasisymmetry error

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS

“Banana” : trapped in primary well

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS

“Ripple trapped” : trapped in secondary well

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS

“Barely trapped” : traverses > 1 field period between bounces

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS

“Passing” : never bounces

Basic trapping classes in magnetic fields close to quasisymmetry

3

ARIES-CS irregular transitioning

4

Class 2: Banana

Class 1: Ripple trapped

Class 3: Barely trapped

ARIES-CS periodic transitioning

5

Class 2: Banana

Class 1: Ripple trapped

Wistell-A periodic transitioning

6

Class 1: Banana

Class 2: Barely trapped

Ku-5 irregular transitioning

7

Class 2: Banana

Class 1: Ripple trapped

Class 3: Barely trapped

Transition-driven alpha transport important on long timescales

Class 2: Banana

Class 1: Ripple trapped

Class 3: Barely trapped

8

Several metrics have been proposed to prevent transitions

9

Isodrasticity calculations in simsopt

”Perfect” QA

10

Concluding thoughts

11

• Non-prompt losses are often dominated by orbits with many class transitions (especially if there are few prompt losses)
• Sometimes periodic, sometimes irregular (”stochastic”)

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• Standard metrics for transitions are less general than isodrasticity

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• Initial implementation made in simsopt
• Further development needed to account for multiple maxima on surface

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