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# Toroidal effect on runaway vortex and avalanche growth rate

## Abstract

The momentum-space dynamics of runaway electrons in a slab geometry, in terms of both the geometry and topological transition of the runaway vortex when synchrotron radiative damping is taking into account, have recently been shown to play a crucial role in runaway mitigation and avoidance. In a tokamak geometry, magnetic trapping arises from parallel motion along the magnetic field that scales as 1/R in strength with R the major radius. Since the transit time for a runaway electron moving along the field is of order 10 ^{-8} sec while the collisional time is of ~ 0.01 sec in ITER-like plasmas, a bounce-averaged formulation can drastically reduce computational cost. Here the LAPS-RFP code’s implementation of a bounce-averaged relativistic Fokker- Planck model, along with the essential physics of synchrotron radiation damping and knock-on collisions, is described. It is found that the magnetic trapping can reduce the volume of the runaway vortex as the momentum-space fluxes are strongly modified inside the trapped-region. As a result, the avalanche growth rate is reduced at off-axis locations. In addition to benchmarking with previous calculations that did not take into account radiation damping, we also clarify how synchrotron radiation damping modifies the avalanche growth rate in amore »

- Authors:

- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

- Publication Date:

- Research Org.:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

- Sponsoring Org.:
- USDOE Office of Science (SC). Fusion Energy Sciences (FES) (SC-24); USDOE

- OSTI Identifier:
- 1558965

- Alternate Identifier(s):
- OSTI ID: 1549543

- Report Number(s):
- LA-UR-18-28552

Journal ID: ISSN 1070-664X

- Grant/Contract Number:
- 89233218CNA000001

- Resource Type:
- Accepted Manuscript

- Journal Name:
- Physics of Plasmas

- Additional Journal Information:
- Journal Volume: 26; Journal Issue: 8; Journal ID: ISSN 1070-664X

- Publisher:
- American Institute of Physics (AIP)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Magnetic Fusion Energy

### Citation Formats

```
Guo, Zehua, Mcdevitt, Chris, and Tang, Xianzhu. Toroidal effect on runaway vortex and avalanche growth rate. United States: N. p., 2019.
Web. doi:10.1063/1.5055874.
```

```
Guo, Zehua, Mcdevitt, Chris, & Tang, Xianzhu. Toroidal effect on runaway vortex and avalanche growth rate. United States. doi:10.1063/1.5055874.
```

```
Guo, Zehua, Mcdevitt, Chris, and Tang, Xianzhu. Fri .
"Toroidal effect on runaway vortex and avalanche growth rate". United States. doi:10.1063/1.5055874.
```

```
@article{osti_1558965,
```

title = {Toroidal effect on runaway vortex and avalanche growth rate},

author = {Guo, Zehua and Mcdevitt, Chris and Tang, Xianzhu},

abstractNote = {The momentum-space dynamics of runaway electrons in a slab geometry, in terms of both the geometry and topological transition of the runaway vortex when synchrotron radiative damping is taking into account, have recently been shown to play a crucial role in runaway mitigation and avoidance. In a tokamak geometry, magnetic trapping arises from parallel motion along the magnetic field that scales as 1/R in strength with R the major radius. Since the transit time for a runaway electron moving along the field is of order 10-8 sec while the collisional time is of ~ 0.01 sec in ITER-like plasmas, a bounce-averaged formulation can drastically reduce computational cost. Here the LAPS-RFP code’s implementation of a bounce-averaged relativistic Fokker- Planck model, along with the essential physics of synchrotron radiation damping and knock-on collisions, is described. It is found that the magnetic trapping can reduce the volume of the runaway vortex as the momentum-space fluxes are strongly modified inside the trapped-region. As a result, the avalanche growth rate is reduced at off-axis locations. In addition to benchmarking with previous calculations that did not take into account radiation damping, we also clarify how synchrotron radiation damping modifies the avalanche growth rate in a tokamak.},

doi = {10.1063/1.5055874},

journal = {Physics of Plasmas},

number = 8,

volume = 26,

place = {United States},

year = {2019},

month = {8}

}

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