# Pivotal issues on relativistic electrons in ITER

## Abstract

The transfer of the plasma current from thermal to relativistic electrons is a threat to ITER achieving its mission. This danger is significantly greater in the nuclear than in the non-nuclear phase of ITER operations. Two issues are pivotal. The first is the extent and duration of magnetic surface breaking in conjunction with the thermal quenches. The second is the exponential sensitivity of the current transfer to three quantities: (1) the poloidal flux change required to e-fold the number of relativistic electrons, (2) the time $$\tau_a$$ after the beginning of the thermal quench before the accelerating electric field exceeds the Connor-Hastie field for runaway, and (3) the duration of the period $$\tau_{\rm op}$$ in which magnetic surfaces remain open. Adequate knowledge does not exist to devise a reliable strategy for the protection of ITER. Uncertainties are sufficiently large that a transfer of neither a negligible nor the full plasma current to relativistic electrons can be ruled out during the non-nuclear phase of ITER. Tritium decay can provide a sufficiently strong seed for a dangerous relativistic-electron current even if $$\tau_a$$ and $$\tau_{\rm op}$$ are sufficiently long to avoid relativistic electrons during non-nuclear operations. The breakup of magnetic surfaces that is associated with thermal quenches occurs on a time scale associated with fast magnetic reconnection, which means reconnection at an Alfvénic rather than a resistive rate. Alfvénic reconnection is well beyond the capabilities of existing computational tools for tokamaks, but its effects can be studied using its property of conserving magnetic helicity. Although the dangers to ITER from relativistic electrons have been known for twenty years, the critical issues have not been defined with sufficient precision to formulate an effective research program. Studies are particularly needed on plasma behavior in existing tokamaks during thermal quenches, behavior which could be clarified using methods developed here.

- Authors:

- Publication Date:

- Research Org.:
- Columbia Univ., New York, NY (United States)

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

- OSTI Identifier:
- 1437785

- Alternate Identifier(s):
- OSTI ID: 1501914

- Grant/Contract Number:
- FG02-03ER54696; SC0016347

- Resource Type:
- Published Article

- Journal Name:
- Nuclear Fusion

- Additional Journal Information:
- Journal Name: Nuclear Fusion Journal Volume: 58 Journal Issue: 3; Journal ID: ISSN 0029-5515

- Publisher:
- IOP Science

- Country of Publication:
- IAEA

- Language:
- English

- Subject:
- 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; runaway electrons; thermal quench; current spike; tokamak disruptions

### Citation Formats

```
Boozer, Allen H. Pivotal issues on relativistic electrons in ITER. IAEA: N. p., 2018.
Web. doi:10.1088/1741-4326/aaa1db.
```

```
Boozer, Allen H. Pivotal issues on relativistic electrons in ITER. IAEA. doi:10.1088/1741-4326/aaa1db.
```

```
Boozer, Allen H. Mon .
"Pivotal issues on relativistic electrons in ITER". IAEA. doi:10.1088/1741-4326/aaa1db.
```

```
@article{osti_1437785,
```

title = {Pivotal issues on relativistic electrons in ITER},

author = {Boozer, Allen H.},

abstractNote = {The transfer of the plasma current from thermal to relativistic electrons is a threat to ITER achieving its mission. This danger is significantly greater in the nuclear than in the non-nuclear phase of ITER operations. Two issues are pivotal. The first is the extent and duration of magnetic surface breaking in conjunction with the thermal quenches. The second is the exponential sensitivity of the current transfer to three quantities: (1) the poloidal flux change required to e-fold the number of relativistic electrons, (2) the time $\tau_a$ after the beginning of the thermal quench before the accelerating electric field exceeds the Connor-Hastie field for runaway, and (3) the duration of the period $\tau_{\rm op}$ in which magnetic surfaces remain open. Adequate knowledge does not exist to devise a reliable strategy for the protection of ITER. Uncertainties are sufficiently large that a transfer of neither a negligible nor the full plasma current to relativistic electrons can be ruled out during the non-nuclear phase of ITER. Tritium decay can provide a sufficiently strong seed for a dangerous relativistic-electron current even if $\tau_a$ and $\tau_{\rm op}$ are sufficiently long to avoid relativistic electrons during non-nuclear operations. The breakup of magnetic surfaces that is associated with thermal quenches occurs on a time scale associated with fast magnetic reconnection, which means reconnection at an Alfvénic rather than a resistive rate. Alfvénic reconnection is well beyond the capabilities of existing computational tools for tokamaks, but its effects can be studied using its property of conserving magnetic helicity. Although the dangers to ITER from relativistic electrons have been known for twenty years, the critical issues have not been defined with sufficient precision to formulate an effective research program. Studies are particularly needed on plasma behavior in existing tokamaks during thermal quenches, behavior which could be clarified using methods developed here.},

doi = {10.1088/1741-4326/aaa1db},

journal = {Nuclear Fusion},

number = 3,

volume = 58,

place = {IAEA},

year = {2018},

month = {1}

}

DOI: 10.1088/1741-4326/aaa1db

*Citation information provided by*

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Web of Science

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