24A04
Experiments on Disruption Runaway Electron Suppression
in the DIII-D Tokamak
E.M. Hollmann1, N. Commaux2, N.W. Eidietis3, D.A. Humphreys3, A.N. James1, T.C. Jernigan2, P.B. Parks3,J.C. Wesley3, J.H. Yu1, J.A. Boedo1, N.H. Brooks3, T.E. Evans3, V.A. Izzo1, R.A. Moyer1, D.L. Rudakov1,E.J. Strait3, C. Tsui4, and M.A. Van Zeeland3
1

University of California-San Diego, La Jolla, California 92093, USA
2
Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
3
General Atomics, PO Box 85608, San Diego, California 92186-5608, USA
4
University of Toronto, Toronto M5S-1A1, CanadaLocalized wall damage from runaway electrons formed during tokamak disruptions is a serious
concern for future large tokamaks like ITER. In the DIII-D tokamak, a large variety of experiments
have been conducted to understand runaway electron formation, amplification, and loss; and to devise
methods for avoiding wall damage due to runaway electrons following disruptions. The experiments
can be broken into two categories: massive impurity injection experiments and runaway electronstudies.

€In massive impurity injection experiments,attempts are made to achieve very high total
electron density ( ncrit ≈ 5 ×1016 cm-3 ) during thecurrent quench to collisionally suppress any
possible runaway electron seeds. Massive gas
injection (MGI) experiments indicate that thelimiting €factors in impurity assimilation are thelimited mixing efficiency of the plasma (~20%during the thermal quench and ~1% during the current quench) as well as the limited rise time of thegas injection impurity delivery relative to thethermal quench (TQ) onset time. Present modelingof MGI in ITER based on DIII-D mixing
efficiencies suggests that MGI will work well in
ITER for heat load and vessel force mitigation, but
may not collisionally suppress runaway electrons,
suggesting the need for investigating alternate
impurity delivery methods. First efforts at
improving assimilation over that achieved with
MGI have been made using either shattered pellets
or shell pellets. Shattered D2 pellets, consisting ofa stream of D2 ice shards, have shown
record-level localized electron densities at theinjection port; however, the toroidally averaged mid€current-quench densities are not much larger than€ MGI experiments (total electron density
best-caseachieved
withsimultaneous
ntot ≈ 0.2n crit
five-valve He MGI), suggesting that particle
confinement during the CQ may be limiting theachievable assimilation. Shell pellets consist of adispersive (powder) payload held together by a thinplastic shell which acts as a sacrificial ablator.
Preliminary experiments at demonstrating the shellpellet concept were successful with small (OD ~1 mm) shell pellets. Large (OD ~ 1 cm) shell pellets
with a t = 1 mm polystyrene shell passed throughthe core without releasing their payload, indicatingthat the plastic shell ablation rate was slower than
expected for large shell pellets, possibly due toanomalous rapid heat transport in the strongly perturbed plasma. The capability to launch large shellpellets with a thinner t = 0.1 mm shell is presently
under development.
Normally, runaway electron (RE) formation is quite
small in DIII-D disruptions and MGI shutdowns. To
study RE formation, control, and dissipation, weintentionally create large (>10 kA) RE seeds by
shutting down high temperature target plasmas withsmall (D = 2.7 mm) cryogenic argon pellet injection.
Typically, the initial RE seeds appear to be largely
lost to the vessel wall during the TQ MHD. Consistent
with NIMROD simulations of TQ MHD mode
structure, this prompt TQ loss of RE seeds is observed
to be much larger in lower single null plasma
discharges than in inner wall limited discharges.
Attempts to intentionally enhance the RE prompt loss
using externally applied non-axisymmetric magnetic
field errors have not shown a clear effect yet.
In some cases, sufficient RE seeds survive theprompt loss phase to amplify by electron avalanche
during the CQ and form a high current (0.1–0.5 MA),
long-lived (>100 ms) RE beam whose current iscarried almost completely by the REs. RE avalanche
gains in DIII-D appear to be of order 50 × , consistent
with theory expectations.
Preliminary experiments at controlling these RE
beam currents with external coils have shown clear€ vertically and hold theability to move the RE beamRE beam vertically stable. Reliable radial control has
been more difficult to achieve, due to the configuration of the DIII-D shaping coils, although radial
motion of the beam in response to control input has
been demonstrated. Additionally, ohmic current coilramps have been used to ramp the runaway current up
and down, demonstrating the possibility of dissipating
RE energy harmlessly into external coils. Typically,
the RE beam is lost suddenly to the wall in a final loss
instability before complete ramp down to zero RE
current is achieved. The cause of this final loss
instability is not understood at present. In several
instances, however, a complete ramp down to zerocurrent without final loss instability has beenachieved.
The RE beam plasmas actually consist of two
components:
acold(T ≈ 1.6 eV),dense(ne ≈ 5–15x1013 cm-3) background plasma co-existingwith the very energetic (Te ≈ 20 MeV), tenuous(nR ≈ 4–18x109 cm-3) RE beam. Collisional drag on
the REs in these experiments is thought to bedominantly due to high-Z impurities in the plasma.
Normally, the RE plasmas contain of order ~20%
argon ions from the argon pellet; however, this high-Z
impurity content and resulting RE collisional dissipation can be increased by additional injection ofhigh-Z gas such as Ne, Ar, or Xe. Comparisons of theRE plateau composition with expected collisional
dissipation rates suggest the presence of an anomalous
loss of runaways to the wall at a rate of ~10/s,
possibly due to drift orbit losses.
Preliminary measurements of the energy balance
during the final loss of RE beams striking the DIII-D
wall have been performed. These studies indicate
quite different dynamics for fast RE-wall strikes (withinteraction times << τ wall ≈ 8 ms where τ wall isthe vessel wall time) compared with slow RE-wallstrikes (with interaction times >> τ wall ). In slow
RE-wall strikes, a significant fraction of the RE beam
€ into RE kinetic
magnetic energy
may be converted€
energy. In fast RE-wall strikes, the RE beam magnetic
energy appears to be dominantly
lost by conversion€into ohmic (cold) plasma current and wall currents.

Acknowledgments
This work was supported in part by the US
Department of Energy under DE-FG02-07ER54917,
DE-FC02-04ER54698,
andDE-AC05-00OR22725.