EX/P8-06
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Overview of runaway electrons control and mitigation experiments
on Tore Supra and lessons learned in view of ITER
F. Saint-Laurent1, G. Martin1, P.B Parks2, S. Putvinski3, J. Bucalossi1, S. Bremond1,
Ph. Moreau1
1
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
General Atomics, San Diego, California, 92186, USA
3
ITER Organization, Route de Vinon sur Verdon, 13115 Saint Paul Lez Durance, France
2
E-mail contact of main author: francois.saint-laurent@cea.fr
Abstract. Runaway electrons (RE) generated during disruption are identified as a major issue for ITER and
reactor-size tokamaks. A seed of decoupled electrons is produced in the plasma core at thermal quench of the
disruption when the current profile flattens due to high MHD activity. These RE are then accelerated by the
toroidal electric field associated to the plasma current quench (CQ) up to relativistic energy. They initiate second
RE generation during current quench (CQ) due to the avalanching process, leading to a multiplication of these
relativistic electrons. The impact of RE on the first wall is well localized due to their very small pitch angle. The
energy deposition may be huge and plasma facing components (PFC) damages are often reported.
Mitigation techniques are thus mandatory to suppress RE formation or/and reduce their heat loads. Two ways are
explored on Tore Supra:
- Suppress the RE beam formation and avalanche amplification by multiple gas jet injections at (CQ).
- Control the RE beam when it is formed and increase the collisionnality to slow-down relativistic
electrons.
1. Introduction
Disruptions and runaway electrons (RE) are identified as a major issue for ITER and reactorsize tokamaks [1]. Disruption produces excessive heat loads on plasma facing components,
induces strong electromagnetic forces in the vessel structures, and generates multi-MeV
runaway electrons. First generation of RE is created in the plasma core during the thermal
quench (TQ) of the disruption, when the plasma current profile flattens producing a huge
toroidal electric field closed to the magnetic axis. Thus fast electrons experiencing low
collisionnality can be freely accelerated. They initiate second RE generation during current
quench (CQ) due to the avalanching process, leading to a multiplication of these relativistic
electrons. The impact of RE on the first wall is well localized due to their very small pitch
angle. The density energy deposition may be huge (10MJ/m2), exceeding the thermal limits of
materials and plasma facing components (PFC) damages are often reported.
Disruption mitigation techniques are actively studied on nowadays tokamaks. The overall
strategy of disruption mitigation for ITER can be described using the defense in depth
concept. The approach, applied in safety studies, is to interpose successive barriers when a
disruptive situation is identified in order to reduce and/or suppress disruption deleterious
effects:
- The 1st barrier requests the detection of disruption precursor as earlier as possible and
deployment of actions to avoid the disruption. Plasma performance could be
temporarily lost.
- When disruption is unavoidable, the 2nd barrier needs to trigger a controlled/mitigated
disruption by massive gas injection (MGI) or shattered pellets to reduce heat loads and
electromagnetic forces.
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Because RE generation may occur, a 3rd barrier must be set in to prevent the
exponential amplification of RE. Studies using resonant magnetic perturbation (RMP)
or multiple gas jet injections are thus undertaken.
- Finally a 4th barrier is mandatory in the case where a RE beam is formed. Active
position control of this RE beam can be helpful to drive RE on dedicated sacrificial
component and/or to save time to perform a collisionnal deceleration up to the
thermalization.
Two ways are explored on Tore Supra which addresses the 3rd and 4th barriers.
-
2. Multiple gas jet injection at current quench
The aim is to deconfine the RE seed at CQ by supersonic gas injection before their
exponential amplification. Such a gas jet might penetrate the cold CQ plasma until q=2
surface, squeeze the current profile and initiate MHD activity which will induce a secondary
disruption [2]. Following the ergodized field lines, RE will be destabilized and spread over
the first wall, thus reducing the density energy deposition. To achieve the requested very fast
injection, a new concept of injector has been developed (FIG.1). A high pressure gas cartridge
(150 bars), located close to the plasma is open by rupture of a bursting disk. The rupture is
initiated by an electric arc inside the gas. The electric impulse is delivered by the discharge of
a capacitor bank (300 J-1500 V) through a resistive spring. The vaporization of the spring
initiates the plasma arc (2000-3000 A) and induces a shock wave inside the pressurized gas.
The opening of the busting disk is thus achieved in less than 300 µs, and releasing the gas is
then completed in about 1.5 ms.
FIG. 1. Detail of a cartridge (left). Cartridges assembly before its installation on Tore Supra (right).
The system has been installed and successfully tested on Tore Supra (FIG.2). Disruptive
plasma was initiated either by Ar puffing or by D2 density limit. The thermal quench
detection (negative loop voltage spike at high field side) was used to trigger Neon or helium
gas injections (240 Pam3) at CQ with an adjustable delay. The propagation of the gas burst in
the plasma is followed using a fast framing camera equipped with an interferential filter
(λ = 580.9 ± 2.7 nm) corresponding mostly to Ne I or He I spectroscopic lines [3].
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6
Cartridges
Vacuum
Vessel
First Wall
2
4
Jet > 500 Pa
Plasma
FIG. 2. Poloidal view showing the location of cartridges with respect to the plasma (left). Top down
view showing the location of relevant diagnostics (right)
Fast camera images show that the high pressure gas jet propagates through the CQ plasma
(FIG.3). The cold gas front is found to penetrate the plasma at 500 m/s for neon and at
1200 m/s for helium. These values correspond to half of gas velocity in vacuum. Even if the
density at CQ is dominated by first wall outgassing, an increase of plasma density is also
observed confirming that the electronic temperature at CQ seems to be greater than 5 eV.
Despite these observations, no robust perturbations on the current decay and on the loop
voltage are recorded [2]. The expected RE suppression has not been observed yet. Moreover,
neither indication of an increase of MHD activity nor RE destabilization is observed.
Modeling of current barycentre trajectory at CQ suggests that the q=2 surface moves to the
magnetic axis faster than the impinging gas front penetration, preventing the secondary
disruption.
TS#47373 @ -8.01ms / Ip = 820 kA
tpeak + 0.96 ms
Ip = 820 kA
TS#47373 @ -7.21ms / Ip = 656 kA
TS#47373 @ -7.85ms / Ip = 786 kA
tpeak + 1.18 ms
Ip = 786 kA
TS#47373 @ -6.97ms / Ip = 607 kA
TS#47373 @ -7.61ms / Ip = 736 kA
tpeak + 1.39 ms
Ip = 736 kA
TS#47373 @ -6.73ms / Ip = 566 kA
TS#47373 @ -7.37ms / Ip = 686 kA
tpeak + 1.60 ms
Ip = 686 kA
TS#47373 @ -6.57ms / Ip = 541 kA
tpeak + 1.81 ms
tpeak + 2.03 ms
tpeak + 2.24ms
tpeak + 2.45ms
Ip = 656 kA
Ip = 607 kA
Ip = 566 kA
Ip = 541 kA
FIG. 3. Fast framing camera images showing the gas jet propagation through the cold CQ plasma.
The first wall and toroidal pumped limiter location (blue lines) and q surface, ranging from q=2 to 6
(yellow lines), are superimposed to the picture.
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The current decay and the current barycentre trajectories for a reference shot without gas jet
injection and with either neon or helium gas jets triggered 200µs after the TQ are compared in
figure 4. Even the gas penetration through the plasma is followed by the fast framing camera,
no plasma perturbation is observed, either on plasma current decay rate, or in the plasma
current trajectory.
1.2
0.2
Ip (MA)
48039 :
Ne injection
0.8
0.6
48037 :
He injection
0.4
ZOOM
0.15
TQ+1ms
Z (m)
1
Reference :
No injection
0.1
TQ+2ms
TQ+3ms
0.05
TQ+4ms
0
0.2
TQ+5ms
-0.05
0
-2
0
2
4
Time (ms)
6
2.2
2.25
2.3
2.35
2.4
R (m)
FIG. 4. Comparison of the current decay rate (left) and plasma current barycentre trajectory (right)
for three shots: a reference disruption and for neon or helium gas jet injections at CQ.
Equilibrium reconstruction using either EFIT or MAESTRO equilibrium codes provides
information on the current profile evolution at CQ. Both codes exhibit the same internal
inductance evolution: a decrease at TQ which is associated to the huge MHD activity and a
subsequent current profile flattening, then a rapid increase at CQ given indication that the
nested magnetic topology is restored and the current is lost from its edge. The predicted q
profiles are superimposed to fast camera frames in figure 2. We find that the q=2 surface
could be reached by the gas jet.
A possible explanation for the absence of the expected magnetic perturbation can be
suggested. An electrical breakdown may occur in the gas jet driven by a high local voltage that
is generated across the jet when plasma current is interrupted by the jet. This would be
possible especially in the case when there is a seed ionization by RE in the jet. Another
possibility is the small estimated variation of the conductivity inside and outside the gas jet
due the already very low electron temperature of the plasma at CQ. The CQ plasma being
highly resistive, the extra resistivity induced by the gas jet ionization would not lead to the
expected current shrinking.
3. Runaway Electrons plateau formation
RE beams lasting several seconds can be observed on Tore Supra for disruption occurring
during the plasma current ramp-up [4-5]. A current of several hundred of kilo amps,
corresponding to 20-60% of the pre-disruptive plasma current can be associated to these
beams. Such a plateau formation is eased with circular plasma in limiter configuration. We
found that multi-seconds duration plateau develops only when the CFC first wall is depleted
of deuterium. Indeed, when the first wall is saturated with deuterium, the RE seed formation
seems to be strongly hindered by the large outgassing consecutive to the thermal quench.
Sufficiently high runaway electron current for vertical equilibrium field matching cannot be
generated to sustain the plateau due to the limited avalanching amplification at CQ on Tore
Supra (Ip < 1.5 MA).
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The capability to sustain long RE plateaus is a unique opportunity to study this unusual
plasma regime. Disruptive plasma is initiated by impurity (neon) puffing at plasma current
ramp-up. Reproducible RE plateau formation is thus achieved. Analysis of magnetics and
infra-red interferometer measurements shows that the RE plateau is characterized by the
superposition of a relativistic electron beam that sustains the current (3 1016 electrons per
100 kA of IRE current on Tore Supra) together with a cold background plasma which gives rise
of the free electron density (few 1019 m-1 line integrated electron density) (fig. 5). The
background plasma originated from the re-ionization of neutrals by RE collisions after the
CQ, and also from the interaction of RE with carbon dusts suspended by mechanical forces
generated at disruption.
This interpretation is supported by the experimental observations (fig. 5):
- The line integrated electron density shows a decrease below 1018 m-2 at the end of the
CQ, and then recovers to 1-1.5×1019 m-2 after 50 ms, without any gas fuelling. This
density is well above the RE density (few 1017 m-2) estimated from the RE current.
- Simultaneously the neutral pressure at plasma edge exhibits a large peak at the end of
CQ followed by a subsequent decrease on the same time scale than the increase of
electron density.
Disruption
TS#33707
0.8
RE plateau
Without any active control
0.7
Pressure of
neutrals
×10 (Pa)
0.6
Ip (MA)
0.5
0.4
0.3
0.2
nl (1020 m-2)
0.1
0
-0.5
0
0.5
1
1.5
t – tdis (s)
2
2.5
FIG. 5: uncontrolled RE plateau generated after a disruption. 100 ms after the RE plasma regime
establishment a free electron density recovers without any gas fuelling associated to a decrease of
the pressure of neutrals.
The parallel resistivity is larger by a factor of two than the one measured for well confined D2
ohmic plasma at the same current. The expected larger Zeff during RE plateau due to the
presence of a large amount of carbon dust as recorded by the fast framing camera could
explain this finding. The RE plateau internal inductance is about twice the pre-disruptive
value as expected when considering seed formation of RE at TQ closed to magnetic axis. Its
value decreases along RE flat-top. This current profile flattening could be attributed to RE
collisions with background plasma. This interpretation is supported by the observed toroidal
symmetry of the photoneutron flux during the RE plateau regime. RE loosed by direct
interaction with prominent first wall component is not the dominant seed for photoneutron on
flattop.
4. Runaway Electrons beam control.
Mastering the RE plateau regime is a key issue to deploy mitigation techniques. An active
control of the RE beam position might be mandatory to drive the RE beam towards a
sacrificial first wall dedicated component. The increase of collisionnality by massive gas
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injection (MGI) could be a promising technique for mitigation [6]. Collisions drive the RE
towards the wall and enhance the RE slowing down. The first effect spreads the REs over the
entire first wall, the second reduces their energy. Both are beneficial to reduce the heat loads.
Nevertheless the thermalization of multi MeV relativistic electron takes time. Thermalization
of 1 MeV electron into 1.7 1020 m-3 electron densities requires 30 ms [7]. This value
extrapolates to half a second for 15 MeV electrons. Thus an active control of RE regime is
required. Together with a beam position control, the RE current must also be sustained to
maintain its value inside the operational domain of the plasma control system.
A control of the RE beams position for several seconds duration has already been
demonstrated [6]. Associated to a position control, the RE current control has also been
demonstrated on Tore Supra. Considering the strong current profile peaking and the reduced
total current, the standard real time plasma controller was modified to feedback on the current
barycentre localization rather than the plasma boundary. Reproducible several seconds
controlled RE plateaus were achieved. Nevertheless this control is found to be not optimum.
A slow (≈ 14 Hz) major radius oscillation may occur associated to a current variation in the
opposite phase (fig. 6). This frequency reduces when decreasing the feedback proportional
gain of the controller.
0.6
2.6
TS#46062
0.5
Ip (MA)
0.4
Major radius position (m)
2.4
Ref.
0.3
0.2
Ref.
2.3
2.2
Meas.
Meas.
2.1
0.1
0
TS#46062
2.5
0
0.5
1
1.5 2
time (s)
2.5
3
3.5
2
0
0.5
1
1.5 2
2.5
time (s)
3
3.5
FIG. 6: position and current controlled RE plateau generated after a disruption induced by Ne
puffing. Reference and measured values are compared. At t = 2.1 s the plasma controller undergoes
an oscillatory instability.
The coupling of position and current controllers is found to be responsible for such instability.
Indeed, for relativistic electrons moving closely to the light velocity, the radial position R and
the runaways current IRE are connected through the total number of runaway electrons,
NRE ∝ R×IRE which is not taken into account by the plasma control system. The amplification
of the instability leads to the loss of runaways beam control after a couple of seconds.
Nevertheless this RE plateau’ duration is long enough to enable dedicated studies for RE
beam mastering.
5. Runaway Electrons beam mitigation.
The capability for decelerating a RE beam through an increase of the electron collisionnality
has thus been investigated on Tore Supra. Massive gas injection (MGI) was triggered on a
controlled RE plateau to increase the electron collisionnality. Previous experiments show that
the amount of injected gas must be modest to avoid a too fast slowing down of RE and a
possible loss of control. Helium gas was chosen due to its better penetration and its lower
slowing down capability [8].
The helium gas penetration leads to a fast increase of the electron density associated to a
larger light emission (fig. 7). The slow oscillation observed on the vertical line integrated
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density is associated to a variation in the plasma radial position. This higher plasma density
prevents the further penetration of neutrals as seen on the edge pressure evolution. The edge
neutral pressure drops rapidly after the RE plateau termination. The final pressure is the one
expected for a uniform filling of the vacuum vessel.
With active position and current control
4
He MGI
2.1 1020 at/m3
e- line integrated
density (1019 m-2)
3.5
TS#46064
3
Pressure of
Neutrals (Pa)
2.5
2
1.5
Ip ×5 (MA)
1
Photoneutron
(1012 s-1)
0.5
0
0
0.2
0.4
0.6
0.8
1
1.2
t-tdis (s)
FIG. 7: MGI on position and current controlled RE plateau generated after a disruption.
Left: fast camera pictures before (top) and after (bottom) helium MGI.
Right: MGI is triggered after 0.5 s. Gas assimilation is followed by electron density raise and the
increase of collisionnality is effective. A partial thermalization of RE beam is deduced from the
photoneutron flux evolution.
Just after the MGI triggering, the flux of photoneutron rises. The photoneutrons are generated
by relativistic electron collisions at energy above 10-15 MeV. This increase is attributed to the
increase of electron collisionnality. A faster increase of photoneutron flux closed to the MGI
localization has also been observed. Then a slowing-down of relativistic electrons is initiated
and a subsequent progressive reduction of the photoneutron flux is observed, down to a factor
10 of reduction. At constant IRE the high energy population (E>10MeV) of RE reduces. The
slowing-down of RE does not affect the high energy tail only. The whole RE spectrum is
narrowed and a beginning of thermalization might be achieved. Finally the RE plateau
terminates abruptly associated with a large photoneutron peak. The bulk of RE current is
rapidly driving towards the first wall CFC components when the position control is loss. A
well localized RE impact is observed on the outboard limiter indicating that energetic
electrons (E ≈ 1 MeV) are still present in the beam. Despite this final observation these results
look like very encouraging. A way for mastering the RE beam regime towards a full
thermalization has been explored, and the first results indicate clearly that the deceleration of
RE beam through an increase of the background plasma density is effective. The remaining
key issue is the development of a better control of the RE plateau regime.
5. Conclusions and lessons for ITER operation.
A reliable suppression of RE avalanching at CQ must be achieved to guarantee that RE effects
are mastered. The increase of the electron density (free + bound) above the Rosenbluth’
critical density is the only nowadays way to cancel this avalanche process [9]. The multiple
gas jet injection at CQ would furnish an alternative mitigation technique with a strongly
reduced gas inventory (a factor 10 below is anticipated). Using a new designed ultra fast gas
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injector, first experiments carried out on Tore Supra do not exhibit the expected effect of RE
destabilization. First theoretical considerations explain the unobserved effect by the apparition
of breakdown currents inside the gas jet or the modest increase of plasma resistivity in the
cloud as compared to the already cold CQ plasma. Both would reduce the expected current
profile shrinking. Nevertheless the new concept using a bursting disk and exploding wire has
been demonstrated to be a reliable technique for massive gas injection. The opening time is
less than 300µs, with a controllable injection time delay, and the gas delivery time is shorter
than 1.5 ms. This technique can be used in ITER radiation environment close to the plasma
edge to provide fast and high pressure gas injection with good gas assimilation while piezo
valves or shattered pellets must be located far away and thus shall be slow and/or less gas
efficient. More experiments are thus mandatory to better determine its capabilities.
Because a reliable RE suppression technique is not still guaranteed, experiments for RE beam
control must be continued. Reproducible RE beam plateaus are now routinely produced on
Tore Supra. RE beams are both position and current controlled. A demonstration of RE
deceleration by MGI is also reported. At constant RE current, a clear reduction of the high
energy (E > 10 MeV) tail is observed.
Despite these promising results, mastering the RE beam regime is still a major issue for ITER.
The necessary time for RE beam control is estimated to be 5-6s, taking into account the
necessity to slow-down the RE not too faster to preserve a toroidal electric field below the
critical limit. More experiments are needed to demonstrate that such a full thermalization of
RE is feasible. In present work the RE plateau terminates often abruptly before thermal
equilibrium achievement. MHD modelling of disruption and CQ decay including the RE
population evolution is requested, as well as modelling of the out of equilibrium unusual RE
plasma regime.
Part of this work was performed under ITER Service Contract 42000000249. The views and
opinions expressed herein do not necessarily reflect those of the ITER Organization.
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