Experimental investigation of runaway electron generation in TEXTOR

Jaspers, R.; Finken, K. H.; Mank, G.; Hoenen, F.; Boedo, J. A.; Cardozo, N.J. Lopes; Schüller, F. C.

doi:10.1088/0029-5515/33/12/i02

Cited by 104 publications

(100 citation statements)

References 13 publications

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“…Experimentally the secondary mechanism was first detected in TEXTOR [22]. Later it was confirmed in a similar way in HT-7 [27] and was found to be necessary for the explanation of the REs in JET disruptions [28,29].…”

Section: Introductionmentioning

confidence: 62%

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Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Bozhenkov¹,

Lehnen²,

Finken³

et al. 2008

Plasma Phys. Control. Fusion

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122

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Abstract. Runaway electrons represent a serious problem for the reliable operation of the future experimental tokamak ITER. Due to the multiplication factor of exp(50) in the avalanche even a few seed runaway electrons will result in a beam of high energetic electrons that is able to damage the machine. Thus suppression of runaway electrons is a task of high importance, for which reason we present here a systematic study of runaway electrons following massive gas injection in TEXTOR. Argon injection can cause generation of runaways carrying up to 30% of the initial plasma current, while disruptions triggered by injection of helium or of mixtures of argon (5, 10, 20%) with deuterium are runaway free. Disruptions caused by argon injection finally become runaway free for very large amounts of injected atoms. The appearance/absence of runaway electrons is related to the fraction of atoms delivered to the plasma center. This so called mixing efficiency is deduced from a 0D model of the current quench. The estimated mixing efficiency is: 3% for argon, 15% for an argon/deuterium mixture and about 40% for helium. A low mixing efficiency of high-Z impurities can have a strong implication for the design of the disruption mitigation system for ITER. However, a quantitative prediction requires a better understanding of the mixing mechanism.

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Section: Introductionmentioning

confidence: 62%

“…The dependence given by equation 3 was confirmed experimentally [22]. It is to be noted that the Dreicer field E D is sometimes called critical.…”

Section: Introductionmentioning

confidence: 64%

Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Bozhenkov¹,

Lehnen²,

Finken³

et al. 2008

Plasma Phys. Control. Fusion

Self Cite

122

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show abstract

“…The latter process results in an exponential growth of the population, as witnessed by the infrared signal ( Fig. 1) [11], and is independent of the plasma density.…”

mentioning

confidence: 76%

Scale Size of Magnetic Turbulence in Tokamaks Probed with 30-MeV Electrons

et al. 2000

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Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" ͑d͒ of magnetic perturbations: d , 0.5 cm in Ohmic discharges, increasing to d 4.4 cm for 0.6 MW NBI. PACS numbers: 52.55.Fa One of the outstanding issues in thermonuclear fusion research remains the anomalous conduction of heat by the electrons in the plasma. In a tokamak, the hot plasma is confined in a toroidal geometry by means of magnetic fields. The field topology is such that field lines lie on nested toroidal surfaces. Transport in the direction perpendicular to the surfaces is reduced by many orders of magnitude by the presence of the field. However, the measured heat fluxes carried by the electrons exceed the theoretically achievable minimum by 1-2 orders of magnitude. This anomaly is generally ascribed to turbulence, which may be of electrostatic or of magnetic nature, or both. There has been extensive research into transport caused by electrostatic fluctuations. Recently, means have been found to greatly reduce the heat loss caused by these [1]. Magnetic turbulence is more difficult to diagnose, since perturbing fields of the orderB͞B 10 25 can already contribute significantly to the heat flux carried by the electrons. The only direct measurements ofB in the core of a tokamak plasma, using the cross-polarization scattering of microwaves, did show the presence ofB at transport relevant levels in Tore Supra [2,3]. Electrons with energy much higher than the thermal energy, in principle, can provide a probe to study magnetic turbulence, since diffusion due to electrostatic turbulence scales with y 21 , whereas the magnetically induced diffusion scales as y, where y is the electron velocity. Moreover, since in a plasma the mean free path of an electron scales as y 4 , collisional transport is negligibly small for high energy electrons. The absence of collisions is also the reason why in tokamak plasmas of sufficiently low density a small fraction of the electrons (so-called runaway electrons) undergo a free fall acceleration and can reach energies in the MeV range, in a background plasma with a temperature of ϳ1 keV.In several studies, runaway electrons have been used to assess magnetic turbulence. One principal difficulty is that runaway electrons in the 1-MeV energy range cannot be diagnosed until they leave the plasma and hit the wall and produce x rays. Thus, in [4] experimental techniques have been used to probe magnetic turbulence in the edge of the plasma. Direct observation of runaway electrons in the center of the plasma column has been performed at...

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“…Recent researches indicate that tokamak, a magnetic confinement fusion energy device, may be the largest artificial positron factory in the world [4,5]. In large tokamaks like JET and JT-60U, above 10 14 positrons are generated in a post-disruption plasma by runaway electrons [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The dynamics of these positrons after birth in tokamaks is a noteworthy question that may yield valuable information about the runaway dynamics and disruption process in tokamaks.…”

mentioning

confidence: 99%

What is the fate of runaway positrons in tokamaks?

et al. 2014

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Massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates the tokamak geometry, loop voltage, radiation and collisional effects. It is found numerically that runaway positrons will drift out of the plasma to annihilate on the first wall, with an in-plasma annihilation possibility less than 0.1%. The dynamics of runaway positrons provides signatures that can be observed as diagnostic tools.

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Experimental investigation of runaway electron generation in TEXTOR

Cited by 104 publications

References 13 publications

Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Scale Size of Magnetic Turbulence in Tokamaks Probed with 30-MeV Electrons

What is the fate of runaway positrons in tokamaks?

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