Measurements of relativistic emission from runaway electrons in Alcator C-Mod: spectrum, polarization, and spatial structure

Transcription

1 Measurements of relativistic emission from runaway electrons in Alcator C-Mod: spectrum, polarization, and spatial structure R. Granetz, R. Mumgaard MIT PSFC APS-DPP New Orleans 2014/10/30

2 Motivation Modeling of ITER 15 MA disruptions leads to predictions of up to 10 MA of current carried by relativistic runaway electrons (RE), with energies of MeV. Potentially very damaging to blanket modules and divertor. Therefore we would like to have an understanding of basic RE parameters, such as: maximum energy; energy distribution pitch angle distribution spatial location/extent density; total number On Alcator C-Mod, we can generate relativistic runaways during the current flattop, which facilitates diagnosis. The purpose of this talk is to describe some of the RE diagnostics we are using, and measurements that we are performing.

3 Runaway electron (RE) diagnostics on Alcator C-Mod I p HXR (ceramic scintillator) V L *MSE total light <n e > *MSE polarized light T e *Z-meter brightness R DD SXR * signal comes from relativistic forward-peaked emission

4 Runaway electron (RE) diagnostics on Alcator C-Mod pixel brightness vs time

5 Relativistic emission can show complicated spatial structure RE forward-peaked emission is often an amorphous blob. But sometimes complicated spatial structure is observed. Crescent shapes are also seen on several other tokamaks (EAST, DIII-D, TEXTOR, )

6 Relativistic emission can show complicated spatial structure RE forward-peaked emission is often an amorphous blob. But sometimes complicated spatial structure is observed. Crescent shapes are also seen on several other tokamaks (EAST, DIII-D, TEXTOR, )

7 Relativistic emission can show complicated spatial structure Images can provide information on RE beam shape, size, motion, etc. In addition, information on pitch angle, θ, and energy can be extracted: forward cone opening angle: θ ~ v /v ( 0.2) drift orbit shift: δ = qγm (γ =1/ 1 v 2 c 2 e c / eb ) 5 cm shift corresponds to γ = 80 ellipticity increases with decreasing θ

8 The RE emission is highly polarized MSE measures the polarization angle of light. Usually the light comes from the interaction of diagnostic neutral beam particles with the plasma. MSE has 10 sightlines on the midplane, spanning from just inside the magnetic axis to almost the outer edge of the plasma. The RE emission on C-Mod is times brighter than typical levels with the DNB We find that the RE emission has a polarization fraction of up to 60%

9 Theory predicts synchrotron emission should be highly polarized and carry information in the polarization Polarization direction coincides with direction of the particle s acceleration Recently explored theoretically due to its importance in astrophysics Synchrotron emission from pulsars Theory extended to tokamak field geometry [Ya.M. Sobolev, ISSN , BAHT, 2013, No 4] Predicts RE beam has spatially varying polarization angle and fraction (0-80%) depending on pitch angle, gamma, etc.

10 C-Mod can measure polarization with temporal and spatial resolution (first ever measurement) Polarization fraction Polarization angle

11 Spectral measurements of relativistic RE emission J. Schwinger, Phys. Rev. 75, 1912 (1949) I. M Pankratov, Plasma Phys. Rep. 25, 145 (1999)

12 Spectral measurements of relativistic RE emission Due to C-Mod s high B, the spectrum should be peaked in the visible, not in the IR as in TEXTOR, EAST, DIII-D Peak location and amplitude depend on RE energy, pitch angle, and population density

13 Spectral measurements of relativistic RE emission Due to C-Mod s high B, the spectrum should be peaked in the visible, not in the IR as in TEXTOR, EAST, DIII-D Peak location and amplitude depend on RE energy, pitch angle, and population density

14 Preliminary spectral measurements Emission starts at t ~ 0.8 s, continues rising until t ~1.48 s, then rapidly falls

15 Preliminary spectral measurements viewing into the RE emission t = 0.5 s

16 Preliminary spectral measurements viewing into the RE emission t = 1.0 s

17 Preliminary spectral measurements viewing into the RE emission t = 1.2 s

18 Preliminary spectral measurements viewing into the RE emission t = 1.3 s

19 Preliminary spectral measurements viewing into the RE emission t = 1.4 s

20 Preliminary spectral measurements viewing into the RE emission t = 1.5 s

21 Preliminary spectral measurements viewing into the RE emission t = 1.6 s

22 Preliminary spectral measurements viewing AWAY from the RE emission t = 1.4 s

23 Preliminary spectral measurements Limited spectral information shows peaking in the visible. dp/dλ is much steeper than expected from singleparticle theory. Modeling of emission from distribution of energies by A. Stahl (Chalmers) does not resolve this issue. Two new spectrometers, which each cover the entire visible range, are now operational. Plan to get absolute intensity calibration during upcoming invessel access. Next experimental campaign is in 2015.

24 Summary Alcator C-Mod has a number of diagnostics which can provide detailed information on runaway electron energies, pitch angles, spatial distribution, polarization, We welcome collaboration with theory and modeling groups to help analyze our image, spectral, polarization data.