18,419 research outputs found
Jets, Blazars and the EBL in the GLAST-EXIST Era
The synergy of GLAST and the proposed EXIST mission as the Black Hole Finder
Probe in the Beyond Einstein Program is remarkable. With its full-sky per orbit
hard X-ray imaging (3-600 keV) and "nuFnu" sensitivity comparable to GLAST,
EXIST could measure variability and spectra of Blazars in the hard X-ray
synchrotron component simultaneous with GLAST (~10-100GeV) measures of the
inverse Compton component, thereby uniquely constraining intrinsic source
spectra and allowing measured high energy spectral breaks to measure the cosmic
diffuse extra-galactic background light (EBL) by determining the intervening
diffuse IR photon field required to yield the observed break from photon-photon
absorption. Such studies also constrain the physics of jets (and parameters and
indeed the validity of SSC models) and the origin of the >100 MeV gamma-ray
diffuse background likely arising from Blazars and jet-dominated sources. An
overview of the EXIST mission, which could fly in the GLAST era, is given
together with a synopsis of other key synergies of GLAST-EXIST science.Comment: 3 pages, 2 Figs., to appear in Proc. First GLAST Symp. (Stanford,
Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan, AIP Conf. Pro
Non-linear effects in electron cyclotron current drive applied for the stabilization of neoclassical tearing modes
Due to the smallness of the volumes associated with the flux surfaces around
the O-point of a magnetic island, the electron cyclotron power density applied
inside the island for the stabilization of neoclassical tearing modes (NTMs)
can exceed the threshold for non-linear effects as derived previously by Harvey
et al, Phys. Rev. Lett. 62 (1989) 426. We study the non-linear electron
cyclotron current drive (ECCD) efficiency through bounce-averaged, quasi-linear
Fokker-Planck calculations in the magnetic geometry as created by the islands.
The calculations are performed for the parameters of a typical NTM
stabilization experiment on ASDEX Upgrade. A particular feature of these
experiments is that the rays of the EC wave beam propagate tangential to the
flux surfaces in the power deposition region. The calculations show significant
non-linear effects on the ECCD efficiency, when the ECCD power is increased
from its experimental value of 1 MW to a larger value of 4 MW. The nonlinear
effects are largest in case of locked islands or when the magnetic island
rotation period is longer than the collisional time scale. The non-linear
effects result in an overall reduction of the current drive efficiency for this
case with absorption of the EC power on the low field side of the electron
cyclotron resonance layer. As a consequence of the non-linear effects, also the
stabilizing effect of the ECCD on the island is reduced from linear
expectations
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