NASA Scientific and Technical Aerospace Reports

20060002430 Princeton Univ., NJ USA
Obliquely Propagating Electromagnetic Drift Instability in the Lower Hydrid Frequency Range
Ji, H.; Kulsrud, R.; Fox, W.; Yamada, M.; Jun. 2005; 52 pp.; In English
Report No.(s): DE2005-841011; PPPL-4078; No Copyright; Avail.: Department of Energy Information Bridge
By employing a local two-fluid theory, we investigate an obliquely propagating electromagnetic instability in the lower
hybrid frequency range driven by cross-field current or relative drifts between electrons and ions. The theory self-consistently
takes into account local cross-field current and accompanying pressure gradients. It is found that the instability is caused by
reactive coupling between the backward propagating whistler (fast) waves in the moving electron frame and the forward
propagating sound (slow) waves in the ion frame when the relative drifts are large. The unstable waves we consider propagate
obliquely to the unperturbed magnetic field and have mixed polarization with significant electromagnetic components. A
physical picture of the instability emerges in the limit of large wavenumber characteristic of the local approximation. The
primary positive feedback mechanism is based on reinforcement of initial electron density perturbations by compression of
electron fluid via induced Lorentz force. The resultant waves are qualitatively consistent with the measured electromagnetic
fluctuations in reconnecting current sheet in a laboratory plasma.
NTIS
Electromagnetic Fields; Frequency Ranges; Magnetohydrodynamic Stability
20060002456 Fermi National Accelerator Lab., Batavia, IL, USA
Magnetic Shielding of an Electron Beamline in a Hadron Accelerator Enclosure
Kroc, T. K.; Schmidt, C. W.; Shemyakin, A.; January 2005; 8 pp.; In English
Report No.(s): DE2005-15017100; FERMILAB-CONF-05/106-AD; No Copyright; Avail.: National Technical Information
Service (NTIS)
The Fermilab Electron Cooling Project requires the operation of a 4.34 MeV electron beam in the same enclosure that
houses the 120, 150 GeV Main Injector. Effective shielding of the magnetic fields from the ramped electrical bus ses and local
static fields is necessary to maintain the high beam quality and recirculation efficiency required by the electron cooling system.
This paper discusses the operational tolerances and the design of the beamline shielding, bus design, and bus shielding as well
as experimental results from the prototype and final installation.
NTIS
Cooling; Electron Accelerators; Electron Beams; Enclosure; Hadrons; Magnetic Fields; Magnetic Shielding; Shielding
20060002462 Lawrence Livermore National Lab., Livermore, CA USA
Numerical Study of On-Axis Dose Rate from Ta and W Bremsstrahlung Converter Targets
McCarrick, J. F.; May 13, 2005; 30 pp.; In English
Report No.(s): DE2005-15016409; UCRL-TR-212255; No Copyright; Avail.: National Technical Information Service (NTIS)
The bremsstrahlung converter target in radiographic accelerators is not, in general, considered a high-technology piece
of equipment. In its essential form it is merely a solid plate of high-Z metal, usually tungsten (W) or tantalum (Ta); electrons
go in, X-rays come out. In this report we characterize the bremsstrahlung performance of Ta and W converter targets over a
range of electron energies (2-20 MeV) and angles. The studies are all performed with the MCNP radiation transport code,
version 4b. A number of steps are involved in the process. First, we must construct the absorption properties of air given
photons of various energies, so that the distribution of photons produced by the incoming electrons can be converted into a
dose rate as commonly used in radiography. Then we study the dose rate of various electron energies in Ta and W, as a function
of target thickness, and find that there is an optimum for a given energy. Following that, we describe how to incorporate
angular dependence in the (rather obtuse) MCNP interface, and then study how the dose rate varies with the distribution of
incoming angles. The major results and useful curve fits are summarized at the end. The appendix contains listings of useful
scripts and MCNP templates. This report does not address the backscattered electron issue. That problem is complex because
it requires self-consistent treatment of the electrons in the external electric fields once they are ejected from the target. Suffice
it to say that a thicker target produces more backscatter and therefore more defocusing. When choosing a target thickness,
rather than selecting from the peak output given below, one may prefer to choose a (usually much thinner) target corresponding
to the 95% output level, or even lower as desired. Of course, a target which must withstand multiple beam pulses has
constraints driving the thickness in the opposite direction, in order to maintain sufficient line density during the hydrodynamic
evolution of the material.
NTIS
Bremsstrahlung; Dosage; Tantalum; Targets; Tungsten
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