Ahmed M. Al-Khateeb

Screening of the resistive-wall impedance by a cylindrical electron plasma

The effect of an electron cloud on the longitudinal coupling impedance is studied by idealizing it as a cold and uniformly distributed non-neutral plasma of electrons. The beam pipe is assumed to be of circular cross section with a thick resistive wall and the beam charge is idealized as a uniform disk. The electron contribution to the charge and current densities is obtained from the collective electron response to the beam passage through the pipe. In the presence of the electron background, a general closed formula for the longitudinal coupling impedance is obtained. The screening of the coupling impedance with the density of the electron plasma is studied and discussed for typical parameters in an accelerator beam pipe for the under-dense and the over-dense plasma regions.

Brillouin backscattering instability in inhomogeneous collisional plasma

Brillouin backscattering instability is investigated in inhomogeneous collisional plasma. The slow-coupling equations for the instability in a medium with linear density ramp are obtained. For large inhomogeneity scale length, the homogeneous growth rate is found to be modified by a factor of [1/square root(2) times square root(ω 0 2 / (ω 0 2 + ν e 2 ) + ω 0 /square root(ω 0 2 + ν e 2 ))] For the convective instability, the amplification factor is found to be [Λ = (|γ B 0 | 2 /2β)(ω 0 2 /ω 0 2 + ν e 2 )] The presence of collisions leads to a reduction in both the growth rate and the amplification factor, where the threshold intensity for the instability to occur increases.

Scenarios of plasma formation with intense fs laser pulses

A non‐standard fluid model is developed to simulate superintense pulse‐solid interaction on the fastest time scale in the collisional regime. Ionization by field emission is the shortest process, occurring during fractions of a laser cycle and leading to ionization dephasing. At later stages ionization by electron impact may become significant in higher‐Z materials. As another significant result collisional absorption is revealed to occur to appreciable degrees.

Enhanced microwave absorption rates in stealth plasma

Summary form only given. After our previous advancement for the Appleton-Hartree theory by extending the existing theory to include the plasma thermal effect, we present enhanced absorption rates for microwaves within the C-band from warm plasma. Thermal effects are important in electromagnetic wave propagation in the ionosphere as well as in the emerging applications of stealth technology.The derivation of the magneto-ionic dispersion equation in warm plasma immersed in a uniform magnetic field was presented in a previous work, where the obtained expressions extend the well known Appleton-Hartree, magneto-ionic formula of stationary cold plasma to the case of warm plasma. In the present work, we study the propagation constant and reflection coefficient for characterizing wave propagation in magnetized, warm plasma. To demonstrate this effect, we give a numerical example of the developed theory for the typical parameters that may exist in stealth technology applications. It is well known that in cold plasma one of the two forward propagating modes is weakly attenuated while the other is weakly amplified. In comparison, in warm plasma, we observed that the weak damping of one mode in the cold plasma is absolutely enhanced by the thermal effect, while the second (which is weakly amplified in cold plasma) follows an anomalous behavior around resonant frequencies with enhanced attenuation and enhanced amplification regions. The present work considers these effects in warm plasma and shows enhanced absorption of the microwaves, due to the thermal effect, from plasma near a metallic wall.

Longitudinal and Transverse Impedances and Shielding Effectiveness of a Resistive Beam Pipe for Arbitrary Energy and Frequency

The longitudinal coupling impedance of a cylindrical beam pipe for arbitrary relativistic energy and mode frequency is obtained analytically for finite wall conductivity and finite wall thickness. Closed form expressions for the electromagnetic fields excited by a beam perturbation are derived analytically. General expressions for the resistive-wall impedance in the presence of a metallic shield and for the rf shielding effectiveness of the beam pipe have been obtained. The results are applied to the GSI synchrotron SIS, where the thickness of the vacuum chamber in the dipole magnets is much smaller than the skin depth at injection energy. In addition, the transverse space-charge and resistive-wall impedances have been investigated analytically of a smooth cylindrical beam pipe of finite conductivity. Transverse beam charge distributions of a hollow beam and of a uniform beam are considered yielding different results. Closed form expressions for the excited electromagnetic fields in the beam-pipe-wall regions and for the corresponding total transverse impedance can be derived analytically for high energy beams.

Convective Two - Plasmon Decay Instability in Inhomogeneous Collisional Plasma

Quantitative description of the two plasmon - decay (TPD) in inhomogeneous collisional plasma is given through the solution of the slow coupling equations of the decay modes. The initial TPD growth rate and the threshold pump intensity for the instability to occur are calculated in a homogeneous unmagnetized collisional plasma. In inhomogeneous plasma with a linear density ramp, the spatial amplification factor and the pump threshold intensity for the convective TPD instability has been calculated.

Coupling impedance of a small hole for particle beams travelling at arbitrary β in a cylindrical beam pipe

Within the Bethe diffraction theory, the impedance of a small circular hole has been calculated for particle beams of arbitrary β and finite size via two approaches. In the first approach we define the impedance in terms of the total work done by the fields excited in the beam pipe, where it finally reduces to a surface integral over the hole region. In the second approach, the hole has been treated as a radiating electric and magnetic dipole with effective electric and magnetic moments resulting from fictitiously introduced surface charge and current densities. The above two approaches lead to exactly the same result for the hole impedance which is consistent with the predictions made by the Bethe theory for wavelengths that are much larger than the hole size.