John G. Siambis

Adiabatic equations of state for intense relativistic particle beams

Adiabatic equations of state for scalar pressure and anisotropic diagonal pressure have been obtained for intense, relativistic, unneutralized electron beams. These adiabatic equations of state complete the set of fluid equations needed to study beam dynamics and interactions from the fluid point of view. In particular, they facilitate the inclusion of thermal effects of momentum spread effects in stability analyses.

Diocotron instability of a relativistic hollow electron beam

The diocotron stability properties of a relativistic hollow electron beam are investigated within the framework of the macroscopic, cold fluid model, in which the electron motion is assumed to be laminar. The stability analysis is carried out for the case of a sharp boundary density profile, including the important influence of relativistic effects and fractional charge neutralization on the stability behavior. It is found that the growth rate of instability is proportional to 1/γb2, thereby being reduced considerably by increasing the electron energy γbmc2. Moreover, the fractional charge neutralization plays a significant role in the stability behavior, particularly for the relativistic electron beam.

Autoacceleration of intense electron beams by coaxial cavity structures

Tonks’ theorem and Faraday’s law are applied to study the autoacceleration interaction of an intense hollow beam with a coaxial transmission line cavity, assuming the constant‐current approximation. The autoacceleration voltage is found proportional to the beam current and the inductance per unit length of the coaxial transmission line. Comparison with experimental data indicates that the coupling coefficient α is of the order 2/3. The creation of a virtual cathode near the autoacceleration region under certain beam and geometry conditions gives rise to a virtual cathode accelerating voltage. This voltage is found to be proportional to the inductance per unit length of the annular region between the hollow beam and the drift tube as well as the amount of charge per unit length reflected from the virtual cathode. The ratio of the virtual cathode voltage to the autoacceleration voltage is a small number and for optimum operating conditions it should be minimized.

Collisional parameters of a test particle and a streaming plasma

The collisional interaction of a test‐charged particle with a streaming Maxwellian distribution of field particles is considered analytically. Values are found for the collisional parameters of dynamical friction force, diffusion tensor, and rate of energy change for a wide range of field particle densities, temperatures, and streaming velocities. Retention of the velocity dependence of the Coulomb logarithm leads to substantial differences in the values of the parallel component of the diffusion tensor and the rate of energy change for the test particle as compared with the Spitzer values. Collision frequencies for momentum transfer, π/2 angular deflection, and energy change for the test particle and the test plasma are defined, simply related to each other, compared with the Spitzer definitions and evaluated for a simple case. The new definitions for the collision frequencies place in evidence the enhancement of collisional momentum and energy transfer in the absence of stationary thermal equilibrium.

Intense Beam Recirculation

The acceleration of intense current (I > 10 kA) electron beams from low voltage (V 1-2 MV) to high voltage (20 MV > V > 10 MV) has been accomplished during the last two decades. In particular, the penetration, of the 10-MV voltage technical barrier that exists in pulsed power acceleration of intense electron beams, has been accomplished by utilizing distributed accelerator concepts and modules from the area of lowcurrent high-voltage traditional linear accelerators. The linear induction accelerator (Astron, ERA, ETA, ATA), the radial pulse line accelerator (LIU-10, Radlac), and the RF linear accelerator (Fermex) are examples of the technology that is being developed in order to accelerate intense electron beams well beyond the 10-MV barrier of pulse power technology. In utilizing the linear accelerator concept of distributed acceleration, exceedingly long systems result. The simplest approach to reducing the length of the system is to fold the accelerator by introducing a 1800 bend in the path of the beam. The next step is to introduce a second 1800 bend and close the system into a racetrack geometry, where acceleration occurs at the linear sections of the geometry and recirculation is accomplished via the two 180° bends, or toroidal sections.

High Current Beam Stability in Linear Accelerators

Equilibrium and stability constraints for high beam current linear accelerators are examined. Three general classes of linear accelerators are established depending on the geometry of the accelerating structure. For the case of klystron type geometry, of broad current interest for many applications, the equilibrium and stability properties are examined in detail. It is found that equilibrium constraints can readily be satisfied in the presence of a uniform guide magnetic field. Stability criteria for the longitudinal bunching mode and the transverse beam break up mode have been obtained and discussed. Stabilization mechanisms are suggested for the stable acceleration of multikiloamp particle beams.

Resistivity, Heating and Radial Transport on the Axis of the Tokamak Discharge

In this work we present analytical and numerical results on the resistivity, heating and radial transport of particles momentum and heat on and near the minor axis of the Tokamak discharge. The crucial role of impurities is emphasized. Asymptotically exact electromagnetics and classical fluid dynamics are utilized to obtain first the case of the straight infinite cylinder. Toroidal effects are then included at the level of the Pfirsch-Schlüter theory. Analytical results from the complete model show the transport coefficients to be classical with enhanced values related to the presence of the impurity ions and the toroidal effects. Numerical results from a simpler simulation model are found to be in reasonable agreement with published experimental results from the ST-Tokamak.