Coaxial Drift Tube Research Articles

Relativistic Charged-Particle Beam Space-Charge Limited Current in Finite Length Coaxial Drift Tube

In the strong axial (guide) magnetic field and uniform beam density approximations, we calculate scalar potential distribution induced by axisymmetric annular relativistic charged-particle beam in a finite-length coaxial drift tube. This scalar potential distribution allows us to find an approximate radial position at which the extremum of the scalar potential is attained in the coaxial drift tube inside the annular relativistic charged-particle beam. By knowing the approximate extremal radial position, we obtain the analytical estimate for the space-charge limited (SCL) current of axisymmetric annular (finite thickness) relativistic charged-particle beam propagating in strong axial magnetic field in a finite length coaxial drift tube and establish a simple correspondence of this result to that in a long coaxial drift tube. A comparison of numerical nonlinear calculations and the analytical estimate for the SCL current in the finite-length coaxial drift tube is also given.

Diagnosis of asymmetric mode competition in triaxial klystron amplifier

In order to meet the diagnosis requirements of asymmetric mode competition in an X-band triaxial klystron amplifier, a novel waveguide coupler with compact structure is designed, and its coupling characteristics for TE61 mode are analyzed with small-hole coupling principle and CST simulation software. A physical model is established by using PIC method to diagnose the spectrum characteristics in the TKA coaxial drift tube. Through simulation, the validity of the designed coupler to diagnose TKA asymmetric mode competition is verified.

Focusing electrode and coaxial reflector used for reducing the guiding magnetic field of the Ku-band foilless transit-time oscillator

Based on the theoretical analysis of the intense relativistic electron beam propagation in the coaxial drift-tube, a focusing electrode and a coaxial reflector is proposed to lessen the demand of the coaxial Ku-band foilless transit-time oscillator (TTO) for the guiding magnetic field. Moreover, a Ku-band TTO with the focusing electrode and the coaxial reflector is designed and studied by particle in cell simulation. When the diode voltage is 390 kV, the beam current 7.8 kA, and the guiding magnetic field is only 0.3 T, the device can output 820 MW microwave pulse at 14.25 GHz by means of the simulation. However, for the device without them, the output power is only 320 MW. The primary experiments are also carried out. When the guiding magnetic field is 0.3 T, the output power of the device with the focusing electrode and the coaxial reflector is double that of the one without them. The simulation and experimental results prove that the focusing electrode and the coaxial reflector are effective on reducing the guiding magnetic field of the device.

Limiting current of axisymmetric relativistic charged-particle beam propagating in strong axial magnetic field in coaxial drift tube

In the strong axial magnetic field approximation, we calculate the space-charge limited (SCL) current of axisymmetric relativistic charged-particle beam in a coaxial drift tube of finite length. Results are compared to analytical estimates and numerical modeling of SCL current in the infinitely long drift tube. For the infinitely long drift tube, which inner conductor is biased and the outer conductor is lined with a finite-width dielectric insert, analytic approximations for the SCL current in the bias voltage are developed.

Upper bound for the space-charge limited current of relativistic electron beams in finite-length coaxial drift tubes

A two-dimensional theory for calculating an upper bound for the space-charge limited current of relativistic electron beams is extended to coaxial drift tubes. The theory applies to annular beams of arbitrary radius, thickness and length travelling in a grounded coaxial drift tube of circular cross section. Poisson’s equation for space-charge limited flow is replaced with a Sturm–Liouville-type eigenvalue problem. The problem reduces to numerically solving a determinantal equation rather than a partial differential equation. Under limiting cases, closed-form analytical expressions are derived.

Operation of a K-band second harmonic coaxial gyroklystron

Amplification studies of a two-cavity second harmonic gyroklystron with a coaxial input cavity and drift tube are reported. The inner conductor is supported by tungsten pins which intercept the beam, and it utilizes lossy dielectric rings to enhance mode suppression. A double-anode magnetron injection gun produces a 440 kV, 200–268 A, 1 μs beam with an average perpendicular-to-parallel velocity ratio near one. The TE011 input cavity is driven near 9.886 GHz and the TE021 output cavity resonates near 19.772 GHz. Peak powers near 30 MW have been achieved, although more easily reproducible peak powers hover closer to 20 MW. While stability is improved over previous devices, beam interception reduces the accessible range in parameter space and thus degrades amplifier performance. Pin erosion is also evident and qualitatively agrees well with theoretical predictions.

High power microwaves at 9 GHz from an extended length cavity in a coaxial beam geometry

Experiment and simulation demonstrate high power microwave generation at 9 GHz using a 9-cm-diam, 400-keV, 7-kA annular electron beam. The beam is propagated in a coaxial drift tube between inner and outer conductors, a configuration which increases the available beam current and reduces the surface fields from existing high power sources. The microwave interaction is provided by an extended length loaded cavity, overcoming the limitations of radiative loss and low quality factor usually imposed by the coaxial geometry. A coupler samples 25 MW of the total 200 MW produced by the beam-cavity interaction. Simulations indicate that the 7% efficiency can be significantly improved by optimizing the interaction length.

Simulation studies of a klystronlike amplifier operating in the 10-100 GW regime

A coaxial drift tube allows propagation of an ultra high-power relativistic electron beam (500 keV, >or=100 kA, 100 ns). The modulation of a large-diameter (12.6 cm) intense, relativistic electronic beam (500 keV, 16 kA) by an external microwave source via particle simulation is studied. The annular beam, enclosed within a coaxial drift tube, is found to be fully modulated by a low-power external RF source at a frequency of 1.3 GHz. It is shown that for such an intense beam, a highly nonlinear interaction takes place at the modulating gap, producing highly coherent bunches of electrons. This finding is similar to earlier research in which such modulations were studied for an intense beam propagating in a hollow drift tube. It is further shown that, unlike the hollow drift tube case, the coaxial configuration is easily scaled to high power. Here, a very large diameter (26 cm) intense beam (460 keV, 100 kA) is fully modulated at 1.3 GHz to obtain 31 GW of RF beam power. >

An experimental study of the formation and reactivity of ionic hydrogen clusters: The first observation and characterization of the even clusters H+4, H+6, H+8, and H+10

A specially designed coaxial drift tube type ion source has been utilized to generate ionic hydrogen clusters. The cluster spectrum observed is dominated by odd clusters (H+5, H+7, H+9, and H+11 ) in line with past studies on hydrogen ion clusters. However, for the first time even hydrogen cluster ions are formed in a high pressure ion source, with relative intensities (H+6≫H+8>H+10>H+4) . The observation of a relatively intense H+6 peak (H+6/H+5 =0.05) was unexpected. The peak at nominal mass 6 was confirmed to be H+6 (and not H4D+) by high resolution studies. A number of possible formation mechanisms for H+6 are discussed. The most likely mechanism is reaction of odd hydrogen cluster ions (H+3, H+5, H+7,... ) with metastable electronically excited odd hydrogen cluster neutrals (H*3, H*5, H*7,...) . The H*3 species has been experimentally characterized by other research groups (Herzberg; Kuppermann) and is probably formed by electron recombination with H+3 (v≠0), H+5 or H+7 in our source. The H+6 ion is observed to lose both H atom and H2 by metastable decomposition in a ratio of H/H2≂4. The kinetic energy distribution for H loss is broad and bimodal with an average release of 38 meV and a maximum release of 140 meV. Collision induced decomposition studies are utilized to characterize both the even and odd clusters. A 50/50 mixture of H2 and D2 yielded a large number of isotopically mixed clusters that were identified by high resolution mass spectrometry.

Fermilab Tevatron High Level RF Accelerating Systems

Eight tuned rf cavities have been installed and operated in the F0 straight section of the Tevatron. Their mechanical placement along the beam line enables them to be operated for colliding beams as two independent groups of four cavities, group 1-4 accelerating antiprotons and group 5-8 accelerating protons. The only difference is that the spacing between cavities 4 and 5 was increased to stay clear of the F0 colliding point. The cavities can easily be rephased by switching cables in a low-level distribution system (fan-out) so that the full accelerating capability of all eight cavities can be used during fixed target operations. Likewise, the cables from capacitive probes on each cavity gap can be switched to proper lengths and summed in a fan-back system to give an rf signal representing the amplitude and phase as ''seen by the beam,'' separately for protons and antiprotons. Such signals have been used to phase lock the Tevatron to the Main Ring for synchronous transfer. A cavity consists of two quarter-wave resonators placed back to back with a coaxial drift tube separating the two accelerating gaps by ..pi.. radians. The cavities are very similar to the prototype which has been previously described/sup 3/ andmore » is operating as Station 8 in the Tevatron. Only additional water cooling around the high current region of the drift tube supports and a double loop used to monitor the unbalance current through the Hipernom mode damping resistor have been added. Each cavity has a Q of about7100, a shunt impedance of 1.2 M..cap omega.., and is capable of running cw with a peak accelerating voltage of 360« less

Intense microwave generation from a non-neutral rotating E layer

The radiation produced by a hollow, non-neutral, rotating, relativistic E layer propagating inside a coaxial cylindrical drift tube has been investigated theoretically and experimentally. The measured radiation spectrum is very broadband in agreement with theory, though a shift in the spectrum can be achieved by a preloaded azimuthal density structure.