Showing papers on "Perveance published in 2018"

Modeling of negative ion extraction from a magnetized plasma source: Derivation of scaling laws and description of the origins of aberrations in the ion beam

Abstract: We model the extraction of negative ions from a high brightness high power magnetized negative ion source. The model is a Particle-In-Cell (PIC) algorithm with Monte-Carlo Collisions. The negative ions are generated only on the plasma grid surface (which separates the plasma from the electrostatic accelerator downstream). The scope of this work is to derive scaling laws for the negative ion beam properties versus the extraction voltage (potential of the first grid of the accelerator) and plasma density and investigate the origins of aberrations on the ion beam. We show that a given value of the negative ion beam perveance correlates rather well with the beam profile on the extraction grid independent of the simulated plasma density. Furthermore, the extracted beam current may be scaled to any value of the plasma density. The scaling factor must be derived numerically but the overall gain of computational cost compared to performing a PIC simulation at the real plasma density is significant. Aberrations ap...

Low emittance growth in a low energy beam transport line with un-neutralized section

Abstract: In a low energy beam transport line (LEBT), the emittance growth due to the beam's space charge is typically suppressed by way of neutralization from either electrons or ions, which originate from ionization of the background gas. In cases where the beam is chopped, the neutralization pattern usually changes throughout the beginning of the pulse, causing the Twiss parameters to differ significantly from their steady state values, which, in turn, may result in beam losses downstream. For a modest beam perveance, there is an alternative solution, in which the beam is kept un-neutralized in the portion of the LEBT that contains the chopper. The emittance can be nearly preserved if the transition to the un-neutralized section occurs where the beam exhibits low transverse tails. This report introduces the rationale for the proposed scheme and formulates the physical arguments for it as well as its limitations. An experimental realization of the scheme was carried out at Fermilab's PIP2IT where low beam emittance dilution was demonstrated for a 5 mA, 30 keV ${\mathrm{H}}^{\ensuremath{-}}$ beam.

High current results from the 2X scaled Penning source

Abstract: To meet the full 2 ms 50 Hz 60 mA beam requirements for the Front End Test Stand (FETS) at the Rutherford Appleton Laboratory (RAL), a new caesiated Penning Surface Plasma Source (SPS) has been developed [1] with a larger plasma volume with 2X the linear dimensions of the standard ISIS source. This paper outlines the design decisions made during the development process and details the 2X source construction. Improved electrode cooling is required to operate at higher duty cycles, this is achieved by changing coolant, adding parallel cooling channels, improving mechanical tolerances, decreasing surface roughness and adding thermal interface gaskets. To operate at higher duty factors the ancillary hardware also required significant upgrading.A clean 75 mA H− beam has been measured from the 2X Scaled Source on a test stand at full 2 ms 50 Hz duty cycle. At lower 800 μs duty cycles H− beam currents of 150 mA have been measured at high discharge currents. Emittance scans and a perveance sweep are shown.

Study of the beam extraction system of a negative ion source with a C12A7 electride plasma electrode

Abstract: Key issues to fabricate a beam extraction system of a H− source with a plasma electrode (PE) made from C12A7 electride which has low work function were studied. Preliminary study of the H− extraction using a compact ECR ion source reveals, that high H− current can be extracted from a source without PE bias of high voltage. A bucket type with ECR source is being designed. Beam simulation for the extraction through the designed electrodes has been carried out using the OPERA code. It predicts that the optimum perveance is 0.6 mA/kV1.5, optimum acceleration to extraction voltage ratio Vacc/Vext is 4, with the minimum beam divergence of 9 mrad, which are enough to demonstrate the capability to produce a high H− current density.Key issues to fabricate a beam extraction system of a H− source with a plasma electrode (PE) made from C12A7 electride which has low work function were studied. Preliminary study of the H− extraction using a compact ECR ion source reveals, that high H− current can be extracted from a source without PE bias of high voltage. A bucket type with ECR source is being designed. Beam simulation for the extraction through the designed electrodes has been carried out using the OPERA code. It predicts that the optimum perveance is 0.6 mA/kV1.5, optimum acceleration to extraction voltage ratio Vacc/Vext is 4, with the minimum beam divergence of 9 mrad, which are enough to demonstrate the capability to produce a high H− current density.

Perveance Measurement of the TLS-Linac Klystron and the Evaluation of Its Operation Performance

Abstract: The high power klystron is a radio frequency amplifier for TLS linac operation. It is a crucial device for electron acceleration in linac. How to evaluate its efficiency, lifetime and performance of klystron in operation is one of the major concern in this report. The key klystron parameter perveance is introduced and used for performance evaluation and operation status monitoring. It is important to execute periodic monitoring on perveance for ensuring a stable linac operation. Klystron characteristics diagnostics can be achieved through perveance measurement. A couple of klystron diagnostic parameters concerning perveance are explored for field examination purpose. Perveance comparison with factory acceptance test data is also presented.

Smoothly Varying Injected Neutral Beam Voltage and Current Provides New Capability on the DIII-D Tokamak

Abstract: A significant engineering upgrade of the DIII-D neutral beam system has recently completed, providing ability to smoothly and independently vary the beam voltage and current. This enables greater control of the injected beam power, torque, and plasma instability drive for fusion experiments. Modifications to the high-voltage equipment and the plasma control system were made and tested over the last year to allow beam energy to vary by as much as 20 kV over a 0.5-s period anywhere within the 45–85 kV operating range of the beams. The beam current can be made to track the voltage (keeping the perveance constant), or the current and voltage can be varied independently (scanning the perveance). The smooth variation of beam energy avoids the extremely perturbative effects of pulsewidth modulation, the only tool previously used for regulating the injected neutral beam power. With independent control of voltage and current, the beam ion velocity space can be tailored to facilitate new experiments that explore, for example, the detrimental effects of Alfven eigenmodes. These modes are driven by energetic particles and can possibly be avoided in steady-state scenarios by timely variation of the beam energy while maintaining constant input power. A description of the modifications made to the power supply and beam controls will be presented, as well as some initial physics results employing the new variable beam energy system.

Design of a High-Efficiency S-band Klystron with a Multi-cell Output Cavity

Abstract: As large-scale scientific facilities develop, the klystron efficiency becomes an important issue to reduce high operating costs. We are developing high-efficiency klystrons for the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) at the Pohang Accelerator Laboratory (PAL). The newly developed klystrons should have the same specifications, including perveance and overall length, to replace existing ones. Under these limited conditions, we designed the klystrons based on a multi-cell output cavity to improve the power-conversion efficiency significatly. We simulated the beam-dynamics using the Field Charge Interaction (FCI) code with a MATLAB script to find the optimum design parameters for high efficiency. We have determined the design parameters, such as cell tuning frequencies, inter-cell distances, couplings between cells and the Qe of the last cell, of the multi-cell output cavity.

Primary Design of Extended Interaction Klystron with Multi-Gap Cavity at 225 GHz

Abstract: The analytical expressions of the beam–wave coupling coefficients and the beam loading conductance for a 2π mode in a multi-gap cavity is proposed as a circuit of the extended interaction klystron (EIK), are derived by space-charge wave theory. The mechanism of the beam–wave synchronization and the coupling in the multi-gap cavity at 225 GHz are studied in detail by calculating the coupling coefficient and the normalized beam loading conductance as a function of gap number, gap dimension, and beam voltage as well as the perveance. The stability of the circuit is analyzed by considering the quality factor of the electron beam. It is found that the stability of the operating 2π mode is more sensitive to the beam voltage and gap number. Based on the theory and analysis, a 5-gap coupled cavity is proposed as a section of EIK’s circuit. A low voltage EIK with a 4-cavity circuit at 225 GHz is designed and is simulated by a particle-in-cell (PIC) code. The EIK can achieve a maximum output power of ~36 W with more than 30 dB gain at 225 GHz.

Parametric 3D Modeling of Low Perveance Elleptical Electron Beams for Devices of THz Range

Abstract: This report presents the results of 3D modeling of an electron optical system with low perveance electron elliptical beams. The comparison of triple elliptical beams with continuous sheet and elliptical beams has been conducted. The studied systems have a triode design with an anode potential of 20 kV and a grid potential of 950 V. The total current of electron beams is a 93 mA. Results of 3D modeling show that using of electron optical system forming triple elliptical beams in THz devices permits to achieve a high current transmission through a beam tunnel of a slow-wave structure with a dimension of 140×2500 ¼m2 and provide interaction with the higher transverse mode of three RF field variations.

Thermal Deformation Analysis and Electron Beam Optics Simulation for a Thermionic Electron Gun Design

Abstract: When the thermionic electron gun of the klystron operates, the structure around the cathode is subjected to thermal deformation due to the high operating temperature of the cathode. A thermal deformation analysis of the existing electron gun with ‘cold’ dimensions (dimension on fabrication) was done using the ANSYS code to get the ‘hot’ dimensions of the electron gun on operation. After that, the optics of the emitted electron beam were simulated using two codes (EGUN and CST-PS) for the two cases of the ‘cold’ dimension and the ‘hot’ dimension. We compared the beam trajectories of the above two cases to investigate the change in the physical quantities, which would have a strong effect on the klystron performance due to the thermal deformation. The thermal deformation was found to cause a perveance change as large as 15%. The scalloping behavior was also investigated with an applied magnetic field for the two cases of cold and hot dimensions.

Low Emittance Growth in a LEBT with Un-Neutralized Section

Abstract: In a Low Energy Beam Transport line (LEBT), the emittance growth due to the beam's space charge is typically suppressed by way of neutralization from either electrons or ions, which originate from ionization of the background gas. In cases where the beam is chopped, the neutralization pattern usually changes throughout the beginning of the pulse, causing the Twiss parameters to differ significantly from their steady state values, which, in turn, may result in beam losses downstream. For a modest beam perveance, there is an alternative solution, in which the beam is kept un-neutralized in the portion of the LEBT that contains the chopper. The emittance can be nearly preserved if the transition to the un-neutralized section occurs where the beam exhibits low transverse tails. This report introduces the rationale for the proposed scheme and formulates the physical arguments for it as well as its limitations. An experimental realization of the scheme was carried out at Fermilab's PIP2IT where low beam emittance dilution was demonstrated for a 5 mA, 30 keV $H^{-}$ beam.