Magnetic shielding and ohmic losses from finite thickness Faraday Shields

A calculation method is developed by which the magnetic field can be calculated for some simple cross‐section shapes of Faraday Shields with finite thickness.(AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87353/2/71_1.pd

Time-Domain Modeling of RF Antennas and Plasma-Surface Interactions

Recent advances in finite-difference time-domain (FDTD) modeling techniques allow plasma-surface interactions such as sheath formation and sputtering to be modeled concurrently with the physics of antenna near- and far-field behavior and ICRF power flow. Although typical sheath length scales (micrometers) are much smaller than the wavelengths of fast (tens of cm) and slow (millimeter) waves excited by the antenna, sheath behavior near plasma-facing antenna components can be represented by a sub-grid kinetic sheath boundary condition, from which RF-rectified sheath potential variation over the surface is computed as a function of current flow and local plasma parameters near the wall. These local time-varying sheath potentials can then be used, in tandem with particle-in-cell (PIC) models of the edge plasma, to study sputtering effects. Particle strike energies at the wall can be computed more accurately, consistent with their passage through the known potential of the sheath, such that correspondingly increased accuracy of sputtering yields and heat/particle fluxes to antenna surfaces is obtained. The new simulation capabilities enable time-domain modeling of plasma-surface interactions and ICRF physics in realistic experimental configurations at unprecedented spatial resolution. We will present results/animations from high-performance (10k-100k core) FDTD/PIC simulations of Alcator C-Mod antenna operation

Simulation of a Rising Sun Magnetron Employing a Faceted Cathode with a Continuous Current Source

It has been proposed that gated field emitters could be used in place of conventional thermionic cathodes to control the current injection in a magnetron, both temporally and spatially. Since gated field emitters have to be fabricated on flat surfaces, a faceted cathode would be used to implement this approach. A 2D ten cavity, rising sun magnetron has been modeled using the particle-in-cell code VORPAL. Cylindrical, five-sided, and ten-sided faceted cathodes were modeled to study the variation of magnetron operation due to the cathode shape. This work shows the results of the device performance employing three different cathode geometries with a typical continuous current source. The cathode voltage is −22.2 kV; magnetic field is 0.09 T; and linear current density is 326 A/m. The three models oscillated at the π-mode, at a frequency of 960 MHz for the cylindrical cathode and 957 MHz for the faceted cathodes. Simulations show a faster start up time for the ten-sided faceted cathode. This resulted in a reduced overall startup time of the device from 200 to 110 ns. A strong current instability was observed in the five-sidedcathode case with a periodicity range from 250 to 350 ns. This instability was limited to the start-up period of the ten-sided cathode model; hence the ten-sided case was more stable

Phase-Control of a Rising Sun Magnetron Using a Modulated, Addressable, Current Source

It has been proposed that the use of gated field emitters with a faceted cathode in place of the conventional thermionic cathode could be used to control the current injection in a magnetron, both temporally and spatially. In this work, this concept is studied using the particle-in-cell code VORPAL. The magnetron studied is a ten-cavity, rising sun magnetron, which can be modeled easily using a 2D simulation. The magnetron has a ten-sided faceted cathode. The electrons are injected from three emitter elements on each of the ten facets. Each emitter is turned ON and OFF in sequence at the oscillating frequency with five emitter elements ON at once to obtain the five electron spokes of the π-mode. The simulation results show that the modulated, addressable cathode reduces startup time from 100 to 35 ns, increases the power density, controls the RF phase, and allows active phase control during oscillation

Phase Control and Fast Start-Up of a Magnetron Using Modulation of an Addressable Faceted Cathode

The use of an addressable, faceted cathode has been proposed as a method of modulating current injection in a magnetron to improve performance and control phase. To implement the controllable electron emission, five-sided and ten-sided faceted planar cathodes employing gated field emitters are considered as these emitters could be fabricated on flat substrates. For demonstration, the conformal finite-difference time-domain particle-in-cell simulation, as implemented in VORPAL, has been used to model a ten-cavity, rising sun magnetron using the modulated current sources and benchmarked against a typical continuous current source. For the modulated, ten-sided faceted cathode case, the electrons are injected from three emitter elements on each of the ten facets. Each emitter is turned ON and OFF in sequence at the oscillating frequency with five emitters ON at one time to drive the five electron spokes of the π-mode. The emitter duty cycle is then 1/6th the Radio-Frequency (RF) period. Simulations show a fast start-up time as low as 35 ns for the modulated case compared to 100 ns for the continuous current cases. Analysis of the RF phase using the electron spoke locations and the RF magnetic field components shows that the phase is controlled for the modulated case while it is random, as typical, for the continuous current case. Active phase control during oscillation was demonstrated by shifting the phase of the electron injection 180° after oscillations started. The 180° phase shift time was approximately 25 RF cycles

Dynamic Phase-Control of a Rising Sun Magnetron Using Modulated and Continuous Current

Phase-control of a magnetron is studied via simulation using a combination of a continuous current source and a modulated current source. The addressable, modulated current source is turned ON and OFF at the magnetron operating frequency in order to control the electron injection and the spoke phase. Prior simulation work using a 2D model of a Rising Sun magnetron showed that the use of 100% modulated current controlled the magnetron phase and allowed for dynamic phase control. In this work, the minimum fraction of modulated current source needed to achieve a phase control is studied. The current fractions (modulated versus continuous) were varied from 10% modulated current to 100% modulated current to study the effects on phase control. Dynamic phase-control, stability, and start up time of the device were studied for all these cases showing that with 10% modulated current and 90% continuous current, a phase shift of 180˚ can be achieved demonstrating dynamic phase control