Magnetic lattice for the HIF neutralized transport experiment (NTX)

Abstract

The NTX experiment at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final focus systems for high perveance heavy ion beams. A pulsed magnetic four-quadrupole transport system for a 400 keV, 80 mA space charge dominated heavy ion beam has been designed, fabricated, tested, measured, and commissioned successfully for the Neutralized Transport Experiment (NTX). We present some generalized multipole decompositions of 3-D finite element calculations, and 2-D transient finite element simulations of eddy currents in the beam tube. Beam envelope calculations along the transport line were performed using superposition of individually 3 -D calculated magnetic field maps. Revised quadrupole design parameters and features, plus fabrication and testing highlights are also presented. Magnetic field measurements were made using both Hall probes (low field DC) and inductive loop coil (high field pulsed). Magnet testing consisted of repetitive full current pulsing to determine reliability. MAGNETIC LATTICEThe transport section is designed to correspond closely to a prototypical HIF driver final focus channel. It consists of a double FODO channel with very short drift regions, including the drift from the source into the channel, and the drift into the plasma neutralization chamber. Fig. 1 shows the X and Y beam profiles through the system, with and without beam neutralization at the exit. The beam is quite large in the two center magnets, which determines the required bore size and winding radius. The magnet has a relatively short center section and a substantial portion of the magnetic field is contained in the end fringe fields, with significant axial components. The heavy ion beam is space charge dominated, having a large cross section, and being highly non-paraxial. As such, the usual method of (beam axis integrated) field quantification into normal multipoles of discrete length does not allow accurate particle tracking simulation, as these hard-edge field simplifications do not contain the axial field components and nonlinear (with radius) gradients which are a significant source of particle deflection. Thus, both normal and pseudomultipoles are important and both must be considered in lattice design, requiring 3D magnetic analysis. MAGNETIC FIELD MODELINGA three-dimensional finite element model of the magnet was generated, using ANSYS/EMAG and solved for the static field in 3D space. A scalar potential formulation was used, with source current elements used for the coil. The space modeled was a 1/8 th section of one half the magnet. Far field (infinite boundary) elements bounded the outer radius and end, at a distance 75 cm from the magnet midplane, which extends well past the source and final focus points. A cylindrical Neumann boundary was used to simulate the steel core, in order to speed analysis for a fine mesh model. An earlier coarse mesh model with steel included (nonlinear B -H curve) showed no significant saturation. The resulting 3D field maps generated ...

Focusing and neutralization of intense beams

“…This is the desired entrance condition for the final neutralized drift experiment. The pulsed magnets [10] were fabricated in-house, and the conductors were arranged to minimize unwanted higher order multipoles. Fields were measured with accurate 3-way B dot probes, and the agreement with ANSYS code calculations was excellent as shown in The beam size and angle at lattice exit were quite sensitive to small changes in quad fields and/or beam energy.…”

Final focus system for high intensity beams

“…The experimental setup ( Fig. 1) consists of three major sections, a low-emittance potassium gun [2], a magnetic transport section with 4 pulsed quadrupoles [3], and a one-meter long drift section with plasma neutralization [4]. The 3-D PIC simulation code WARP3D has been used to design the gun and the magnetic lattice, and predictions of nonlinear forces have been made and are compared against the NTX experiments.…”

Neutralized transport of high intensity beams

“…The experimental setup ( Fig. 1) consists of three major sections, a low-emittance potassium gun [2], a magnetic transport section with 4 pulsed quadrupoles [3,4], and a one-meter long drift section with plasma neutralization. The MRC electromagnetic code LSP has been used to specify the requirements for the plasma source, as well as the predicted degree of neutralization as a function of the various beam and plasma parameters.…”