Abstract
Longitudinal bunch compression of intense ion beams for warm dense matter and heavy ion fusion applications occurs by imposing an axial velocity tilt onto an ion beam across the acceleration gap of a linear induction accelerator, and subsequently allowing the beam to drift through plasma in order to neutralize its space-charge and current as the pulse compresses. The detailed physics and implications of acceleration gap effects and focusing aberration on optimum longitudinal compression are quantitatively reviewed using particle-in-cell simulations, showing their dependence on many system parameters. Finite-size gap effects are shown to result in compression reduction, due to an increase in the effective longitudinal temperature imparted to the beam, and a decrease in intended fractional tilt. Sensitivity of the focal plane quality to initial longitudinal beam temperature is explored, where slower particles are shown to experience increased levels of focusing aberration compared to faster particles. A plateau effect in axial compression is shown to occur for larger initial pulse lengths, where the increases in focusing aberration over the longer drift lengths involved dominate the increases in relative compression, indicating a trade-off between current compression and pulse duration. The dependence on intended fractional tilt is also discussed and agrees well with theory. A balance between longer initial pulse lengths and larger tilts is suggested, since both increase the current compression, but have opposite effects on the final pulse length, drift length, and amount of longitudinal focusing aberration. Quantitative examples are outlined that explore the sensitive dependence of compression on the initial kinetic energy and thermal distribution of the beam particles. Simultaneous transverse and longitudinal current density compression can be achieved in the laboratory using a strong final-focus solenoid, and simulations addressing the effects of focusing aberration in both directions are presented.
Highlights
Of the challenges encountered in developing heavy ion drivers [1] for warm dense matter and heavy ion fusion applications [2 – 4], one of the most significant is found in the final transport section leading to the target, where ion beam compression in space and time is required in order to achieve the necessary high intensities for striking the target
IVA), a strong final-focus solenoid is situated near the end of the drift region in order to transversely focus the beam to a submillimeter spot size coincident with the longitudinal focal plane, and the compression dependence on beam radius rb t entering the solenoid is explored
In the classical limit of point particles, there is no upper bound on the longitudinal current compression under the assumptions mentioned, since an ideal tilt will cause all of the beam particles to arrive at the focal plane at the exact same time
Summary
Of the challenges encountered in developing heavy ion drivers [1] for warm dense matter and heavy ion fusion applications [2 – 4], one of the most significant is found in the final transport section leading to the target, where ion beam compression in space and time is required in order to achieve the necessary high intensities for striking the target [5,6]. The induction module employs a time-dependent voltage waveform to modify the longitudinal velocity profile of the beam, in order to decrease the initial pulse length and increase the current of the beam Such focusing can be longitudinal emittance dominated, due to the inherent longitudinal temperature of the beam Tk , provided that the accuracy of the imposed velocity tilt and the level of neutralization by the plasma are both high. Since imposing the velocity tilt results in the application of a time-dependent radial divergence to the ion beam (Sec. IVA), a strong final-focus solenoid is situated near the end of the drift region in order to transversely focus the beam to a submillimeter spot size coincident with the longitudinal focal plane, and the compression dependence on beam radius rb t entering the solenoid is explored (Sec. IV B).
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