Ion Effects in the DARHT-II Downstream Transport
The DARHT-II accelerator produces an 18-MeV, 2-kA, 2-μs electron beam pulse. After the accelerator, the pulse is delivered to the final focus on an x-ray producing target via a beam transport section called the Downstream Transport. Ions produced due to beam ionization of residual gases in the Downstream Transport can affect the beam dynamics. Ions generated by the head of the pulse will cause modification of space-charge forces at the tail of the pulse so that the beam head and tail will have different beam envelopes. They may also induce ion-hose instability at the tail of the pulse. If these effects are significant, the focusing requirements of beam head and tail at the final focus will become very different. The focusing of the complete beam pulse will be time dependent and difficult to achieve, leading to less efficient x-ray production. In this paper, we will describe the results of our calculations of these ion effects at different residual-gas pressure levels. Our goal is to determine the maximum residual-gas pressure allowable in DARHT-II Downstream Transport such that the required final beam focus is achievable over the entire beam pulse under these deleterious ion effects.
A Study of the Ion Hose Instability in the DARHT-II Downstream Transport Region
The second axis of the DARHT flash X-ray facility at Los Alamos National Laboratory (''DARHT-II'') is a multiple-pulse, 18.4 MeV, 2 kA induction electron linear accelerator [1]. A train of short ({approx}50 ns) pulses are converted via bremsstrahlung to X-rays, which are then used to make radiographic images at various times (nominally four) during a ''hydrotest'' experiment. The train of pulses is created by carving them out of a two microsecond long macropulse, using a fast switching element called a kicker [2]. The unused portion of the macropulse is absorbed in a beam dump. Thus, upstream of the kicker, two microseconds of beam are transported through a vacuum system roughly sixty meters long. These conditions involve length and, specifically, time scales which are new to the transport of high-current beams. A concern under such conditions are the macroscopic interactions between the electron beam and positive ions created by impact ionization of the residual gas in the vacuum system. Over two microseconds, the ion density can develop to a hundredth or even a tenth of a percent of the beam density--small, to be sure, but large enough to have cumulative effects over such a long transport distance. Two such effects willmore » be considered here: the ion hose instability, where transverse forces conspire to pull the electron beam farther and farther off axis, and background gas focusing, where radial forces (with respect to the beam) change the beam envelope during the course of the macropulse. The former effect can cause beam emittance growth (affecting the ability to focus the beam on the target) and eventually catastrophic beam loss; the latter can cause either serious degradation of the statically tuned final focus on the converter target, or a pinching of the beam on the surface of the main dump to the point where the heat flux causes damage. The beam transport upstream of the kicker has two distinct phases. First, the beam is created and accelerated up to 18.4 MeV over a distance of about fifty meters. Then the true downstream transport begins: the beam drifts through a matching section in preparation for the kicker, over some ten meters; the long-pulse beam then travels about four more meters from the kicker to the main dump. In the accelerator, the beam energy is obviously not constant; the transport is emittance-dominated and done through nearly continuous solenoidal focusing. In the downstream section, there are only two discrete solenoids over the entire fourteen meters and the transport is largely ballistic. Since ion hose has been studied in the accelerator [3] and since the lack of continuous focusing is considered a concern with respect to ion hose in the downstream section, the focus of this study is only from the exit of the accelerator to the main dump. A more in-depth description of the baseline (ion-free) DARHT-II downstream transport, including description of the actual transport elements and their use, will not be presented in this document; such details can be found in the documents cited in the References. The study of these effects will be done in stages. In the next section, the nature of the residual gas in the vacuum system will be considered, along with the various assumptions made in characterizing the creation of ions. Then the ion hose instability will be described in its simplest form. In the fourth section, additional features of ion hose will then be presented which attempt to capture some of the key behavior. Then a much more complete model using particle-in-cell (PIC) numerical techniques will be described, followed by details of the specific implementation used here. In section seven, the code will be benchmarked against results published in the literature. Section eight has the most relevant material: the actual study of the effects of ion hose and background gas focusing in the DARHT-II downstream transport region. In section nine, a simple experiment which can be tacked on to existing experiments is proposed in order to verify the modeling. Finally, the results are summarized and the very last section lists references.« less
Electron beam propagation in the ion‐focused regime
In this paper the features of propagation of intense relativistic beams are discussed in the environment of low‐density (≲1 Torr), partially ionized gas. Beam phenomena in this ion‐focused regime (IFR) are found to be analogous to those at much higher density. Several instabilities and dissipative mechanisms have been identified and are compared to their high‐density counterparts. Scaling laws are derived for quantitative estimation of these effects. In general, it is found that long propagation distances are possible in the laboratory suggesting that this regime may be useful for controlled beam transport and conditioning.