Dynamics of ion beam charge neutralization by ferroelectric plasma sources

Intense ion beam neutralization by sufficiently cold, co-moving and co-injected electrons is crucial to Light Ion and important to Heavy Ion Inertial Confinement Fusion (ICF) drivers for the ballistically focused propagation onto ~ 5 mm radius targets located some 10 m downstream. Methods of generating the beam neutralization electrons with required properties are given in the context of a Light Ion Fusion Experiment (LIFE) designed accelerator. Recently derived envelope equations for neutralized and ballistically focused intense ion beams are applied to the LIFE geometry in which 10 MeV He+ multiple beamlets coalesce and undergo 45:1 radial compression while beam pulses experience a 20:1 axial compression in the propagation range of 10 m. For these representative conditions, the theoretical analysis produces a requirement of the initial electron temperature neutralizing the ions as Teo ≤ 35 eV where also the initial ion temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">io</sub> is correlated. Both active and auto <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> -neutralization methods are examined and found to produce initial electron temperatures consistent with the requirement of the envelope equation for both radial and axial adiabatic beam pulse compressions. The stability of neutralized beam propagation is also examined concerning the Pierce type electrostatic instability and for the case of LIFE beams it is found to have insignificant effect. A scaled experimental setup is presented which can serve to perform near term tests on the ballistically focused propagation of neutralized light ion beams.

Summary form only given. Ferroelectric plasma sources are known as sources of surface discharge plasma. Main advantage these sources is the ability to create layers of plasma on surfaces with large area and arbitrary shape. In vacuum, numerous discharges on the surface of ferroelectric ceramics appear in response of a driving pulse applied across a sample of ferroelectric ceramics. Electron avalanches can be initiated by field emission from triple junctions at the edges of patterned electrodes in electric fields, induced by inhomogeneities of the polarization charge. These avalanches along the surface are driven by secondary electron emission form the ferroelectric ceramics. Surface ablation and gas desorption supply neutrals, which are further ionized by avalanching electrons, resulting in a surface plasma. The depicted scenario is the same as for surface flashover; however, discharge on ferroelectrics does not turn to vacuum arc because of separation of electrodes by the ceramic bulk. This feature makes the discharge on ferroelectrics very interesting for study of the plasma formation process in the surface discharge. For instance, initial plasma density and electron temperature in the stem of the discharge can be deduced from the temporal profiles of the density in expanding plasma. For PZT ceramics, the initial plasma density was found in the range of (0.4-4)/spl times/10/sup 17/ cm/sup -3/ and the initial electron temperature of about 16-20 eV for the driving electric fields of 16-30 kV/cm. It is shown that the discharge on ferroelectrics can be realized also in a gas mode. Transition from vacuum surface discharge to the gas discharge begins at the background pressure of 1-20 Torr, and the surface discharge in this pressure range sustains the gas discharge. At the pressures between 20 and 80 Torr, formation of the discharge stems is completely suppressed, which indicates the range of neutral pressure in the stem of the surface discharge in vacuum. The gas discharge is uniform and it appears on the open surface of the ferroelectric ceramics. In Argon at a distance of 5.5 mm from the surface, the maximal plasma density of /spl sim/10/sup 13/ cm/sup -3/ was observed at pressure of /spl sim/8 Torr. Preliminary study of secondary electron emission from ferroelectrics shows that at low energies of primary electrons the total yield of electron-induced SEE from ferroelectrics is high comparing with other dielectrics and it depends on polarization of the ferroelectric material. Low value of energy which corresponds to the unity yield of SEE from ferroelectrics leads to higher desorption rate and consequently to lower threshold of the surface discharge on ferroelectrics.

The results of computer simulations of time-dependent charge and current neutralization of high-current ion beams by cold electrons emitted from a plane to a half-space are presented. The cases of long and short injected ion pulses with sharp and linear current risetimes are considered. It is shown that the system of cold electrons and ions passes to the same steady state independently of the initial and boundary conditions. The results are of interest in connection with the application of ion beams in inertial fusion research. >

ALCBEAM – Neutral beam formation and propagation code for beam-based plasma diagnostics

In heavy ion inertial confinement fusion systems, intense beams of ions must be transported from the exit of the final focus magnet system through the target chamber to hit millimeter spot sizes on the target. Effective plasma neutralization of intense ion beams through the target chamber is essential for the viability of an economically competitive heavy ion fusion power plant. The physics of neutralized drift has been studied extensively with PIC simulations. To provide quantitative comparisons of theoretical predictions with experiment, the Heavy Ion Fusion Virtual National Laboratory has completed the construction and has begun experimentation with the NTX (Neutralized Transport Experiment) as shown in Figure 1. The experiment consists of 3 phases, each with physics issues of its own. Phase 1 is designed to generate a very high brightness potassium beam with variable perveance, using a beam aperturing technique. Phase 2 consists of magnetic transport through four pulsed quadrupoles. Here, beam tuning as well as the effects of phase space dilution through higher order nonlinear fields must be understood. In Phase 3, a converging ion beam at the exit of the magnetic section is transported through a drift section with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we present first results from all 3 phases of the experiment.

Parameters of the plasma produced by a ferroelectric plasma source (FPS) assisted high-current (∼1 kA), hollow anode (HA), low-pressure (10−4 Torr) Ar gas discharge were studied using a time- and space-resolved laser induced fluorescence (LIF) diagnostic technique. It was shown that the plasma filling of the HA cavity occurs due to the expansion of the plasma flows generated by the FPSs with a gradual time-dependent increase in the plasma density along the length of the HA cavity. These data were verified by the results of one-dimensional modeling of the plasma expansion in vacuum and time-dependent collisional-radiative modeling. Also, LIF diagnostics, which was tested on the saturation effect both experimentally and numerically, showed that the plasma ion temperature gradually increases during the discharge reaching ∼9 eV at its end. Various phenomena (plasma kinetic instabilities, charge-exchange processes during the plasma ion interaction with the HA walls, and the model of neutral atoms ionization in...

Parameters of the plasma produced by a ferroelectric plasma source (FPS) assisted high-current (∼1 kA) hollow anode (HA), low-pressure (10-4 Torr) Ar gas discharge were studied using a time- and space-resolved laser induced fluorescence (LIF) diagnostic technique. It was shown that the plasma filling of the HA cavity occurs due to the expansion of the plasma flows generated by the FPSs with a gradual time-dependent increase in the plasma density along the length of the HA cavity. These data were verified by the results of 1D modeling of the plasma expansion in vacuum and time-dependent collisional-radiative modeling. Also, LIF diagnostics, which was tested on the saturation effect both experimentally and numerically, showed that the plasma ion temperature gradually increases during the discharge reaching ∼9 eV at its end. Various phenomena (plasma kinetic instabilities, charge-exchange processes during the plasma ion interaction with the HA walls and the model of neutral atoms ionization inside the Debay's sphere) which could be responsible for such ion temperature were considered.

Dynamics of electromagnetic two-stream interaction processes during longitudinal and transverse compression of an intense ion beam pulse propagating through background plasma

Plasma neutralization of an intense ion pulse is of interest for many applications, including plasma lenses, heavy ion fusion, cosmic ray propagation, etc. An analytical electron fluid model has been developed based on the assumption of long charge bunches (l/sub b//spl Gt/r/sub b/). Theoretical predictions are compared with the results of calculations utilizing a particle-in-cell (PIC) code. The cold electron fluid results agree well with the PIC simulations for ion beam propagation through a background plasma. The analytical predictions for the degree of ion beam charge and current neutralization also agree well with the results of the numerical simulations. The model predicts very good charge neutralization (>99%) during quasi-steady-state propagation, provided the beam pulse duration /spl tau//sub b/ is much longer than the electron plasma period 2/spl pi///spl omega//sub p/, where /spl omega//sub p/ = (4/spl pi/e/sup 2/n/sub p/ / m)/sup 1/2/ is the electron plasma frequency, and n/sub p/ is the background plasma density. In the opposite limit, the beam pulse excites large-amplitude plasma waves. The analytical formulas derived in this paper can provide an important benchmark for numerical codes, and provide scaling relations for different beam and plasma parameters.

Development of a 1-m plasma source for heavy ion beam charge neutralization

Neutralization and focusing of intense charged particle beam pulses by electrons forms the basis for a wide range of applications to high energy accelerators and colliders, heavy ion fusion, and astrophysics. For example, for ballistic propagation of intense ion beam pulses, background plasma can be used to effectively neutralize the beam charge and current, so that the self-electric and self- magnetic fields do not affect the ballistic propagation of the beam. From the practical perspective of designing advanced plasma sources for beam neutralization, a robust theory should be able to predict the self-electric and self-magnetic fields during beam propagation through the background plasma. The major scaling relations for the self-electric and self-magnetic fields of intense ion charge bunches propagating through background plasma have been determined taking into account the effects of transients during beam entry into the plasma, the excitation of collective plasma waves, the effects of gas ionization, finite electron temperature, and applied solenoidal and dipole magnetic fields. Accounting for plasma production by gas ionization yields a larger self-magnetic field of the ion beam compared to the case without ionization, and a wake of current density and self-magnetic field perturbations is generated behind the beam pulse. A solenoidal magnetic field can be applied for controlling the beam propagation. Making use of theoretical models and advanced numerical simulations, it is shown that even a small applied magnetic field of about 100G can strongly affect the beam neutralization. It has also been demonstrated that in the presence of an applied magnetic field the ion beam pulse can excite large-amplitude whistler waves, thereby producing a complex structure of self-electric and self-magnetic fields. The presence of an applied solenoidal magnetic field may also cause a strong enhancement of the radial self-electric field of the beam pulse propagating through the background plasma. If controlled, this physical effect can be used for optimized beam transport over long distances.

Neutralization and focusing of intense charged particle beam pulses by electrons form the basis for a wide range of applications to high energy accelerators and colliders, heavy ion fusion, and astrophysics. For example, for ballistic propagation of intense ion beam pulses, background plasma can be used to effectively neutralize the beam charge and current, so that the self-electric and self-magnetic fields do not affect the ballistic propagation of the beam. From the practical perspective of designing advanced plasma sources for beam neutralization, a robust theory should be able to predict the self-electric and self-magnetic fields during beam propagation through the background plasma. The major scaling relations for the self-electric and self-magnetic fields of intense ion charge bunches propagating through background plasma have been determined taking into account the effects of transients during beam entry into the plasma, the excitation of collective plasma waves, the effects of gas ionization, finite electron temperature, and applied solenoidal and dipole magnetic fields. Accounting for plasma production by gas ionization yields a larger self-magnetic field of the ion beam compared to the case without ionization, and a wake of current density and self-magnetic field perturbations is generated behind the beam pulse. A solenoidal magnetic field can be applied for controlling the beam propagation. Making use of theoretical models and advanced numerical simulations, it is shown that even a small applied magnetic field of about 100 G can strongly affect the beam neutralization. It has also been demonstrated that in the presence of an applied magnetic field the ion beam pulse can excite large-amplitude whistler waves, thereby producing a complex structure of self-electric and self-magnetic fields. The presence of an applied solenoidal magnetic field may also cause a strong enhancement of the radial self-electric field of the beam pulse propagating through the background plasma. If controlled, this physical effect can be used for optimized beam transport over long distances.

Simulations suggest that the plasma density must exceed the beam density throughout the drift compression and focusing section in order to inhibit the space charge forces that would limit the spot size and beam intensity on the target. WDM experiments will therefore require plasma densities up to 10{sup 14}/cm{sup 3}, with the highest density in the last few centimeters before the target. This work was guided by the simulations performed for the FY09 Q1 milestone. This milestone has been met and we report results of modifications made to the NDCX beamline to improve the longitudinal and radial distribution of the neutralizing plasma in the region near the target plane. In Section 2, we review pertinent simulation results from the FY09 Q1 milestone. Section 3 describes the design, and beam measurements following installation, of a biased, self-supporting metal grid that produces neutralizing electrons from glancing interception of beam ions. Section 4 describes the design and initial testing of a compact Ferro-Electric Plasma Source (FEPS) that will remove the remaining 'exclusion zone' in the neutralizing plasma close to the target plane. Section 5 describes the modification of the beamline to decrease the gap between the FEPS section exit and the final focus solenoid (FFS). Section 6 presents a summary and conclusions.

The spherical torus (ST) concept underlies several contemporary plasma physics experiments, in which relatively low magnetic fields, high plasma edge q, and low aspect ratio combine for potentially compact, high beta and high performance fusion reactors. An important issue for the ST is the calculation of energetic ion confinement, as large Larmor radius makes conventional guiding center codes of limited usefulness and efficient plasma heating by RF and neutral beam ion technology requires minimal fast ion losses. The National Spherical Torus Experiment (NSTX) is a medium-sized, low aspect ratio ST, with R=0.85 m, a=0.67 m, R/a=1.26, Ip*1.4 MA, Bt*0.6 T, 5 MW of neutral beam heating and 6 MW of RF heating. 80 keV neutral beam ions at tangency radii of 0.5, 0.6 and 0.7 m are routinely used to achieve plasma betas above 30%. Transport analyses for experiments on NSTX often exhibit a puzzling ion power balance. It will be necessary to have reliable beam ion calculations to distinguish among the source and loss channels, and to explore the possibilities for new physics phenomena, such as the recently proposed compressional Alfven eigenmode ion heating.

Plasma neutralization of an intense ion pulse is of interest for many applications, including plasma lenses, heavy ion fusion, cosmic ray propagation, etc. An analytical electron fluid model has been developed to describe the plasma response to a propagating ion beam. The model predicts very good charge neutralization during quasi-steady-state propagation, provided the beam pulse duration τb is much longer than the electron plasma period 2π/ωp, where ωp=(4πe2np/m)1/2 is the electron plasma frequency, and np is the background plasma density. In the opposite limit, the beam pulse excites large-amplitude plasma waves. If the beam density is larger than the background plasma density, the plasma waves break. Theoretical predictions are compared with the results of calculations utilizing a particle-in-cell (PIC) code. The cold electron fluid results agree well with the PIC simulations for ion beam propagation through a background plasma. The reduced fluid description derived in this paper can provide an important benchmark for numerical codes and yield scaling relations for different beam and plasma parameters. The visualization of numerical simulation data shows complex collective phenomena during beam entry and exit from the plasma.