Long plasma source for heavy ion beam charge neutralization

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

Ferroelectric Plasma Sources (FEPSs) can generate plasma that provides effective space-charge neutralization of intense high-perveance ion beams, as has been demonstrated on the Neutralized Drift Compression Experiment NDCX-I and NDCX-II. This article presents experimental results on charge neutralization of a high-perveance 38 keV Ar+ beam by a plasma produced in a FEPS discharge. By comparing the measured beam radius with the envelope model for space-charge expansion, it is shown that a charge neutralization fraction of 98% is attainable with sufficiently dense FEPS plasma. The transverse electrostatic potential of the ion beam is reduced from 15 V before neutralization to 0.3 V, implying that the energy of the neutralizing electrons is below 0.3 eV. Measurements of the time-evolution of beam radius show that near-complete charge neutralization is established ∼5 μs after the driving pulse is applied to the FEPS and can last for 35 μs. It is argued that the duration of neutralization is much longer than a reasonable lifetime of the plasma produced in the sub-μs surface discharge. Measurements of current flow in the driving circuit of the FEPS show the existence of electron emission into vacuum, which lasts for tens of μs after the high voltage pulse is applied. It is argued that the beam is neutralized by the plasma produced by this process and not by a surface discharge plasma that is produced at the instant the high-voltage pulse is applied.

Highly ionized plasmas are being used as a medium for charge neutralizing heavy ion beams in order to focus the ion beam to a small spot size. A radio frequency (RF) plasma source has been built at the Princeton Plasma Physics Laboratory (PPPL) in support of the joint Neutralized Transport Experiment (NTX) at the Lawrence Berkeley National Laboratory (LBNL) to study ion beam neutralization with plasma. The goal is to operate the source at pressures /spl sim/10/sup -5/ Torr at full ionization. The initial operation of the source has been at pressures of 10/sup -4/-10/sup -5/ Torr and electron densities in the range of 10/sup 8/-10/sup 11/ cm/sup -3/. Recently, pulsed operation of the source has enabled operation at pressures in the 10/sup -6/ Torr range with densities of 10/sup 11/ cm/sup -3/. Near 100% ionization has been achieved. The source has been integrated with the NTX facility and experiments have begun.

The performance of a novel tracking detector developed for the focal plane of the NSCL/FRIB S800 magnetic spectrometer is presented. The detector comprises a large-area drift chamber equipped with a hybrid Micro-Pattern Gaseous Detector (MPGD)-based readout. The latter consists of a position-sensitive Micromegas detector preceded by a two-layer M-THGEM multiplier as a pre-amplification stage. The signals from the Micromegas readout are processed by a data acquisition system based on the General Electronics for TPC (GET). The drift chamber has an effective area of around 60 × 30 cm2, which matches to the very large acceptance of the S800 spectrometer. This work discusses in detail the results of performance evaluation tests carried out with a low-energy alpha-particles source, and with high-energy heavy-ion beams with the detector installed at the S800 focal plane. In this latter case, the detector was irradiated with a 150 MeV/u 78Kr36+ beam as well as a heavy-ion fragmentation cocktail beam produced by the 78Kr36+ beam impinging on a thin beryllium target. The new detector allows sub-millimeter position resolution in both dispersive and non-dispersive directions. The new read out scheme, coupled to the GET electronics, has several advantages compared to the old Cathode Readout Drift Chamber (CRDC), including a four time higher detection rate (of around 20 kHz), multi-hit capability, and a better operation stability. The new improved performance are needed to take full advantage of FRIB's rare-isotope beam portfolio and expected high intensity.

The aspect ratio (AR) of discharge geometry is an important parameter in view of inductively coupled plasma (ICP) source design. AR is defined as the ratio of chamber radius (R) and chamber height (L). The effects of the AR on plasma parameters and uniformity are investigated in a 2 MHz ICP source. The argon discharge is performed in two chambers of AR = 0.72 and 0.35 with different heights, and the effects of AR on electron loss mechanisms are studied using a global model. The results show a tendency for generation of higher density plasma with better uniformity in the case of AR = 0.72 at 0.5–2 Pa. The difference in electron density is caused by the differences in the volume. The observations on plasma uniformity are explained by the study of the electron loss mechanism. The accumulation of electrons is weakened by axial diffusion at the chamber center in the case of AR = 0.72. The difference in plasma uniformity between the two chambers diminishes with the increase in the gas pressure from 0.5 to 2 Pa, which results from the fact that the electron diffusion along the axial direction becomes more difficult. At higher pressures from 5–10 Pa, the ambipolar diffusion loss of electrons to the chamber wall becomes more difficult with the increase in pressure due to frequent collisions. Therefore, the electron accumulation in the discharge center is more evident in the chamber with AR = 0.72, which deteriorates the plasma uniformity. The above study can give a reference to the design of cylindrical ICP sources for practical applications.

ABSTRACTWe employ intense and short pulses of energetic lithium (Li+) ions to investigate the relaxation dynamics of radiation induced defects in single crystal silicon samples. Ions both create damage and track damage evolution simultaneously at short time scales when we use the channeling effect as a diagnostic tool. Ion pulses, ∼20 to 600 ns long and with peak currents of up to ∼1 A are formed in an induction type linear accelerator, the Neutralized Drift Compression eXperiment at Lawrence Berkeley National Laboratory. By rotating silicon (<100>) membranes of different thicknesses and changing the incident ion energy, the fraction of channeled ions in the transmitted beam could be varied. In preliminary experiments we find that the Li ion intensity is not high enough to generate overlapping cascades (in time and space) that would be necessary to measure a change in the shape of the current waveform of the transmitted ion beam. We discuss the concept of pump-probe type experiments with short ion beam pulses to access defect dynamics in materials and outline a path to increasing damage rates with heavier ions and by the application of longitudinal and lateral pulse compression techniques.

Ion beam heated target simulations for warm dense matter physics and inertial fusion energy

Design of a large area fast RICH detector with CsI photocathode for particle identification at ALICE-LHC

A new luminescence beam profile monitor is realized in the polarized hydrogen gas jet target at the Relativistic Heavy Ion Collider (RHIC) facility. In addition to the spin polarization of the proton beam being routinely measured by the hydrogen gas jet, the luminescence produced by beam-hydrogen excitation leads to a strong Balmer series lines emission. A selected hydrogen Balmer line is spectrally filtered and imaged to produce the transverse RHIC proton beam shape with unprecedented details on the RHIC beam profile. Alternatively, when the passage of the high energy RHIC gold ion beam excited only the residual gas molecules in the beam path, sufficient ion beam induced luminescence is produced and the transverse gold ion beam profile is obtained. The measured transverse beam sizes and the calculated emittances provide an independent confirmation of the RHIC beam characteristics and to verify the emittance conservation along the RHIC accelerator. This optical beam diagnostic technique by making use of the beam induced fluorescence from injected or residual gas offers a truly noninvasive particle beam characterization, and provides a visual observation of proton and heavy ion beams. Combined with a longitudinal bunch measurement system, a 3-dimensional spatial particle beam profile can be reconstructed tomographically.

Chemical conversions in lead thin films induced by heavy-ion beams at Coulomb barrier energies

Studies of equation of state properties of high-energy density matter using intense heavy ion beams at the future FAIR facility: The HEDgeHOB collaboration

High-energy-density physics experiments with intense heavy ion beams

Summary form only given, as follows. Intense pulsed heavy ion beams (HIB) of nitrogen, carbon, aluminum, etc. are expected to be applied to materials processing such as surface heat treatment, ion implantation etc. To apply the pulsed power ion diode to those applications we are developing two types of Br type magnetically insulated ion diode, i.e. a high impedance diode and a low impedance diode. In the high impedance diode a coaxial plasma gun was used as an ion source. The diode was driven by a high impedance pulsed power system (output parameter; 200 kV, 2 kA, 200 ns) where a pulse-forming network and step up transformer are used. On the other hand, in the low impedance diode a flashboard anode is used and the anode plasma is produced by the irradiation of electrons. The diode was drive by a Blumlein pulse forming line (output parameter; 600 kV, 24 kA, 60 ns). The ion current density, ion species, energy, and brightness were evaluated for both type of ion diode. A Thomson parabola spectrometer, filtered ion pinhole camera and biased ion corrector are used in the experiment.

Optimization of the plasma parameters for the high current and uniform large-scale pulse arc ion source of the VEST-NBI system

There are several applications which require plasma to be produced over large volumes (~few cubic meters) and where it is desirable to have high radial/axial uniformity and high plasma density. Although electron cyclotron resonance (ECR) plasma sources are efficient and yield high plasma densities, these do not scale well with the volume. This paper presents a scheme for large volume plasma production that retains the advantages of ECR plasma sources without sacrificing on uniformity or high plasma density. It uses multiple, compact ECR plasma sources (CEPS) that are highly portable and easily mountable on any chamber. The large volume plasma system (LVPS) described in the paper is a scalable, circular cylindrical vessel of diameter ≈ 1m, consisting of source sections (SS) and spacer sections (SPS). The CEPS were mounted in azimuthally symmetric manner on matching ports along the periphery of the source sections, and the system was configured for 4/6 CEPS (one SS, two SPS, height ≈ 1.1 m) and 12 CEPS (two SS and three SPS, height ≈ 1.55 m). Since plasma filling in this scheme is radially inwards, the decrease in the volume to be filled as the plasma flows inwards, partially offsets the depletion of plasma as it flows away from the sources. This results in a more uniform plasma density over the system volume. The scheme also takes advantage of the magnetic field of the CEPS (produced by ring magnets) to produce a cusp magnetic field inside the chamber aiding confinement and uniformity. The paper discusses the development of the LVPS and presents plasma production results for two different configurations of the LVPS. The mechanism of plasma production is discussed in the light of results obtained for single CEPS.