Ion Diode Research Articles

Telescopic refractive index gradient diagnostic of an ion diode anode plasma

We present a general analysis of possible variants of refractive index gradient (RING) diagnostics with a laser beam probe. Using a differential bicell photodiode as a detector, the sensitivity, dynamic range, and geometric restrictions of RING deflectometry have been found for lensless, one lens, and “three-telescope” optical schemes. The three-telescope method is found to be the most flexible and easily aligned. If the refracted/deflected laser beam cross section in the back focal plane of the output lens is also recorded using a fast framing camera, measurements of the beam deflection and its spatial frequency spectrum after passing through the refractive medium can be obtained simultaneously. A general relation is presented between the Fourier transform of a Gaussian beam by the output lens and the spatial frequency spectrum of the inhomogeneities. From these considerations, we present the specific design of a RING diagnostic for study of anode plasma evolution in a magnetically insulated ion diode on the Cornell Beam Research Accelerator (COBRA) (800 kV, 80 ns pulse). The maximum density gradient was found to be located about 0.4 mm from the anode surface at the peak of the diode voltage pulse. The electron density at this position was about 2×1015 cm−3. The transverse spatial frequency distribution of the probe laser beam after passing through the anode plasma was recorded using the optical Fourier transform technique. This experiment demonstrates that the combined integrated deflection. (RING) and Fourier transform optical techniques can give a great deal of information about this thin (∼1 mm) anode plasma layer.

VUV absorption spectroscopy measurements of the role of fast neutral atoms in a high-power gap breakdown

The maximum power achieved in a wide variety of high-power devices, including electron and ion diodes, z pinches, and microwave generators, is presently limited by anode-cathode gap breakdown. A frequently discussed hypothesis for this effect is ionization of fast neutral atoms injected throughout the anode-cathode gap during the power pulse. We describe a newly developed diagnostic tool that provides a direct test of this hypothesis. Time-resolved vacuum-ultraviolet absorption spectroscopy is used to directly probe fast neutral atoms with 1-mm spatial resolution in the 10-mm anode-cathode gap of the SABRE 5 MV, 1 TW applied-B ion diode. Absorption spectra collected during Ar RF glow discharges and with CO2 gas fills confirm the reliability of the diagnostic technique. Throughout the 50-100 ns ion diode pulses no measurable neutral absorption was seen, setting upper limits of (0.12-1.5)x10(14) cm(-3) for ground-state fast neutral atom densities of H, C, N, O, and F. The absence of molecular absorption bands also sets upper limits of (0.16-1.2)x10(15) cm(-3) for common simple molecules. These limits are low enough to rule out ionization of fast neutral atoms as a breakdown mechanism. Breakdown due to ionization of molecules is also found to be unlikely. This technique can now be applied to quantify the role of neutral atoms in other high-power devices.

A sensitive dispersion interferometer with high temporal resolution for electron density measurements

A dispersion interferometer with a sensitivity of up to 10−4 fringe shifts and a time resolution of about 1 ns was developed. This time resolution exceeds previous systems by three orders. The operation of the dispersion interferometer and its characteristic properties are discussed and the experimental setup of the installation is explained in detail. The stability of the interferometer was tested and it was shown that its operation point drifts less than 0.25 rad/h. The interferometer was used to investigate the high power ion diodes at the pulse generator KALIF, where focused proton beams with power densities of up to 1 TW/cm2 are generated. For the very first time the electron line density inside the diode gap, where the protons are accelerated, could be directly measured. The line density was on the order of several 1013 cm−2. A straightforward estimation of the line density using a simplified diode model proved to be in excellent agreement with the results. The dispersion interferometer can be used for sensitive electron density measurements on nanosecond time scales. Further developments to improve the sensitivity are discussed.

8–10sς, 8–10dς 1Σ+g, and 10–15dπ 1Πg Rydberg States of Na2

With a sensitive space-charge-limited diode ionization detector, we studied 8–10sς, 8–10dς 1Σ+g, and 10–15dπ 1Πg Rydberg states of Na2 and obtained their molecular constants for the first time. It is the first successful example of a diode ionization detector that is used in a pulsed OODR molecular spectroscopy experiment.

A new approach to film dosimetry for high-energy photon beams using organic plastic scintillators

Successful radiotherapy relies on accurate dose measurement. Traditional dosimeters such as ion chambers, TLDs and diodes have disadvantages such as relatively long measurement time and poor spatial resolution. These drawbacks become more serious problems for dynamic beams (i.e. with the use of dynamic wedges or even the intensity modulation technique). X-ray film, an integrating dosimeter, may not be associated with the above disadvantages and problems. However, there are several major issues regarding use of x-ray film for routine dosimetry, including the over-response of the film to low-energy photons, variations in the dose response curve (nonlinearity), lack of reproducibility due to variation in processing, etc. This paper addresses the first problem. That is, x-ray film over-responds to low-energy photons (energies below 400 keV), and thus generates unacceptably inaccurate dosimetric data compared with ion-chamber data. To overcome the over-response problem of x-ray film in a phantom, a scintillation method has been investigated. In this method, a film is sandwiched by two plastic scintillation screens to enhance the film response to upstream electrons, and therefore minimize the over-response caused by low-energy photons. The sandwiched system was tested with a 4 MV linac beam. The result shows that, depending on the uniformity of the scintillation screens, the depth-dose distribution obtained from the sandwich system can be made to agree well with that obtained from ion chambers. However, the required high degree of uniformity remains a challenge for the scintillation screen manufacturers.

Development of the laser evaporation ion source for lithium beam generation on the Particle Beam Fusion Accelerator (PBFA-II)

The Laser Evaporation Ion Source (LEVIS) active lithium ion source has been developed for use on the focusing ion diode operated on the 10 TW Particle Beam Fusion Accelerator-II (PBFA-II) [J. P. VanDevender and D. L. Cook, Science 232, 831 (1986)] at Sandia National Laboratories. The source configuration consists of two laser pulses impinging on a heated (200 °C) thin-film LiAg layer on the anode surface. A short-pulse Nd:YAG laser creates a high-density vapor, which is then ionized by a long-pulse dye laser using the LIBORS (laser ionization based on resonant saturation) ionization method. Small-scale experiments determined that this dual laser-based approach can produce a source plasma of adequate density and confinement for acceleration and transport. Hardware modifications were undertaken to correct problems of premature impedance collapse and lack of beam lithium seen on previous PBFA-II experiments. As much as 85 kJ of Li is measured at the beam focus, but the source may not have been operating in a fully active (i.e., preformed) manner. Focusing performance appears superior to a passive LiF ion source operated on PBFA-II with the same magnetic field topology.

Angular distribution and polarization of atomic radiative emission in electric and magnetic fields

A density-matrix approach has been developed for the angular distribution and polarization of radiative emission during single-photon atomic transitions for a general set of steady-state excitation processes in an arbitrary arrangement of static (or quasistatic) electric and magnetic fields. Particular attention has been directed at spectroscopic observations in the intense fields of the high-power ion diodes on the Particle Beam Fusion Accelerator II (PBFA II) and SABRE devices at Sandia National Laboratories and at magnetic-field measurements in tokamak plasmas. The field-dependent atomic eigenstates are represented as expansions in a complete basis set of field-free bound and continuum eigenstates. Particular emphasis has been given to directed-electron collisional excitations, which may be produced by an anisotropic incident-electron velocity distribution. We have allowed for the possibility of the coherent excitation of the nearly degenerate field-dependent atomic substates, which can give rise to a complex spectral pattern of overlapping Stark-Zeeman components. Coherent excitations may be produced by a beam of electrons that are spin-polarized at an angle with respect to the propagation direction or by nonparallel electric and magnetic fields. Our main result is a general expression for the matrix elements of the photon-polarization density operator representing the total intensity, angular distribution, and polarizationmore » of the atomic radiative emission. For the observation of radiative emission in the direction of the magnetic field, the detection of linearly polarized emission, in addition to the usual circularly polarized radiation, can reveal the presence of a perpendicular electric field or a coherent excitation mechanism.« less

Electron–anode interactions in particle-in-cell simulations of applied-B ion diodes

Particle-in-cell simulations of applied-B ion diodes using the QUICKSILVER code [D. B. Seidel et al., in Proceedings of the Europhysics Conference on Computational Physics, Amsterdam, 1990, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] have been augmented with Monte Carlo calculations of electron–anode interactions (reflection and energy deposition). Extraction diode simulations demonstrate a link between the instability evolution and increased electron loss and anode heating. Simulations of radial and extraction ion diodes show spatial nonuniformity in the predicted electron loss profile leading to hot spots on the anode that rapidly exceed the 350 °C–450 °C range, known to be sufficient for plasma formation on electron-bombarded surfaces. Thermal desorption calculations indicate complete desorption of contaminants with 15–20 kcal/mole binding energies in high-dose regions of the anode during the power pulse. Comparisons of parasitic ion emission simulations and experiment show agreement in some aspects, but also highlight the need for better ion source, plasma, and neutral gas models.

Measurements of head-scatter factors with cylindrical build-up caps and columnar miniphantoms.

Head-scatter factors, Sh, also referred to as output factors, are measured in-air with an ion chamber and a semiconductor diode fitted with cylindrical build-up caps and columnar miniphantoms fabricated from materials of different atomic number. Sh increases with field size less rapidly when cylindrical build-up caps are constructed from high atomic number materials. This is a consequence of a net scatter of contamination electrons away from the detector. Ion chambers and diodes give identical results when the same type of build-up caps are used. Contamination electrons can be avoided by the use of columnar miniphantoms that have sufficient wall thickness in the radial direction. This radial wall thickness is characterized in this work for 6, 10, and 18 MV x-ray beams. Sh increases with field size less rapidly when columnar miniphantoms are constructed from high atomic number materials. This is due to the decrease in the average energy of photons at large field sizes. It is concluded that to obtain Sh for dosimetry in water, cylindrical build-up caps and columnar miniphantoms should be constructed from material with an atomic number close to that of water.

New method for thin-film formation with cryogenic diode and cryogenic target

In this paper we propose a new method to produce thin films by the irradiation of targets using focused ion beams. The starting point is to use the cryogenic targets with or without cryogenic ion diodes. As a first trial, we built a cryogenic target which was cooled by liquid nitrogen and we irradiated the ice-covered target by proton beams. We measured the luminosity of the ablated region to investigate the fundamental processes concerned. The preliminary results are shown here.

Production, diagnostic and application of pulsed ion beams with light and medium mass; LIB (and MIB) program in Japan

This is a combined outline paper of many groups who are interested in inertial confinement fusion by light or medium mass ion beams. The first (Tokyo Institute of Technology) group presents (1) a direct measurement of pulsed ion beams with CCD element, (2) preliminary observation of beam divergence with pin hole camera and (3) application of light ion beam to thin film formation and related diagnostic of ablation process. The second (Osaka University and Himeji Institute of Technology) group presents two-dimensional beam diagnostics for two-stage ion diode by an advanced arrayed pin-hole camera, and the theoretical or simulation (Nagaoka University of Technology, Tokyo University of Agriculture and Technology, and Teikyo Heisei University) group presents the transport of light ion beams, and pellet implosion in ion beam ICF. Here, the results which are not described in other papers in this Proceedings are shown minutely, while the others are shown briefly.

A finite‐volume particle‐in‐cell method for the numerical treatment of Maxwell‐Lorentz equations on boundary‐fitted meshes

A new conceptual framework solving numerically the time-dependent Maxwell–Lorentz equations on a non-rectangular quadrilateral mesh in two space dimensions is presented. Beyond a short review of the applied particle treatment based on the particle-in-cell method, a finite-volume scheme for the numerical approximation of the Maxwell equations is introduced using non-rectangular quadrilateral grid arrangements. The coupling of a high-resolution FV Maxwell solver with the PIC method is a new approach in the context of self-consistent charged particle simulation in electromagnetic fields. Furthermore, first simulation results of the time-dependent behaviour of an externally applied-B ion diode developed at the Forschungszentrum in Karlsruhe are presented. Copyright © 1999 John Wiley & Sons, Ltd.

Applied-B ion diode theory: The screening scale, relaxation of a Lagrangian invariant, and the diode stability

The assumption of relaxation to uniformity for the electron drift Lagrangian invariant ne/B results in a voltage–current characteristic U(ji) of an ion diode. The effect of the electric field screening at the magnetic Debye scale rB=B/4πene causes the existence of a maximum in the U(ji) curve at a certain threshold ion current density ji=jcrit. The diode can be stable only at the rising part of the U(ji) curve (ji<jcrit). The theoretical threshold ion current density jcrit is in very good agreement with the ion current density at which the slow two-fluid mode arises in the three-dimensional, particle-in-cell (3D PIC) simulations. This can be an important key to suppression of this mode, which is the main source of the beam divergence.

Crossed E×B-Fields Back-Bombardment Instability and Ion Cyclotron Heating of Gas-Discharge Plasma Streams

The article present our recent results of computer simulations and theory analysis concerning of dynamics of the crossed E×B-fields back-bombardment (BKB) instability in magnetron diodes. The concept of the BKB-instability applied to show new explanation for known experimental data of an intense ion heating in accelerated plasma streams. Main features of the BKB-instability into ion diodes are following: 1) ion magnetic insulation is d0 > ri where d0 is accelerating gap across applied magnetic field B0, ri = 5V00.5/B0 is proton Larmor radius, cm; V0 – applied voltage, kV; B0 – kG; 2) EA ≠ 0 is high electric field on self or secondary emission anode electrodes; 3) main frequency oscillation of fs = fci/2 = 0.76 B0 MHz; 4) convert power efficiency of ηB = P~/P0 is up to 50%.Radial proton HF-oscillations current density of the Jir provided by the BKB-instability excites magnetohydrodynamics waves. They are propagated in the tube plasma with azimuthal velocity v∼9 ~ 107 cm/s (Miv∼92 ~ eV0) and along the B0 with the vA ~ 4×107 cm/s -Alfven velocity. Known experimental data was demonstrated a level of the HF-oscillation power which was absorbed by proton streams by ion cyclotron heating equals up to 30% total supply power of the 1 MW.Work supported by RFFI under grant 96-02-19215.

The effect of electrode contamination, cleaning and conditioning on high-energy pulsed-power device performance

High-energy pulsed-power devices routinely use field strengths above those at which broad-area, cathode-initiated, HV vacuum-breakdown occur (>10/sup 7/ to 3/spl times/10/sup 7/ V/m). Examples include magnetically-insulated transmission lines and current convolutes, high-current-density electron and ion diodes, high-power microwave devices and cavities and other structures for electrostatic and RF accelerators. Energy deposited in anode surfaces may exceed anode plasma thermal-desorption creation thresholds on the time scale of the pulse. Stimulated desorption by electron or photon bombardment also can lead to plasma formation on electrode or insulator surfaces. Device performance is limited above these thresholds, particularly in pulse length and energy, by the formation and expansion of neutral and plasma layers formed primarily from electrode contaminants. In-situ conditioning techniques to modify and eliminate the contaminants through multiple HV pulses, low base pressures, RF discharge cleaning, heating, surface coatings and ion- and electron-beam surface treatment allow access to new regimes of performance through control of plasma formation and modification of the plasma properties. Experimental and theoretical progress from a variety of devices and small scale experiments with a variety of treatment methods are reviewed and recommendations given for future work.

Radiachromic film as a detector for intense MeV proton beams

A simple technique has been developed to image intense pulsed proton beams with radiachromic film and to measure their spatial distribution. The optical density (OD) of 50 μm thick film, sufficient to stop 1.5 MeV protons, is measured to infer the incident beam fluence. The OD increases nearly linearly with fluence up to 0.25 cal/cm2. This film is used to measure the uniformity of large-area beams as well as the detector in a multiple-pinhole camera to determine the source uniformity and divergence of applied-magnetic field and pinched-beam ion diodes.

Simulations of the plasma dynamics in high-current ion diodes

Our time-implicit fluid/Particle-In-Cell (PIC) code DYNAID [1]is applied to problems relevant for applied- B ion diode operation. We present simulations of the laser ion source, which will soon be employed on the SABRE accelerator at SNL, and of the dynamics of the anode source plasma in the applied electric and magnetic fields. DYNAID is still a test-bed for a higher-dimensional simulation code. Nevertheless, the code can already give new theoretical insight into the dynamics of plasmas in pulsed power devices.

Ion pressure induced bending of the virtual-cathode in multistage ion diodes

The electric force that accelerates ions in a magnetically insulated diode is transferred to the electrodes by the magnetic field. This force, which appears as an ion pressure term in the pressure balance equation, causes a bending of the virtual-cathode and consequently a lensing effect. Analytic calculations of the shape of the virtual-cathode resulting from this ion pressure are presented and a simple formula for the effective focal length resulting from the bending of the virtual-cathode is given. The focussing is a function of the accelerating voltage and the injected beam kinetic energy and current density. Thus a relationship between these parameters must be maintained to keep the beam on target.

A mechanism for ion acceleration near the anode of a magnetically insulated ion diode

We propose a theoretical model for the ion acceleration near the anode in the anode plasma of a magnetically insulated ion diode (MID). The applied diode voltage is screened by the anode plasma. We assume that as a result of the strong magnetization of the electrons and nonmagnetization of the ions in the anode plasma, a narrow space charge sheath is formed near the anode, of a width close to the electron Larmor radius. The potential drop that is formed across the sheath accelerates the ions born inside the sheath into the plasma. We solve a system of equations for the sheath and derive an approximate expression for the potential drop across the sheath. This expression exhibits the dependence of the ion energy on the magnetic field intensity and on the electron temperature and density. We also discuss an influence of the sheath width to the electron Larmor radius ratio as a parameter. The theoretical results are found to be in good agreement with the experimental measurements.

Observations of virtual cathode formation and electron density profile measurements in an intense ion beam diode

A recently developed high pulse rate intense ion beam diode has been exploited to measure the relative density profile of electrons in the ion diode’s acceleration gap as a function of time. This diode was magnetically insulated and it produced 100 keV Ar+ beams at up to 4 A/cm2 using an active anode plasma ion source. The diode also had a backfill of 10−2 Torr of He that was collisionally excited by energetic electrons in the acceleration gap. When the excited He atoms radiatively decayed, the emitted light was measured and used to deduce the relative electron density profile with space and time resolution. The profile was peaked toward the center of the gap and dropped off significantly toward both the anode and the cathode. The absence of electron density near the physical cathode is evidence for magnetic field profile modification from the diamagnetic drift of the electrons in the diode. With a macroscopic electric field of order 100 kV/cm, the electron current takes many tens of nanoseconds, from the moment the cathode begins to emit, to reach the Child–Langmuir current. The extent to which this limit is exceeded depends strongly on the profile of the insulating magnetic field and appears to be dominated by the dynamics of the electron flow in the diode gap. In contrast, the delay between when electrons are first observed in the gap spectroscopically and when the ion beam is first formed is independent of the magnetic field profile. This delay is about the time that it takes an argon ion to transit the acceleration gap, suggesting that ion inertia is the rate limiting factor.