An ion source utilizing a cathode and anode for producing an electric arc therebetween. The arc is sufficient to vaporize a portion of the cathode to form a plasma. The plasma leaves the generation region and expands through another regon. The density profile of the plasma may be flattened using a magnetic field formed within a vacuum chamber. Ions are extracted from the plasma to produce a high current broad on beam.

High current ion source

A high‐current ion source is described in which a metal vapor vacuum arc is used to create the plasma from which the ion beam is extracted. An ion beam current of up to about 1 A has been measured at an extraction voltage of 25 kV for a range of metallic species spanning the periodic table from lithium to uranium. Multiply ionized charge states are seen up to as high as Q=6+ in the case of uranium. In the metal vapor vacuum arc, regions of intense current concentration are formed at the cathode, and it is at these regions, the cathode spots, that the cathode material is vaporized and ionized to form a dense, quasi‐neutral plasma which plumes away from the cathode surface. The plasma plume is caused to impinge upon a set of extractor grids, and the ion beam is extracted from the streaming metal plasma. A preliminary survey of the performance of the source has been made. We describe here the principle of operation of the source, the source design, and the results of those measurements that we have made to date.

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Metal vapor vacuum arc ion source

The subject of this review is the field of intense pulsed ion beam generation and its potential application to fusion research Considerable progress has been made in the past six years Power levels of the order of 1000 MW/cm2 have been obtained for pulse-lengths from 10−8 to 10−6 s Light-ion beams with currents approaching 1 MA and energies exceeding 1 MeV have been produced The first half of the paper treats the physics and technology of beam generation Following a unified discussion of the theory of ion injectors, experimental work since 1973 is reviewed Diagnostic techniques appropriate to high-flux ion beams are summarized The general problem of high-current ion beam transport in vacuum and through plasmas is also considered The second part of the paper is devoted to applications to fusion research Applications in both magnetic confinement and inertial confinement fusion are considered Intense ion beams have a number of features which make them ideally suited as inertial fusion drivers Discussions are given of the physics of inertial fusion targets, options for ion beam production, experiments on light-ion beam focusing, and high-current multi-stage accelerators

https://iopscience.iop.org/article/10.1088/0029-5515/20/12/006/pdf

Intense pulsed ion beams for fusion applications

Diagnostic methods pertinent to the probing of intense pulsed ion beams are described. Special emphasis is given to the measurement of delayed radioactivity from nuclear reactions induced in a target by the ions. This nuclear activation analysis provides the most precise, unambiguous means of determining the number of ions per pulse that is presently available. A compilation of yields and cross sections for several nuclear reactions is given. Included among the nuclear techniques is the measurement of prompt radiation such as γ rays and neutrons from ion‐induced reactions. In addition, a scintillator–photodiode detector is described which has proven to be useful for the measurement of time of flight and which may be used to observe the relative pulse shape. Biased ion collectors and difficulties associated with their use are also discussed.

Diagnostics for intense pulsed ion beams

Inertial confinement fusion with ion beams requires the efficient delivery of high energy (≳1 MJ), high power (≳100 TW) ion beams to a small fusion target. The propagation and focusing of such beams is the subject of this paper. Fundamental constraints on ion beam propagation and focusing are discussed, and ion beam propagation modes are categorized. For light ion fusion (LIF), large currents (2–33 MA) of moderate energy (3–50 MeV) ions of low atomic number (1⩽A≲12) must be directed to a target of radius ≲1 cm. The development of pulsed power ion diodes for LIF is discussed, and the necessity for virtually complete charge neutralization during transport and focusing is emphasized. Fornear-term LIF experiments, the goal is to produce pellet ignition without the standoff needed for the ultimate reactor application. Ion diodes for use on Sandia National Laboratories Particle Beam Fusion Accelerators PBFA-I (2–4 MV, 1 MJ, 30 TW, operational) and PBFA-II (2–16 MV, 3.5 MJ, 100 TW, scheduled for operation in 1985) are discussed. Ion beam transport from these diodes to the pellet is examined in reference to the power brightness ℬ. While values of ℬ=2–5 TW/cm2/sr have been achieved to date, a value of ℬ≈100 TW/cm2/sr is needed for breakeven. Research is now directed toward increasing ℬ, and means already exist (e.g., scaling to higher voltages, enhanced ion diode current densities, and bunching), which indicate that the required goal should be attainable. Forfar-term LIF applications, the goal is to produce net energy gain with standoff suitable for a reactor. This may be achieved by ion beam transport in preformed, current-carrying plasma channels. Channel transport research is discussed, including experiments with wire-initiated, wall-initiated, and laser-initiated discharge channels, all of which have demonstrated transport with high efficiency (50–100%). Alternate approaches to LIF are also discussed, including comoving electron beam schemes and a neutralized beam scheme. For heavy ion fusion (HIF), moderate currents (∼10 kA) of high energy (∼10 GeV) ions of high atomic number (A≳200) must be directed to a target of radius ≲0.3 cm. Conventional accelerator drivers for HIF are noted. For a baseline HIF reactor system, the optimum transport mode for low charge state beams is ballistic transport in near vacuum (10−4–10−3 Torr lithium), although a host of other possibilities exists. Development of transport modes suitable for higher charge state HIF beams may ultimately result in more economical HIF accelerator schemes. Alternate approaches to HIF are also discussed which involve collective effects accelerators. The status of the various ion beam transport and focusing modes for LIF and HIF are summarized, and the directions of future research are indicated.

Ion beam propagation and focusing

A magnetically insulated diode has been used to produce cylindrically converging intense proton beams. By providing electron neutralization along field lines, the beams can be propagated across the magnetic field to within 1 cm of the axis. Proton currents up to 5 kA have been propagated to achieve current densities up to 70 A/cm2. Divergences less than 3° have been achieved with new plasma anode designs. Calculations are presented on the extrapolation of magnetic diodes to achieve power densities needed for ion‐beam pellet‐fusion breakeven.

S. Humphries

Papers

Extraction and focusing of intense ion beams from a magnetically insulated diode