T. Weis

Matching of a cylindrical ion beam to a periodic quadrupole channel

Abstract A procedure to match a cylindrically symmetric ion beam to a periodic electrostatic quadrupole channel is described. He + beams are extracted from a plasma source with a voltage of 40 kV. Behind the ion source the beam current and emittance are measured. The extracted beam current is variable from 1 to 20 mA, while the measured emittance is 10–20 π mm mrad. For a chosen zero-current phase advance per cell μ 0 in the focusing channel the so-called periodic solution is calculated for the measured current and emittance. μ 0 is varied between 43° and 114°. The model, which is used for the calculations, is based on the Kapchinsky-Vladimir-sky equations. The quadrupole lens forces are calculated separately by solving the Laplace equation in 2 dimensions. Furthermore, the voltages on the first five quadrupole singlet lenses, which adapt the extracted beam to the periodic solution, are determined. The beam current and emittance are again measured after the sixth quadrupole singlet lens, which is the first lens of the periodic channel. These measured values are compared with the calculated beam parameters. A reasonable agreement is found.

Multichannel acceleration of intense, low-energy ion beams

We present measurements on multichannel acceleration of intense, low‐energy ion beams with a multiple electrostatic quadrupole array linear accelerator. The beam properties are investigated for different transverse and longitudinal focusing strengths, which can be adjusted independently in the accelerator. A maximum He+ ion current of four times 2 mA has been accelerated from 40 to 115 keV with an acceleration efficiency of 50%. Measurements and computer simulations suggest that the current is limited by transverse fields and by mismatch and misalignment of the beams, whereas longitudinal fields are of minor importance. The energy spread in the bunches is roughly three times the gap voltage.

A study of the electromagnetic properties of a resonator for multichannel acceleration of light ions

Abstract We present an experimental study of a modified Interdigital-H-resonator which can be used for multichannel acceleration of light ions. The distribution of the electromagnetic fields is investigated with a perturbation method. Furthermore, measurements of the resonance frequency, resonator parallel resistance and quality factor are discussed. The measurements have been done on three types of accelerating structures, which have a different capacitance per unit length. The results are compared with an elementary model, in which the resonator is described in terms of discrete circuit elements. It is found that the resonance frequency can be calculated with good accuracy. For the other parameters scaling laws are derived.

Bunching of intense, low-energy ion beams in a periodic focusing quadrupole channel

Abstract Presented are investigations on bunching of intense, low-energy ion beams in a periodic focusing channel, which is part of a novel type of multichannel accelerator. Analytic models are derived to describe the bunching process in the presence and in the absence of space charge. These models show a good qualitative agreement with multiparticle simulations and with measurements.

Design study of high energy, high current rf accelerators for ion implantation

Abstract Ion implantation is of great importance in semiconductor device fabrication. Due to the increasing interest of the microelectronic industry in the implantation of ions in the MeV energy range, high energy beams are required. Furthermore, for several applications the implanted dose is as high as some 1018 ions/cm2, which implies that high currents are needed also. For both requirements the rf linac is well suited. The presented design studies are based on new linac structures (RFQ, MEQALAC, Spiral Loaded Cavity), which fulfill the specific demands of various applications. The discussed systems cover a current range from 1 to 150 mA and an energy range from 0.3 to 6 MeV for ion masses between 10 and 133.

Simultaneous transport of N+ and N2+ ions through a periodic focusing quadrupole channel

Abstract The transport of a mixed beam containing N+ and N2+ ions through a periodic focusing electrostatic quadrupole structure is investigated for 20 and 40 keV beam energy and for a wide range of the zero-current phase advance per cell. The current limit for a mixed beam has been derived by means of the Kapchinski-Vladimirski equations and the matrix formalism. Both the theory and experimental results show that ion beams which contain various ion species are transported in essentially the same way as ion beams which contain only one ion species. The dependence of the current limit on the ion mass and energy, as predicted by the theory, is encountered experimentally. For high injected currents the maximum transmission, which is 85%, is reached for a phase advance per cell of 60°–84°, independent of the particle mass and energy.

Comparison of two types of low-ß RF linacs: MEQALAC and RFQ

Abstract The current limit and the power consumption of a Multiple Electrostatic Quadrupole Array Linear Accelerator (MEQALAC) and a Radio Frequency Quadrupole (RFQ) accelerator are compared theoretically. Design studies are presented for 1 MeV, 24 mA N + accelerators for ion-implantation purposes and for 1.5 MeV, 100 mA D − accelerators. The latter are specified to be used for fusion plasma diagnostics.

MEQALAC: A 1-MeV multichannel rf-accelerator for light ions

Abstract The MEQALAC (Multiple Electrostatic Quadrupole Array Linear Accelerator) project deals with multichannel acceleration of light ions in a resonator cavity. An advantage of the MEQALAC over the RFQ is its higher acceleration efficiency. The total accelerated current can be increased via an increase of the number of beams. The space charge dominated beams are focused by arrays of miniaturized electrostatic quadrupoles; the channel radius is 2.5 mm. In a proof-of-principle experiment we accelerated four He+ ion beams of 2.2 mA per channel from 40 to 120 keV. The objective for the next stage which presently is under construction is to accelerate four N+ ion beams from 40 keV to 1 MeV. With the same acceleration structure both the ion mass and the exit energy can be varied up to a factor of 2 via a variation of the resonator inductance. Performance tests on various components are presented.

Tailoring of the RF properties of a multichannel accelerator for MeV energies

Abstract Measurements on the rf properties of a modified Interdigital-H-resonator equipped with a multichannel, low-β acceleration structure (MEQALAC) are presented. They are compared with model calculations. The standard description of such a capacitively loaded resonator by a lumped-circuit model is improved by a more extended model. In this model the resonator is treated as a series of inductively coupled circuits containing the components of the separate resonator cells. In this way, the local properties of the resonator are taken into account, so that the functional dependence of the voltage distribution over the rf gaps on the resonator geometry can be found. The results are reasonably consistent with measurements.

Space-charge-limited current in a miniaturized quadrupole channel

We present measurements on the transport of low‐energy ions through a miniaturized electrostatic quadrupole channel. These are done at a zero‐current phase advance per cell of 60°. The limiting current is 50% smaller than is calculated with the Kapchinsky–Vladimirsky equations. This reduction is attributed to mismatch and misalignment, which have a pronounced influence in a miniaturized channel.