O. Tarvainen

Measurements of bremsstrahlung production and x-ray cryostat heating in VENUS

The VENUS superconducting electron cyclotron resonance (ECR) ion source is designed to operate at 28 GHz with up to 10 kW of rf power. Most of this power is absorbed by the plasma electrons and then dumped onto the plasma chamber wall. The distribution of heating and bremsstrahlung production is highly nonuniform and reflects the geometry of the magnetic confinement fields. The nonuniform distribution of electron losses to the wall results in localized heating on the aluminum chamber walls, which can lead to burnout. In addition, part of the bremsstrahlung produced by the collision of the hot-electrons with the walls is absorbed by the cold mass of the superconducting magnet leading to an additional heat load in the cryostat in the order of several watts. Therefore a new plasma chamber has been installed that incorporates a high-Z tantalum shield to reduce the cryostat heating and enhance water cooling to minimize the chance of burnout. In order to better understand the heat load, the spectrum of the brem...

Electron cyclotron resonance ion source related development work for heavy-ion irradiation tests

The European Space Agency (ESA) uses the facilities at the Accelerator Laboratory (Department of Physics, University of Jyvaskyla: JYFL) for heavy-ion irradiation tests of electronic components. Electron cyclotron resonance ion source related development work has been carried out in order to meet the requirements set by the project. During the irradiation tests several beam changes are performed during the day. Therefore, the time needed for the beam changes has to be minimized. As a consequence, a beam cocktail having nearly the same m∕q ratio is used. This makes it possible a quick tuning of the cyclotron to select the required ion for the irradiation. In addition to this requirement, very high charge states for the heavy elements are needed to reach a penetration depth of 100μm in silicon. In this article we present some procedures to optimize the ion source operation. We also present results of the first three-frequency heating tests. The main frequency of 14GHz was fed from a klystron and both second...

A-PHOENIX, an electron cyclotron resonance ion source for the Spiral 2 facility

Spiral 2, the radioactive ion-beam facility of GANIL, will start its commissioning in 2009. After a brief recall of Spiral 2 beam requirements, emittance measurements of preliminary 1mA O6+ test beams, done with the PHOENIX source for Spiral 2, are shown and discussed. The 28GHz A-PHOENIX source, designed to better meet the Spiral 2 requirements, is presented and a progress report of its construction is proposed.

Effect of the gas mixing technique on the plasma potential and emittance of the JYFL 14 GHz electron cyclotron resonance ion source

The effect of the gas mixing technique on the plasma potential, energy spread, and emittance of ion beams extracted from the JYFL 14 GHz electron cyclotron resonance ion source has been studied under various gas mixing conditions. The plasma potential and energy spread of the ion beams were studied with a plasma potential instrument developed at the Department of Physics, University of Jyvaskyla (JYFL). With the instrument the effects of the gas mixing on different plasma parameters such as plasma potential and the energy distribution of the ions can be studied. The purpose of this work was to confirm that ion cooling can explain the beneficial effect of the gas mixing on the production of highly charged ion beams. This was done by measuring the ion-beam current as a function of a stopping voltage in conjunction with emittance measurements. It was observed that gas mixing affects the shape of the beam current decay curves measured with low charge-state ion beams indicating that the temperature and∕or the ...

A modified permanent magnet structure for a stronger multipole magnetic field

The performance of the electron cyclotron resonance ion source (ECRIS) increases proportionally to the microwave frequency squared. This behavior encourages the use of higher microwave frequencies. However, a higher frequency would require a stronger magnetic field for the efficient operation of ECRIS. A rather complicated magnetic field configuration results from the combination of solenoids for the axial confinement and a multipolar radial field usually provided by permanent magnets. These fields produce the so-called B-minimum structure which is required for a stable and efficient operation of ECRIS. The highest multipole field achieved so far in an ECR ion source by using permanent magnets is about 1.3 T. This makes the efficient operation at a microwave frequency of about 18 GHz possible. We introduce here a new approach to further increase the magnetic multipole field provided by permanent magnets. According to our two-dimensional (2D) simulations, a remarkable improvement in the radial magnetic fie...

Optimization of the Halbach-type magnetic multipole for an electron cyclotron resonance ion source

An efficient electron cyclotron resonance ion source requires microwave frequency as high as possible. At the same time, the magnetic field has to meet experimentally found rules. The magnetic field configuration required for efficient operation is provided by a combination of solenoid and multipole fields. A so-called Halbach array is used to produce as high a magnetic field as possible using permanent magnets. In this work the above-mentioned structure has been studied in detail in order to maximize the strength of the multipole field. Simulations showed that the so-called offset structure, described in this article, gives stronger fields at magnetic poles. As one result of this work, simple equations were developed to easily calculate the value of the magnetic field for different size hexapolar Halbach structures.

Effect of broadband microwave radiation on the performance of a conventional B-minimum geometry electron cyclotron resonance ion source

The performances of electron cyclotron resonance ion sources can be enhanced by increasing the physical sizes or numbers of resonant zones embedded within their plasma volumes. Broadband rf power offers a simple, cost effective alternative for increasing the sizes of electron cyclotron resonance (ECR) zones in conventional minimum-B geometry sources over independently powered narrow bandwidth, multiple discrete frequency schemes. In this report, the charge-state enhancing effects of broadband microwave radiation are first demonstrated by comparing the high charge states of Ar ion beams produced by powering a conventional minimum-B geometry, 6.4 GHz ECR ion source with broadband microwave radiation {200 MHz [full width at half maximum (FWHM)]} with those produced by conventional bandwidth [∼1.5MHz (FWHM)] radiation. The results of these studies show that high-charge-state beams (e.g., Ar11+) can be enhanced by factors >2 with broadband microwave radiation over those powered with narrow bandwidth radiation ...

Emittance and plasma potential measurements in double-frequency heating mode with the 14GHz electron cyclotron resonance ion source at the university of Jyvýskylý

The performance of electron cyclotron resonance ion sources can be improved through the use of multiple-frequency heating. However, the physical processes leading into enhanced production of highly charged ions are still mainly unknown. This gave us a strong motivation to perform a set of emittance and plasma potential measurements with the 14GHz electron cyclotron resonance ion source at the university of Jyvýskylý to compare the results obtained in single- and double-frequency heating modes. The measurements were performed with different microwave frequencies and combinations of primary and secondary powers. It was observed that both the emittance of different ion beams and the plasma potential decreased in the single-frequency heating mode as the microwave frequency was increased. The emittance of highly charged ion beams and the plasma potential was slightly lower in double-frequency heating mode than in single-frequency mode with the same source settings and total power.

Effect of Ion Escape Velocity and Conversion Surface Material on H− Production

According to generally accepted models surface production of negative ions depends on ion escape velocity and work function of the surface. We have conducted an experimental study addressing the role of the ion escape velocity on H{sup -} production. A converter-type ion source at Los Alamos Neutron Science Center was employed for the experiment. The ion escape velocity was changed by varying the bias voltage of the converter electrode. It was observed that due to enhanced stripping of H{sup -} no direct gain of extracted beam current can be achieved by increasing the converter voltage. At the same time the conversion efficiency of H{sup -} was observed to vary with converter voltage and follow the existing theories in qualitative manner. We discuss the role of surface material on H{sup -} formation probability and present calculations predicting relative H{sup -} yields from different cesiated surfaces. These calculations are compared with experimental observations from different types of H{sup -} ion sources. The effects caused by varying cesium coverage are also discussed. Finally, we present a novel idea of utilizing materials exhibiting so-called negative electron affinity in H{sup -}/D{sup -} production under UV-light exposure.

H− Ion Source Development for the LANSCE Accelerator Systems

Employment of H‐ ion sources for the LANSCE accelerator systems goes back about 20 years, to the construction of the Proton Storage Ring (PSR). The standard ion source consists of a filament driven multi‐cusp discharge vessel with a biased converter electrode for negative‐ion production and an 80‐kV extraction system feeding into a 670‐kV electrostatic pre‐accelerator. The source typically delivers 18 mA pulsed beam current into the pre‐accelerator column and reaches up to 35 days between services at 60 Hz pulse repetition rate. Recent development efforts with this source have been dedicated to improved filament material, improved cesium oven geometry and operating the source at elevated temperatures. A second line of development focuses on filament‐less devices driven by a helicon discharge. Performance data obtained with the standard source as well as key results for the helicon experiments are given in this paper; the helicon work is discussed in a separate paper in much greater detail.