V. Toivanen

Effect of electron cyclotron resonance ion source frequency tuning on ion beam intensity and quality at Department of Physics, University of Jyväskylä

Ion beam intensity and quality have a crucial effect on the operation efficiency of the accelerator facilities. This paper presents the investigations on the ion beam intensity and quality after the mass separation performed with the Department of Physics, University of Jyväskylä 14 GHz electron cyclotron resonance ion source by sweeping the microwave in the 14.05-14.13 GHz range. In many cases a clear variation in the ion beam intensity and quality as a function of the frequency was observed. The effect of frequency tuning increased with the charge state. In addition, clear changes in the beam structure seen with the beam viewer were observed. The results confirmed that frequency tuning can have a remarkable effect on ion beam intensity and quality especially in the case of highly charged ion beams. The examples presented here represent the typical charge state behavior observed during the measurements.

Plasma breakdown diagnostics with the biased disc of electron cyclotron resonance ion source

The electron cyclotron resonance ion sources at the JYFL (University of Jyvaskyla, Department of Physics) accelerator laboratory have been operated in pulsed mode to study the time-resolved current signal from the biased discs of the ion sources. The purpose of the experiments is to gain an understanding of the ion source parameters affecting the time required for the transition from neutral gas to plasma. It was observed that the plasma breakdown time depends strongly on the neutral gas density, gas species and density of seed electrons. In particular, it was observed that a low power microwave signal at secondary frequency makes the breakdown time virtually independent of the neutral gas density. The results can be utilized for operation of ECR ion sources in the so-called preglow mode. A simple qualitative model, which is in good agreement with the experiments, has been developed to interpret the results.

Studies of plasma breakdown and electron heating on a 14?GHz ECR ion source through measurement of plasma bremsstrahlung

Temporal evolution of plasma bremsstrahlung emitted by a 14?GHz electron cyclotron resonance ion source (ECRIS) operated in pulsed mode is presented in the energy range 1.5?400?keV with 100??s resolution. Such a high temporal resolution together with this energy range has never been measured before with an ECRIS. Data are presented as a function of microwave power, neutral gas pressure, magnetic field configuration and seed electron density. The saturation time of the bremsstrahlung count rate is almost independent of the photon energy up to 100?keV and exhibits similar characteristics with the neutral gas balance. The average photon energy during the plasma breakdown is significantly higher than that during the steady state and depends strongly on the density of seed electrons. The results are consistent with a theoretical model describing the evolution of the electron energy distribution function during the preglow transient.

Diagnostics of plasma decay and afterglow transient of an electron cyclotron resonance ion source

The electron cyclotron resonance ion sources at the JYFL (University of Jyvaskyla, Department of Physics) accelerator laboratory have been operated in pulsed mode to study the decay of bremsstrahlung emission and ion beam currents of different charge states. The purpose of the experiments is to gain understanding on the ion source parameters affecting the afterglow. It was observed that the bremsstrahlung emission characteristics during the afterglow and decay times of extracted ion beam currents are virtually independent of the ion source tuning parameters. The decay time of different charge states was found to be almost inversely proportional to the square of the ion charge. The result is in good agreement with a simple theoretical model based on diffusion of ions from the magnetic field of the ion source. It was observed that the plasma decay time is shorter in the case of the ion source with lower operation frequency and, thus, lower magnetic field strength. The scaling between the ion sources supports a model based on Bohm diffusion, arising from non-linear effects such as instabilities and fluctuating fields in turbulent plasma. The experiments provide information on the mechanisms causing instabilities during the plasma decay.

The role of seed electrons on the plasma breakdown and preglow of electron cyclotron resonance ion source.

The 14 GHz Electron Cyclotron Resonance Ion Source at University of Jyväskylä, Department of Physics (JYFL) has been operated in pulsed mode in order to study the plasma breakdown and preglow effect. It was observed that the plasma breakdown time and preglow characteristics are affected by seed electrons provided by a continuous low power microwave signal at secondary frequency. Sustaining low density plasma during the off-period of high power microwave pulses at the primary frequency shifts the charge state distribution of the preglow transient toward higher charge states. This could be exploited for applications requiring fast and efficient ionization of radioactive elements as proposed for the Beta Beam project within the EURISOL design study, for example. In this article we present results measured with helium and neon.

Ion beam development for the needs of the JYFL nuclear physics programme.

The increased requirements towards the use of higher ion beam intensities motivated us to initiate the project to improve the overall transmission of the K130 cyclotron facility. With the facility the transport efficiency decreases rapidly as a function of total beam intensity extracted from the JYFL ECR ion sources. According to statistics, the total transmission efficiency is of the order of 10% for low beam intensities (I(total)< or =0.7 mA) and only about 2% for high beam intensities (I(total)>1.5 mA). Requirements towards the use of new metal ion beams for the nuclear physics experiments have also increased. The miniature oven used for the production of metal ion beams at the JYFL is not able to reach the temperature needed for the requested metal ion beams. In order to fulfill these requirements intensive development work has been performed. An inductively and a resistively heated oven has successfully been developed and both are capable of reaching temperatures of about 2000 degrees C. In addition, sputtering technique has been tested. GEANT4 simulations have been started in order to better understand the processes involved with the bremsstrahlung, which gives an extra heat load to cryostat in the case of superconducting ECR ion source. Parallel with this work, a new advanced ECR heating simulation program has been developed. In this article we present the latest results of the above-mentioned projects.

The effect of rf pulse pattern on bremsstrahlung and ion current time evolution of an ECRIS.

Time-resolved helium ion production and bremsstrahlung emission from JYFL 14 GHz ECRIS is presented with different radio frequency pulse lengths. rf on times are varied from 5 to 50 ms and rf off times from 10 to 1000 ms between different measurement sets. It is observed that the plasma breakdown occurs a few milliseconds after launching the rf power into the plasma chamber, and in the beginning of the rf pulses a preglow transient is seen. During this transient the ion beam currents are increased by several factors compared to a steady state situation. By adjusting the rf pulse separation the maximum ion beam currents can be maintained during the so-called preglow regime while the amount of bremsstrahlung radiation is significantly decreased.