Charge breeding time investigations of electron cyclotron resonance charge breeders

Abstract

To qualify electron cyclotron resonance charge breeders, the method that is traditionally used to evaluate the charge breeding time consists in generating a rising edge of the injected beam current and measuring the time in which the extracted multicharged ion beam reaches 90% of its final current. It is demonstrated in the present paper that charge breeding times can be more accurately measured by injecting short pulses of 1 þ ions and recording the time resolved responses of Nþ ions. This method is used to probe the effect of the 1 þ ion accumulation in the plasma known to disturb the buffer gas plasma equilibrium and is a step further in understanding the large discrepancies reported in charge breeding times. The experiments are conducted injecting a 85 Rb þ ion beam into the Laboratoire de Physique Subatomique et de Cosmologie (LPSC) charge breeder operated with helium as a buffer gas. The time needed for the extraction of 90% of the multicharged ions in short pulse mode is found slightly shorter (9%) than the charge breeding time measured with the traditional method. The charge breeding efficiency is identical with both methods. The pulse width and amplitude of the 1þ injection pulses have been varied to study their influence on the Nþ response and temporal parameters are proposed to qualify the time response. The short pulse method can be used to study the influence of the ion source tuning parameters on the charge breeding temporal characteristics. For example, it is shown here that an increase of the minimum magnetic field strength of the LPSC charge breeder in the range 0.432-0.444 T improves the multicharged ion confinement in the electron cyclotron resonance plasma, and so increases the charge breeding efficiency. The short pulse method is also used to estimate the charge breeding efficiencies of radioactive Rb isotopes taking into account their half-lives and charge states. The neutron-rich heavy isotopes have short half-lives, which makes the charge state distributions shift to lower charge states and the estimated charge breeding efficiencies of high charge states being close to three times less in comparison to the stable ions.

Plasma diagnostic tools for ECR ion sources—What can we learn from these experiments for the next generation sources

“…These measurements together allow quantifying the expected charge breeding efficiency of the radioactive isotopes with varying half-lives from the efficiency measured for stable isotopes of the same element. 37 The comparison of ion confinement times in conventional and charge breeder ECR ion sources yielding similar results 23 implies that the discharge properties are comparable and, thus, supports the use of stable ion beams in the development of sources for radioactive beams where optimizing the ion confinement time is important to first produce the high charge state ions by electron impact ionization and then extract them before a significant loss of ions through radioactive decay. This is further motivated by the similarity of the simultaneously measured charge breeding times and cumulative confinement times.…”

“…The difference can be explained by the fact that the first method probes the confinement of the escaping ion population, whereas the second one must be interpreted as a cumulative confinement time measuring the total residence time of a particle extracted at the charge state q. Two experimental observations made with charge breeder ECRISs support using the latter definition: (i) experiments with a short pulse injection of 1+ ions confirm that the cumulative confinement time of ions is indeed measured in tens of milliseconds 37 and (ii) there is a strong correlation between the confinement time deduced from the beam current decay transient and the extracted beam current, 23 which directly supports the validity of the last term of Eq. (8).…”

Method for estimating charge breeder ECR ion source plasma parameters with short pulse 1+ injection of metal ions

“…Short 1+ pulses with a width of 5 ms, corresponding to 2.2 × 10 10 particles per pulse, were injected using a square signal to drive the pulsing system. The 5 ms duration was chosen to obtain N+ pulse responses with a good signal-to-noise ratio, without accumulation effect [7]. The repetition rate was carefully tuned to leave N+ pulse responses enough time to recover between consecutive pulses.…”

“…The performance improvements of these installations are dependent in part on the ECRIS development, which is 1 arXiv:2103.03546v1 [physics.plasm-ph] 5 Mar 2021 driven by empirical laws obtained from decades of research and development. Despite recent studies using optical spectroscopy methods [4], [5] and 1+/n+ diagnostics [6], [7], understanding of the fundamental ECRIS plasma behavior, such as the ion confinement, the electron energy distribution or the plasma electrostatic potential distribution is still imprecise.…”

“…Several techniques are used for pulsed material injection, e.g. laser ablation [11], fast gas injection [12] sputtering [12]- [14], and 1+ injection [7]. A 0D code is then generally used to reproduce the measured beam currents by numerically optimising the plasma parameters involved in a system of differential equations formed by physical models describing the plasma [10], [15], [16].…”