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

Abstract

A new method for determining plasma parameters from beam current transients resulting from short pulse 1+ injection of metal ions into a charge breeder electron cyclotron resonance ion source has been developed. The proposed method relies on few assumptions, and yields local values for the ionisation times 1 / n e σ v q → q + 1 inz , charge exchange times 1 / n 0 σ v q → q − 1 cx , the ion confinement times τ q , as well as estimates for the minimum plasma energy contents n e E e and the plasma triple products n e E e τ q . The method is based on fitting the current balance equation on the extracted beam currents of high charge state ions, and using the fitting coefficients to determine the postdictions for the plasma parameters via an optimisation routine. The method has been applied for the charge breeding of injected K+ ions in helium plasma. It is shown that the confinement times of K q+ charge states range from 2. 6 − 0.4 + 0.8 ms to 16. 4 − 6.8 + 18.3 ms increasing with the charge state. The ionisation and charge exchange times for the high charge state ions are 2. 6 − 0.5 + 0.5 ms–12. 6 − 3.2 + 2.6 ms and 3. 7 − 1.6 + 5.0 ms–357. 7 − 242.4 + 406.7 ms, respectively. The plasma energy content is found to be 2 . 5 − 1.8 + 4.3 × 1 0 15 eV cm−3.

Measurement of ionization, charge exchange and ion confinement times in charge breeder ECR ion sources with short pulse 1+ injection of metal ions

“…The optimum efficiency of K 10+ charge state measured in June 2020 was 10.6 % with τ 10+ CB of 132 ms. The first data using the CT-method to obtain the plasma parameters were taken in this configuration [1] with He support plasma.…”

“…The method for analyzing the measured beam current transients of the q+ ions has been extensively described in Ref. [1] and is only briefly recapitulated here: The balance equations [2,3] governing the evolution in time of the ion population densities are defined by dn q dt = +n e σv inz q−1→q n q−1 − n e σv inz q→q+1 n q…”

“…While the numerical method has remained unchanged in principle since the introduction of the method [1], the code has been updated improving the precision and speed of the algorithm: (i) The rate coefficients are now calculated as a function of the average energy E e of the EED (not T e ), which facilitates the comparison between different EEDs in the future. (ii) In Ref.…”

“…(ii) In Ref. [1] we adopted an "umbrella" uncertainty bound of 60 % for the rate coefficient as reported by Voronov [11]. The code now uses the experimental uncertainties of the cross section data [12].…”

“…The Consecutive Transients (CT) method has been used to obtain postdictions for the ionization, charge exchange and confinement times (τ q inz , τ q cx and τ q ) of charge state q ion populations in a CB-ECRIS plasma [1]. The method is based on measuring the extracted current transients prompted by short pulse injection of metal ions into the plasma, making fits to the transients of (minimum) five consecutive charge states, and an optimisation procedure to obtain the plasma electron density n e and average energy E e of the presumed Electron Energy Distribution (EED) as well as the times τ q inz , τ q cx and τ q .…”