Correlation of bremsstrahlung and energy distribution of escaping electrons to study the dynamics of magnetically confined plasma

Bhaskar, B. S.; Koivisto, H.; Tarvainen, O.; Thuillier, T.; Toivanen, V.; Kalvas, Taneli; Izotov, I. V.; Skalyga, V. A.; Kronholm, Risto; Marttinen, M.

doi:10.1088/1361-6587/ac156a

“…These are in turn affected by the plasma density, power absorption and electron confinement in an inherently complex manner. It has been shown experimentally 23,24 that adjusting B ext by 10% (similar to the experiment reported here) has a significant impact on the electron flux escaping the ion source through the extraction aperture. This is arguably due to the mirror ratio R ¼ Bmax Bmin of the magnetic configuration affecting the escape probability p of electrons as…”

Section: Discussionsupporting

confidence: 79%

“…Þ-phase space is the extraction field. It has been shown that the electron flux escaping through the extraction aperture (mirror) of an ECRIS can be increased by more than a factor of two by decreasing the extraction field B ext by 15%, 23,24 which suggests that the total electron loss rate and/or the distribution of electron losses can be controlled by adjusting the weakest mirror. Based on the field topology and experimental evidence on electron losses it is argued that the extraction field could presumably affect the instability threshold, i.e., the exact B min =B ECR -ratio at which instabilities appear.…”

Section: Introductionmentioning

confidence: 99%

Influence of axial mirror ratios on the kinetic instability threshold in electron cyclotron resonance ion source plasma

Toivanen

¹

,

Bhaskar

²

,

Koivisto

³

et al. 2022

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Electron Cyclotron Resonance (ECR) ion source plasmas are prone to kinetic instabilities. The onset of the instabilities manifests as emission of microwaves, bursts of electrons expelled from the plasma volume, and the collapse of the extracted highly charged ion (HCI) currents. Consequently, the instabilities limit the HCI performance of ECR ion sources by limiting the parameter space available for ion source optimization. Previous studies have shown that the transition from stable to unstable plasma regime is strongly influenced by the magnetic field structure, especially the minimum field value inside the magnetic trap (B min ). This work focuses to study the role of the magnetic confinement on the onset of the kinetic instabilities by probing the influence of the injection and extraction mirror field variation on the instability threshold. The experiments have been performed with a room-temperature 14.5 GHz ECR ion source with an axially movable middle coil that provides flexible control over the axial field profile and especially the B min , which was used to quantify the variation in the instability threshold. The experimental results show that variation of the extraction field B ext , which defines the weakest magnetic mirror, correlates systematically with the variation of the instability threshold; decreasing the B ext allows higher threshold B min . The result demonstrates the importance of electron confinement and losses on the plasma stability. The connection between the weakest mirror field and the onset of instabilities is discussed taking into account the variation of magnetic field gradient and resonance plasma volume.

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“…in the frequency range f < 14 GHz, both the emissions start to grow nonlinearly. It is worth mentioning both low (detected by SDD) and high (detected by HPGe) energy fluxes are enhanced by B increase (up to now only the high-energy part has been observed, for example in [11]), even though the HPGe detected flux grows more rapidly than the SDD one.…”

Section: Frequency Scan: Stable and Unstable Regime Characterization ...mentioning

confidence: 97%

“…The underlying hypothesis of this paper is thus that, if such a regime is established, we should observe an increasing amount of x-ray radiation coming from the plasma chamber walls (due to the lost electrons) compared with the plasma emission. Past papers have tried to investigate in more detail the evolution of x-ray fluxes from ECR ion sources in turbulent regimes, such as the interesting results reported in [11]. In these papers, the authors have underlined the difficulty of separating the actual plasma x-ray emission from the bremsstrahlung radiation coming from the electrons impacting the chamber walls.…”

Section: Introductionmentioning

confidence: 99%

Experimental study of single- vs two-close-frequency heating impact on confinement and loss dynamics in ECR ion source plasmas by means of x-ray spectroscopy and imaging

Naselli¹,

Rácz²,

Biri³

et al. 2022

Plasma Phys. Control. Fusion

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We hereby report the study of confinement and electron losses dynamics in the magnetic trap of an Electron Cyclotron Resonance Ion Source (ECRIS) using a special multi-diagnostic setup that has allowed the simultaneous collection of plasma radio-self-emission and X-ray images in the range 500 eV - 20 keV. Argon plasmas were generated in single and two close frequency heating (TCFH) modes. Evidences of turbulent regimes have been found: for stable and unstable configurations quantitative characterizations of the plasma radio self-emission have been carried out, then compared with local measurement of plasma energy content evaluated by X-ray imaging. This imaging method is the only one able to clearly separate X-ray radiation coming from the plasma from the one coming from the plasma chamber walls. X-ray imaging has been also supported and benchmarked by volumetric spectroscopy performed via SDD and HPGe detectors. The obtained results in terms of X-ray intensity signal coming from the plasma core and from the plasma chamber walls have permitted to estimate the average ratio: plasma vs. walls (i.e., plasma losses) as a function of input RF power and pumping wave frequency, showing an evident increase (above the experimental errors) of the intensity in the 2-20 keV energy range due to the plasma losses in case of unstable plasma. This ratio was well correlated with the strength of the instabilities, in single frequency heating (SFH) operation mode; in TCFH mode, under specific power balance conditions and frequency combinations, it was possible to damp the instabilities, thus the plasma losses were observed to decrease and a general reconfiguration of the spatial plasma structure occurred (the X-ray emission was more concentrated in the center of the plasma chamber). In the end, a simplified model has been used to simulate electron heating under different pumping frequencies, discussing the impact of velocity anisotropy vs. the onset of the instability, and the mechanism of particles diffusion in the velocity space in stable and unstable regimes.

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“…Refs. [24,26]), (iii) the magnetic field minimum B min as it affects the tail of the EED and the occurrence of kinetic plasma instabilities [27][28][29], and (iv) the extraction magnetic field B ext , which allegedly affects the trapping of the hot electrons and the global plasma confinement [30]. For each parameter sweep, a 500 −900 nA K + beam was produced with the 1+ ion source.…”

Section: Experimental Setup and Proceduresmentioning

confidence: 99%

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

Angot

¹

,

Luntinen

²

,

Kalvas

³

et al. 2020

Plasma Sources Sci. Technol.

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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.

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Correlation of bremsstrahlung and energy distribution of escaping electrons to study the dynamics of magnetically confined plasma

Cited by 10 publications

References 60 publications

Influence of axial mirror ratios on the kinetic instability threshold in electron cyclotron resonance ion source plasma

Influence of axial mirror ratios on the kinetic instability threshold in electron cyclotron resonance ion source plasma

Experimental study of single- vs two-close-frequency heating impact on confinement and loss dynamics in ECR ion source plasmas by means of x-ray spectroscopy and imaging

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

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