Multidiagnostics investigation of the role of the magnetic field profile in a simple mirror trap

Abstract

The impact of the magnetic field profile on the operations of plasma-based Electron Cyclotron Resonance Ion Sources (ECRIS) has been debated and investigated for a long time, and a summary is given by the collection of the so-called "magnetic scaling laws", i.e., some rules-of-thumb providing the optimal configuration of the axial and radial confining magnetic fields. Anyway, the optimal configurations have been found just looking at the output beam currents and charge states. Only recently, more advanced experiments have shown that the optimal performances are related to some "islands" in the parameters space leading to a dense, energetic but stable plasma. A general, systematic characterization of plasma density and temperature as a function of the magnetic scaling, and for different energy domains (i.e., for cold, warm, and hot electrons) is not yet available. This paper presents a multidiagnostics characterization of a hydrogen plasma heated by microwaves at around 6.8 GHz via Electron Cyclotron Resonance, and confined in an axisymmetric simple mirror magnetic trap. Investigation of plasma response to solely the axial field profile gives the unique opportunity to decouple the effect of simple mirror field from the radial components that are, usually, provided by a hexapole. Optical emission spectroscopy, Langmuir probe, and an x-ray silicon drift detector were simultaneously used to evaluate the plasma density and temperature in different energy domains, thus exploring the electron energy distribution function from a few eV to hundreds of keV. Plasma parameters have been measured as a function of the magnetic field profile (in particular by varying the ratio B min =B ECR), of the microwave power and of the neutral pressure. Furthermore, for the same scaling, the relative abundances of H atoms and H 2 have been estimated. Data show smooth trends of electron density and temperatures versus microwave power and neutral pressure, but strong and even nonlinear responses as a function of the magnetic field profile variation, allowing us to connect semiempirical scaling laws to changes in plasma parameters.

Charge breeding at GANIL: Improvements, results, and comparison with the other facilities

“…Let us remind the main modifications done and the outcomes of these experiments. On the SP1-CB, the injection part has been significantly modified by adding a second RF port to inject second RF heating wave within a broad frequency range (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18); the injection iron plug has been symmetrized; a new gas injection with an outlet close to the ECR plasma has been added; the waveguides orientation has been changed to target the core of the ECR plasma. The majority of the o-Rings have been replaced by metallic gaskets and the plasma chamber is made of aluminum.…”

“…The objective was to study and find a set of SP1-CB parameters which minimizes the charge breeding time while maximizing the charge breeding efficiency; that set matches the best RIB production environments to maximize the yields on the physicist's target regarding the SP1-CB device [14]. to be at the edge of the plasma stability / instability region and to lead to the production of highly charged ions [15].…”

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

“…These studies can be characterised based on whether or not the obtained results provide spatially and/or temporally resolved information, as well as based on the origin of the measured bremsstrahlung (plasma emission, thick target radiation induced by escaping electrons). Techniques resulting to volume integrated and time averaged results have been used most widely to probe the general plasma properties [6][7][8][9][10][11][12][13][14][15][16], but there has been also significant development in the use of spatially [17,18] and temporally [19] resolved methods. However, unambiguous de-convolution of either thick-target (wall) or plasma bremsstrahlung spectra to obtain the electron energy distribution (EED) of the escaping or confined electrons is seemingly impossible despite some notable efforts [13,20,21].…”

Time resolved X-ray emission diagnostics in an axis-symmetric simple mirror trap

“…Typical operating frequencies are in the range 4-7 GHz. It has been experimentally demonstrated that, when FPT is operated in simple mirror magnetic configuration, there exists a threshold value of the B min /B ECR ratio beyond which X-ray emission is increased by orders of magnitude independently of the operative frequency [6,7]. Moreover, several experiments have shown that, when B min /B ECR > 0.75, CM instabilities can be revealed [3,8].…”