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![Emissivity vs. temperature as a function of the surface state for: (top left) alumina (Al 2 O 3 ), (top right) copper, (bottom left) stainless steel and (bottom right) tantalum [10,11].](profile/Federico-Carra/publication/325560055/figure/fig8/AS:638771678699535@1529306444812/Emissivity-vs-temperature-as-a-function-of-the-surface-state-for-top-left-alumina-Al.png)
Emissivity vs. temperature as a function of the surface state for: (top left) alumina (Al 2 O 3 ), (top right) copper, (bottom left) stainless steel and (bottom right) tantalum [10,11].
Source publication
The Linac3 ion source at CERN produces lead ion beams by the vaporization of solid samples inside the internal ovens and the consequent ionization of the evaporated material in the plasma. The geometry, materials and surface state of the oven elements are critical parameters influencing the oven temperature characteristics and consequently the evap...
Contexts in source publication
Context 1
... seen in Section 4, the heat flow and the temperature gradient in steady-state conditions of the problem under study depend on the thermal conductivity and the emissivity of the materials. These properties are temperature-dependent, and available in literature for all the materials adopted in the analysis [9][10][11]. The emissivity, on the other hand, is strictly related to the surface state of the radiating bodies [10]. Fig. 8 shows the emissivity values for alumina, copper, stainless steel and tantalum as a function of temperature and surface state. It is important to underline that in the numerical analysis the data is linearly extrapolated for the higher temperatures. It is evident that, in general, the surface state consistently influences the emissivity. Nevertheless, the surface state can be challenging to assess accurately considering that it usually changes with time. The metal parts of the oven are machined without applying a finishing polishing and are then operated at high temperatures in a residual gas atmosphere with always some low level oxygen residue. Therefore, the surface conditions of the materials are expected to be between the polished and oxidized limits. ...
Context 2
... the first case study (Case 1), the surface state was considered polished and cleaned for all the components. The emissivities used, extracted from Fig. 8, are reported in Table ...
Context 3
... heat transfer between the components was modelled imposing a perfect surface-to-surface radiation, i.e. the total amount of energy exchanged inside a defined enclosure. In this case, the perfect enclosure is the whole area inside the simplified external frame, where surface- to-surface radiation occurs between the main system elements. In such enclosure the net total radiation is zero. The emissivity was imposed to the materials as a non-linear function of the temperature, according to the data from literature (Fig. 8). The boundary conditions are summa- rized in Fig. 10. Finally, the convection contribution was neglected for the reasons mentioned in Section 4, while the conduction through the thermal interfaces was estimated according to (3) (see Section ...
Context 4
... conditions of the problem under study depend on the thermal conductivity and the emissivity of the materials. These properties are temperature-dependent, and available in literature for all the materials adopted in the analysis [9][10][11]. The emissivity, on the other hand, is strictly related to the surface state of the radiating bodies [10]. Fig. 8 shows the emissivity values for alumina, copper, stainless steel and tantalum as a function of temperature and surface state. It is important to underline that in the numerical analysis the data is linearly extrapolated for the higher temperatures. It is evident that, in general, the surface state consistently influences the ...
Context 5
... In this case, the perfect enclosure is the whole area inside the simplified external frame, where surface- to-surface radiation occurs between the main system elements. In such enclosure the net total radiation is zero. The emissivity was imposed to the materials as a non-linear function of the temperature, according to the data from literature (Fig. 8). The boundary conditions are summa- rized in Fig. 10. Finally, the convection contribution was neglected for the reasons mentioned in Section 4, while the conduction through the thermal interfaces was estimated according to (3) (see Section ...
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Citations
... When the gaseous element is not available, the most popular methods are the utilization of gaseous compounds [8], low or high-temperature oven techniques [9,10], and insertion techniques [11]. Among these technologies, low-temperature oven techniques are very commonly used to produce low-melting-point metal vapors in ECRIS [12,13]. For example, in 1989, 60 μA of Li 3+ were delivered by a 7.5 GHz ECRIS combined with a low-temperature oven in Karlsruhe [14]. ...
At Peking University (PKU), a compact 2.45 GHz microwave ion source was developed to obtain high-intensity Li ⁺ beam. To improve the generation efficiency of Li ⁺ ions, we developed a built-in lithium oven named surface ionization oven (SIO), which consists of a heating oven for lithium vapor production and a surface ionizer for the ionization of lithium neutrals. Consequently, the surface ionization and electron cyclotron resonance heating (ECRH) principles are combined within the compact microwave ion source. In this work, the structure and materials of the SIO are investigated systemically to lower thermal risk and increase ionization efficiency. The oven is heated by resistance heating. To minimize the heat loss, polished stainless steel shields are used to surround the oven. The surface ionizer is installed at the exit of the nozzle of the heating oven. It consists of six rhenium wires with the diameter of 0.25 mm inside the ionizer tube. Limited space near the microwave window, the dimension of SIO is ⊘ 13.5 mm × 40 mm. This source has been tested with only heating oven and SIO. With only heating oven, which means the absence of surface ionization, and a 200 eμA pulsed Li ⁺ ion beam was stably extracted from this ion source. When the surface ionizer and the heating oven work together, a pulsed Li ⁺ ion beam of 800 eμA is obtained, which increased by about 300%. The experiment proved that surface ionization could dramatically increase the discharge efficiency of lithium plasma in ECRIS.
... A dedicated study to understand the reasons for the necessity of the frequent tuning and the failure mechanisms of the oven was conducted, including thermal simulations, gas flow simulations and measurements at an offline test stand [3], [5], [6]. Trials at the test stand showed the main failure mechanism to be the formation of a lead oxide blockage starting from lead condensate on the outer oven cover (also in [7]). ...
The GTS-LHC 14.5 GHz ECR ion source provides the ion beam, which after acceleration in the ion injector complex, is injected into the LHC, as well as being sent to fixed target experiments at CERN. Two experiments have been performed on the source. The stainless steel plasma chamber has been sputter-coated with a rather thick layer of aluminium on the surface facing the plasma; and the lead micro-oven has been modified to avoid the build-up of lead-oxide on the oven outlet. Details of the changes will be given, and results of beam measurements will be shown, with particular attention on how the stability and oven-refill schedule is impacted by these changes.
The GTS-LHC ECR ion source (named after the Grenoble Test Source and the Large Hadron Collider) at CERN provides heavy ion beams for the chain of accelerators from Linac3 up to the LHC for high energy collision experiments and to the Super Proton Synchrotron for fixed target experiments. During the standard operation, the oven technique is used to evaporate lead into the source plasma to produce multiple charged lead ion beams. Intensity and stability are key parameters for the beam, and the operational experience is that some of the source instabilities can be linked to the oven performance. Over long operation periods of several weeks, the evaporation is not stable which makes the tuning of the oven unpredictable and nonreproducible. A dedicated test stand is used to study the oven performance and possible improvements independently of the source operation. It was observed that the measured evaporation rate of the oven can vary spontaneously in a wide range even when stable operating conditions are applied to the oven controls. Data collected at the test stand hint that these fluctuations are caused by temperature instabilities of the oven itself. Several ways to improve the oven stability were tested, including insulation changes and modifications of the oven crucible. Some of the most promising results regarding the stability of the evaporation will be presented in this paper.
