|Present status of UST_1 and planned improvements|
Abstract : Description of the status of the stellarator, present enhancements and near future improvements to obtain an acceptable quality stellarator.
* In May and June about 44 pulse were produced. A 30eV electron beam was confined after the upgrading to 2 batteries. Two poitns of one magnetic surface were obtained.
* The vacuum system has been out of order for ten days because the heater of the diffusion pump melted. It was repaired and a customised spare part has been design and made. A thermocouple has been added to the heater itself and a simple electronic OPAM board has been design and installed to warn of overheating with a buzzer. Previously only a thermocouple was installed at the external surface of the boiler. It is useless to protect the heater against water supply failure and lack of oil evaporation (overpressure, mistaken operation...)
* One or more small leaks remain on the vacuum vessel. The leaks + outgassing is 7 x 10-5Pa m3/s. One leak has been detected in one port. It should be easy to seal but the exact position is unknown because of the small size and the tendency to block
* In spite of the leaks the powerful diffusion pump reaches 3 x 10-2Pa.
* Some small glow discharges in Argon and Air were produced in June. The breakdown currents were calculated and compared with the real values, given reasonable accuracy.
Improvements in vacuum system
* Several experiences and improvements are related with the improvement of the vacuum system :
- Argon and Helium was introduced through a prepared leak (1 x 10-3Pa m3/s) and the electron beam was activated. It could help the detection of small leaks if the ray changes the colour. Strangely the beam remained blue. It will be analysed.
- A low cost RGA is searched in ebay but it might last some time.
- A set of NW50 pipes, clamps and bellow are been shipped to significantly increase the conductance from the diffusion pump to the VV. An even more accurate calculation of the ultimate pressure according to the leaks-outgassing-conductance has been done. The present Varian angle vacuum valve will be removed. A gate valve is searched but it will take long time. Till then the system will work without inlet valve.
* A visible spectroscope from ebay has been received and a handheld spectroscope is being shipped. The aim is to adapt a sensitive firewire camera or photodetector to record some main spectrum lines. In any case it can be used for visual inspection.
B0 magnetic field
* A third battery will be installed soon. A new set of provisional coils, perhaps with 3 turns of 6mm2 copper conductor will be installed. It will achieve 23mT. An extra battery will give 30mT at the magnetic axis. Second harmonic resonance at 2.45Hz starts to be possible at this level.
* The mean free path of 70eV electrons in Argon is 5.4m for the present pressure, so the increase in the magnetic field will also allow more turns of the e-beam even if the vacuum is not improved.
* The heating system is being calculated and tested. I am studying some basic data in RF (transmission, impendance matching, antennae). The first setback was the cut-off frequency of a waveguide small enough to pass through the port. Even the node TE10 cannot pass through 3.2cm circular hole which measures the port. A coaxial cable + antenna or maybe a high dielectric constant waveguide will be used. The is being calculated and some cables and connectors are being ordered. A microwave oven is used. The same system is utilized in the Korean Tokamk for pre-ionization and useful explanations are available. WEGA stellarator also uses the same frequence and phylosophy.
The planned heating system will be reported soon.
Date of publication 03-07-2006