Annual report 2001
This year the H-1NF was operated for 93 days over 32 weeks, recording 44 Gigabytes of data over 4,300 shots. Of this, approximately 3,400 shots over 78 days of operation were plasma physics shots, the balance being power supply and machine test shots. The high-precision 12 Megawatt magnet dual power supply will ultimately increase the magnetic field of the H-1 NF device from its original operating value of 0.2T to its design value of 1T. This year, a program of tests at 0.5T, with very sensitive magnetic diagnostics, have demonstrated control to a small fraction of one Ampere, and extended the sensitivity of measurements to equivalent currents about one order lower. The high precision of the power supply ensures highly accurate magnetic geometry, avoids interference with measurement systems, and minimises induction of current into this inherently current-free plasma configuration.
The flexibility of the H-1 heliac is obtained though a secondary supply which powers the control windings and allows the plasma shape to be varied, under computer control, over a much wider range than possible in conventional stellarators or tokamaks. This year, by using the new power supplies, the flexibility of magnetic configurations was explored over a much wider range than ever before, in much more detail and much more straightforwardly. Initial results show an optimum configuration for plasma density at about 20% higher vertical field than the original design. The effect of magnetic configuration changes on magnetic fluctuations is being explored as part of the PhD thesis work of a new student, Mr. David Pretty.
Regular plasma operation at 0.5 Tesla (~80O pulses) provided the highest densities in hydrogen and helium so far obtained in H-1NF - more than 10 times higher than before ( > The best results were with RF heating near the ion cyclotron frequency (7MHz) in hydrogen and a hydrogen-helium mixture. As well as being of intrinsic interest, this provides a target plasma for the first phase of high temperature plasma operation in which the plasma is heated at the second harmonic of the electron cyclotron resonance frequency. This year, the 200kW electron cyclotron heating system was successfully recommissioned and thoroughly tested, as part of the collaboration with Kyoto University and the Japanese National Institute for Fusion Science (NIFS). After some initial problems with the high-voltage oil tank, the gyrotron was successfully operated very close to full ratings in power and pulse length. Work on the ion cyclotron range heating system included installation of DC isolation components, and cabling with high power coaxial cable to the launching port and some pulse control electronics. A full remote control system will be installed next year. Both the electron heating system and the new ion cyclotron heating system are now ready for first high-temperature plasma experiments.
A cryopump, which allows rapid pump down after a vacuum break, was commissioned in early 2001, producing an order of magnitude improvement in vacuum quality in normal operation. This pump is designed to have a high capacity to pump water vapour, and uses a simple, large-area cooling coil instead of critical activated surfaces which can be easily contaminated. In addition to the ability to pump down quickly after breaks to install new equipment or collaborators experiments, the Facility now has a degree of redundancy in power, heating and vacuum systems. This has provided a high level of availability this year, and facilitated the remaining upgrades to the heating and launching systems. The final step, that is bringing the magnetic field up to full strength (1 Tesla), is expected to take less than three months.
A number of new plasma measurement systems have been installed or commissioned. The Modulated Optical Solid State (MOSS) camera has been operating routinely since early 2000 and has produced a wealth of new information pertaining to the H-1 heliac plasma dynamics. After some early difficulties, the tomographic MOSS (TOMOSS) spectroscopy system was fully installed and commissioned in September 2000. A new multi-channel spectroscopy system for measurement of electron temperature and plasma fluctuations is also operational while a general survey spectrometer completes the spectroscopic diagnostic suite.
The far-infrared scanning interferometer has been extensively upgraded this year, and is now productive. The 2mm sweep-frequency interferometer (a standard diagnostic) has been relocated to allow toroidal cross-correlation with the FIR system measurements. The installation of a ruby-laser-based Thomson scattering system for electron temperature measurements is also nearing completion.