A. Nishizawa

H. Ogawa

K. Tsukada

M. Yokota

S. Kato

T. Ido

T. Inoue

Y. Hamada

Y. Kawasumi

National Institute for Fusion Science (NIFS-Repository)

§ 16. Development of a Heavy Ion BeamProbe for LH DIda, T., Nishizawa, A., Kawasumi, Y., Tsukada, K.,Kato, S., Yokota, M., Ogawa, H., Inoue, T.,Hamada, Y.A heavy ion beam probe (HIBP) is being developedto measure potential and density fluctuation in plasmas.The LHD-HIBP system is composed of a negative ionsource, a tandem accelerator, beam lines, and an energyanalyzer. We progressed the development of all compo-nents, but we could not attain the potential measurementin this campaign. The progress of each component is de-scribed below.AcceleratorAt the end of August,the operation of 3 MV tandemaccelerator was finally permitted by the authorities con-cerned. Then, we started the check of its performancein high voltage range. The accelerator was installed inthe LHD Building in 2000 after it had been tested in theDiagnostics Building. At this time, the pressure vesselmade from iron was remade with stainless steel so thatit does not affect the magnetic field of LHD. Thus, theperformance check is for the first time after the recon-struction.The aging of the accelerator was continued for a fewweeks and the output voltage reached to 2MV. How-ever, the load current of the Schenckel-circuit increasesextraordinarily in the output voltage range of 1 MV ormore. The anomaly in the load current is reciproducible,so the accelerator can not reach to its rated voltage inpresent.Moreover, SF6 gas leaks from the gas cell into thebeam line, so the vacuum of the beam line is not goodand a part of the injected beam damps in the accelerator.It needed to be repaired, but it was operated in thevoltage range of 2 MV or less in this campaign of LHDbecause of no time. We must repair it in next year.Ion SourceIn LHD-HIBP, a spatter type Au- ion source is usedfor the tandem accelerator. It had been developed inNIFS and the energy spread is under 10 eV for HIBP. Theoutput current of Au- is 3.6 J-tA , that is small for theplasma diagnostics but enough for the beam trajectorytrace.It is operated with a remote control system developedby using PLC and COACK (Component Oriented Ad-vanced Control Kernel), that is a kind of programinglanguage developed to control various hardware in theHigh Energy Accelerator Research Organization (KEK).The ion source system works substantially, but weshould improve the ion source itself to obtain more out-put current for plasma diagnostics.Beam LineThe performance of the beam line was continued to bechecked.The stray field of LHD affects beam line componentsas well as the beam trajectory. In a electrostatic deflector(7.8CD), which is located about 5 m apart from the cen-ter of LHD, the breakdown was enhanced by stray field ofLHD. Although the deflector can be applied the voltageof 38 kV without the stray field of LHD, its breakdownvoltage becomes a few kV in the stray field.We persistently continued the aging of the deflectorin the stray field. We use other power supplies becausethey can work even in the overcurrent by the Controlof the output voltage. As the result, the break downvoltage increased little by little and .reached to 20 kV,which is limited by the performance of power supplies.The aging in the stray field is necessary because the pathof the free electron should be affected by the field. Weneed new 40 kV power supplies and we will check thebreakdown voltage in next year.We tried to trace the beam trajectory with detectors inthe LHD vacuum vessel. Unfortunately, the controller ofthe electrostatic steerer, which was used to compensatethe deflection of the beam trajectory due to the strayfield, had something wrong, so we could not inject thebeam into the LHD vacuum vessel. It has been alreadyfixed , so the trajectory trace will be carried out in nextcampaign.Energy analyzerA new type of energy analyzer is being developed inthe Diagnostics Building. It has two anodes so as toreduce the required voltages ; the voltages of 56.5 kVand 113.6 kV for 6 MeV beam are applied on the 1stand 2nd anode respectively.It has been already constructed and the performanceis being examined.165

Development of a Heavy Ion Beam Probe for LHD

Development of a Heavy Ion Beam Probe for LHD

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