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Institute of Applied Physics, Russia

Analysis of Gas Efficiency for ECR Ion Source Operating in either Singly or Multiply Charged Ions Mode by Way of Example of Helium. Modeling and Experiments. Institute of Applied Physics, Russia Laboratoire de Physique Subatomique et de Cosmologie , France. Gas composition.

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Institute of Applied Physics, Russia

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  1. Analysis of Gas Efficiency for ECR Ion Source Operating in either Singly or Multiply Charged Ions Mode by Way of Example of Helium. Modeling and Experiments. Institute of Applied Physics, Russia Laboratoire de Physique Subatomique et de Cosmologie, France

  2. Gas composition Production: 10^13 particle per second Frequency repetition: 10 Hz Plasma chamber volume > 10 cm^3 So gas density in the chamber is less than 10^11 cm^-3 It is mach less than ion density in ECR plasma sources Buffer gas is needed Gas composition in our consideration: small number of He atoms and a lot of buffer gas

  3. Radioactive particle losses 1 Pulse temporal structure (i.e. duration >100 us) 2 Unsuitable charge state distribution, partial ionization 3 Nonoptimalextraction system Total efficiency couldn’t be higher than

  4. Some experimental and theoretical work was done at IAP RAS in order to find a way to reduce every of losses mentioned above SMIS 37 facility. 1 – gyrotron, 2 – MW beam, 3 – pulsed gas valve, 4 – vacuum chamber, 5 – magnetic trap coils,6 – quartz MW window, 7 – diagnostic chamber, 8 – extraction system, 9 – Faradey cup, 10 – beam charge state analyzer

  5. 1 Pulse temporal structure (i.e. duration >100 us)

  6. There are two regimes of plasma confinement

  7. Three approaches for creation of short pulse multicharged ion beams Short pulse ECR ion source Steady state generation Non-steady state (preglow & afterglow effects) By mean of high current extraction

  8. Steady state approach on generation of short pulses Plasma confinement time << pulse duration ~ 100µs Steady state generation Plasma confinement time ~ 10 – 20µs Quasi-gasdynamic plasma confinement

  9. Oscillograms of total ion current observed with different MW frequencies along with numerically calculated currents, showing good agreement between theory and experiments 75 GHz, 150 kW, mirror magnetic trap with effective length of 30cm, pressure2.5·10-4Torr, plug field 4Tesla 37.5 GHz, 100 kW, CUSP magnetic trap with effective length of 28 cm, pressure1·10-4Torr, plug field 1.7 Tesla Fast ionization!! All gas is ionazed Rising time of total extracted ion current is ~15 s !!!

  10. Non-steady state approach Plasma confinement time ~ pulse duration ~ 100µs Non-steady state (preglow & afterglow effects) Spring experiments 37 GHz gyrotron MW pulse 20 µs Microwave duration = 50 µs Duration of ion current = 20 µs Ion current of N3+ =2 мА Ion current of N3+

  11. Modeling of short pulses Modeling Experiment Simple mirror trap, L=37 cm Mirror Ratio = 4 MW=10 kW/cm2 Extraction voltage = 25 kV MW duration ~ 70 µs

  12. Fast ionization in high frequency ECR ion sources was predicted theoretically and obtained experimentally

  13. 2 Unsuitable charge state distribution, partial ionization

  14. Charge state distribution under quasi-gasdynamic confinement. • Steady state approach 37.5 GHz, 100 kW, CUSP magnetic trap with effective length of 28 cm, pressure1·10-4Torr, plug field 1.7 Tesla 75 GHz, 150 kW, mirror magnetic trap with effective length of 30cm, pressure2.5·10-4Torr, plug field 4Tesla

  15. N3+ N2+ Nitrogen H+ O2+ O3+ Ar4+ C2+ C3+ Ar5+ N+ Argon N4+ C+ O+ Ar3+ C2+ N2+ Ar2+ O+ C+ Non-steady state approach Charge state distribution in short pulses C2+

  16. Analyzer signal, a.u. Magnet current, A Steady state vs non-steady stateCharge of ions Nitrogen Steady state Non-steady state N3+ N2+ H+ O2+ O3+ C2+ C3+ N+ N4+ C+ O+ Magnet current, eA Average charge is higher

  17. 3 Nonoptimal extraction system

  18. Number of radioactive atoms is very low. Buffer gas should be added for discharge ignition All radioactive atoms have to be extracted for short time Number of particles > 10^14, they have to be extracted for less than 10^-4 s One has to extract extremely high currents: up to several eA. High current ion separation Experiments on forming of high-intensity beams with low emittance were performed at IAP RAS as well.

  19. High current ion beams 13aperture extracting system diameter of an aperture 3 mm 2 см Imax>150 mA

  20. Emittancemeasurement Nitrogen beam <Z>=2 >150 meAof total current normalized emmitance <1 πmm mrad !!!

  21. Short pulse ECR ion source By mean of high current extraction

  22. The following scheme of the source was considered in simulations Field lines Plasma flow considered to be “one-side”: flow 2>>flow1 (due to biased disk, magnetic field, etc.) Extraction coefficient = Se/Sp, Sp – plasma cross-section in the extraction plug Se – extraction hole cross-section. Biased disk Sp Se PLASMA Plasma Flow 2 Plasma Flow 1 6He Particles which haven’t been not extracted, returns back to a discharge as a neutrals after collision with a wall and recombination. Working gas (6He) is injected once before the discharge, buffer gas (Nitrogen) is being injected during the discharge in order to maintain ~constant plasma density. Gas flow Supporting gas Time RF pulse

  23. Temporal evolution of He and buffer gas. Determination of gas efficiency. 100 µs Utilized particles Integration for 100 µs =Nu Lost particles Over100 µs gas efficiency = Nu N

  24. A plenty of simulations were run in order to optimize the future source for reaching high gas efficiency. 6He was used as a working gas, Nitrogen – as a buffer gas. Different buffer gas density Optimization of microwave power 100% extraction (all ions which left the trap are considered as utilized) 90% efficiency could be reached for 6He+ 70% efficiency for 6He++

  25. Which frequency do we need? High frequency for 6He++ is needed because high efficiency is reached with plasma density higher than cut-off density for i.e. 37 GHz. (Following the simulation, 6He++ efficiency could be increased using frequency higher than 60 GHz along with higher buffer gas density)

  26. Ion current oscillogramsfor 6He+ and 6He++ maximum efficiencies 100% extraction means one needs to extract every ion comes from the trap. Along with total current densities shown above it means up to 10 eA of total current (assuming 1 cm^2 plasma cross-section at the plug) needed to be extracted!

  27. Let’s try to reduce extraction coefficient! Reducing of extraction coefficient by 70% (and total current as well) reduces gas efficiency only by 20%.

  28. Plans for further study Creation of short pulses of ions flow in non-steady state mode with microwave pumping 75 GHz – December 2010. 2. We can’t measure gas efficiency because of gas pumping from plasma chamber 3. We can’t increase extracted ion current because we haven’ proper power supply. 4. Calculation of gas efficiency with consideration of pulse periodic regime. Decay time is much longer than duration between pulses. We hope for decreasing of extracting current

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