Limitation of the ECRIS performance by kinetic plasma instabilities O. Tarvainen, T. Kalvas, H. Koivisto, J. Komppula, R. Kronholm, J. Laulainen University of Jyväskylä I. Izotov, D. Mansfeld, V. Skalyga Institute of Applied Physics – Russian Academy of Sciences V. Toivanen CERN G. Machicoane National Superconducting Cyclotron Laboratory, Michigan State University The 16 th International Conference on Ion Sources New York, NY, USA, August 2015

Content ECRIS magnetic field configuration and semiempirical scaling laws Instabilities in ECRIS plasmas Experimental setup and diagnostics signals Instabilities limiting ECRIS performance

ECRIS magnetic field Superposition of solenoid and sextupole fields The magnetic field configuration serves for three purposes The figure is by courtesy of C. Lyneis

Resonance condition Electrons gain sufficient energy for the ionization of highly charged ions and cause significant wall bremsstrahlung

Plasma (electron) confinement B inj = magnetic field at injection B ext = magnetic field at extraction B min = minimum magnetic field B rad = radial magnetic field at the pole Result: electron confinement B inj B ext B min B rad

Suppression of MHD instabilities ∂B/∂r < 0 ∂B/∂r ≥ 0 JYFL 14 GHz ECRIS OK Sextupole Solenoid

Empirical scaling laws of ECRIS B-field D. Hitz, A. Girard, G. Melin, S. Gammino, G. Ciavola, L. Celona Rev. Sci. Instrum. 73, (2002), p B rad affected by the solenoid field!

ECRIS performance vs. B min /B ECR VENUS 18 GHz Xe 26+ D. Leitner, C.M. Lyneis, T. Loew, D.S. Todd, S. Virostek and O. Tarvainen, Rev. Sci. Instrum. 77(3), (2006). 0.75

ECRIS performance vs. B min /B ECR H. Arai, M. Imanaka, S.M. Lee, Y. Higurashi, T. Nakagawa, M. Kidera, T. Kageyama, M. Kase, Y. Yano and T. Aihara, Nucl.Instrum. Meth. A 491, 1-2, (2002). RAMSES 18 GHz Kr

ECRIS performance vs. B min /B ECR Previously unpublished SuSI 18 GHz Xe

ECRIS performance vs. B min /B ECR O. Tarvainen, J. Laulainen, J. Komppula, R. Kronholm, T. Kalvas, H. Koivisto, I. Izotov, D. Mansfeld and V. Skalyga, Rev. Sci. Instrum. 86, , (2015). JYFL 14 GHz A-ECR O

ECRIS performance vs. B min /B ECR 0.7 – 0.8

ECRIS performance vs. B min /B ECR A clue from the temporal stability of the extracted beam current (JYFL 14 GHz A-ECR) Below the optimum B min /B ECR Above the optimum B min /B ECR

ECRIS performance vs. B min /B ECR A clue from the temporal stability of the extracted beam current (SuSI 18 GHz) Below the optimum B min /B ECR Above the optimum B min /B ECR Xe 35+

Periodic oscillations of the beam current

30 Hz – 1.4 kHz 1 – 65 %

What causes the beam current oscillations?

Content ECRIS magnetic field configuration and semiempirical scaling laws Instabilities in ECRIS plasmas Experimental setup and diagnostics signals Instabilities limiting ECRIS performance

Electron velocity distribution in ECRIS Resonant heating Anisotropic EVDF due to hot electron population with v  > v ||

Electron energy distribution in ECRIS Kinetic instabilities! Cold (non-relativistic) electrons Warm and hot electrons carrying most of the plasma energy

Fingerprints of electron cyclotron instabilities Hot electrons interacting with the excited plasma wave Bursts of microwave emission and hot electrons from the plasma S.A. Hokin, R.S. Post, and D.L. Smatlak, Phys. Fluids B: Plasma Physics 1, 862 (1989). M. Viktorov, D. Mansfeld and S. Golubev, EPL, 109 (2015),

Content ECRIS magnetic field configuration and semiempirical scaling laws Instabilities in ECRIS plasmas Experimental setup and diagnostics signals Instabilities limiting ECRIS performance

Experimental setup – JYFL 14 GHz ECRIS (A-ECR) 1.Injection coil 2.Extraction coil 3.PM hexapole 4.Plasma chamber 5.Waveguides 6.Extraction 7.Pumping 8.Radial viewport

Experimental setup – diagnostics 10 MHz – 50 GHz microwave detector diode connected to WR-75 waveguide WR-75 ’diagnostics port’ Biased disc

Experimental setup – diagnostics BGO scintillator + PMT measuring bremstrahlung power flux

Experimental setup – diagnostics Faraday cup  5 m downstream in the beam line

Typical diagnostics signals Further details on the microwave emission in the poster by Ivan Izotov Note different time scales!

Why we observe a perturbation of the beam current? Biased disc current with -150 V applied voltage 0.2 mA -22 mA electron peak 14 mA ion peak Transient current  100 x steady-state current!

Why we observe a perturbation of the beam current? Biased disc current with -150 V applied voltage Electron losses overcome the ion losses for  1 µs Build-up of significant plasma potential

Ion beam energy spread Faraday cup  5 m downstream in the beam line Ramp the dipole current I(t,B)

Ion beam energy spread Peaks in m/q spectrum overlap following an instability event  10% energy spread equals to 1 kV plasma potential

Content ECRIS magnetic field configuration and semiempirical scaling laws Instabilities in ECRIS plasmas Experimental setup and diagnostics signals Instabilities limiting ECRIS performance

When do the instabilities occur and limit the ECRIS performance (current and stability)?

Electron cyclotron instabilities The energy of the microwave emission, E µ, is described by mode- dependent growth and damping rates,  and  Exponential growth of the instability amplitude when  >  Threshold between stable and unstable regimes  is proportional to the anisotropy of the EVDF  is proportional to the (inelastic) electron collision frequency

Transition from stable to unstable regime

Beam current oscillations in unstable regime

Instabilities limiting the source perfomance Solid symbols: stable operating regime Open symbols: unstable operating regime

Instabilities limiting the source perfomance Solid symbols: stable operating regime Open symbols: unstable operating regime Instabilities limit the parameter space available for optimizing the extracted currents of high charge states!

Instabilities limiting the source perfomance Temporal evolution of O 6+ ion current in stable (red) and unstable (black) regime

Repetition rate of the periodic instabilities Production of highly charged ions: Good confinement High microwave power Low neutral gas pressure

Ionization times vs. instabilities Ion current rise times (ionization + confinement) are on the order of 10 ms for high charge states (e.g. > Ar 8+ ) R. C. Vondrasek, R. H. Scott, R. C. Pardo, and D. Edgell, Rev. Sci. Instrum. 73, 548 (2002). Typical repetition rate of the instabilities is 1 kHz which corresponds to 1 ms temporal interval Ions must survive 10 instability events in order to become highly charged! 10 ms 1 ms

Do all ECRISs suffer from electron cyclotron instabilities? VENUS: studying plasma-microwave coupling Peaks of microwave emission coincident with dropping beam current (poor resolution)

Do all ECRIS suffer from electron cyclotron instabilities? 14.5 GHz PHOENIX charge breeder at LPSC Periodic ripple of beam current and bursts of bremsstrahlung Microwave emission coincident with the leading edge of the burst of bremsstrahlung emission ++

Why is B min /B ECR linked to instabilities?

Unstable Why is B min /B ECR linked to instabilities? JYFL 14 GHz ECRIS B min /B ECR = 0.66B min /B ECR = 0.73B min /B ECR = 0.80 Stable

How to prevent instabilities? Minimize the zero gradient regions on the ECR-surface? Are those regions important or is all the action on axis? To be studied with a superconducting ECRIS

How to prevent instabilities? Single frequency heating Two frequency heating Stabilizing effect of two-frequency heating is related to the suppression of electron cyclotron instabilities Further details in poster by V. Skalyga

“Tuning an ECR ion source is searching for an island of stability in a sea of turbulence” - R. Geller Thank you for your attention

Future plans A study of instabilities vs. gas mixing Measurement of the ion beam energy spread in unstable operation mode together with estimating the electron charge repelled by the instability Better understanding of the two-frequency stabilization