Limitation of the ECRIS performance by kinetic plasma

instabilitiesO. 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. ToivanenCERN

G. MachicoaneNational Superconducting Cyclotron Laboratory, Michigan State University

The 16th 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

Binj = magnetic field at injection

Bext = magnetic field at extraction

Bmin = minimum magnetic field

Brad = radial magnetic field at the pole

Result: electron confinement

Binj

Bext

Bmin

Brad

Suppression of MHD instabilities

∂B/∂r < 0

∂B/∂r ≥ 0

JYFL 14 GHz ECRIS

Conditions for suppressing MHD instabilities

and ∂B/∂r ≥ 0

OK

Sextupole

Solenoid

Empirical scaling laws of ECRIS B-fieldD. Hitz, A. Girard, G. Melin, S. Gammino, G. Ciavola, L. CelonaRev. Sci. Instrum. 73, (2002), p. 509.

Brad affected by the solenoid field!

ECRIS performance vs. Bmin/BECR

VENUS 18 GHz Xe26+

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. Bmin/BECR

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 Kr15+

0.78

ECRIS performance vs. Bmin/BECR

Previously unpublished

SuSI 18 GHz Xe35+

0.80

ECRIS performance vs. Bmin/BECR

O. Tarvainen, J. Laulainen, J. Komppula, R. Kronholm, T.Kalvas, H. Koivisto, I. Izotov, D. Mansfeld and V. Skalyga, Rev. Sci. Instrum. 86, 023301, (2015).

JYFL 14 GHz A-ECR O5+

0.70

ECRIS performance vs. Bmin/BECR

0.7 – 0.8

What limits the ECRIS performance at ?

ECRIS performance vs. Bmin/BECR

A clue from the temporal stability of the extracted beam current (JYFL 14 GHz A-ECR)

Below the optimum Bmin/BECR Above the optimum Bmin/BECR

ECRIS performance vs. Bmin/BECR

A clue from the temporal stability of the extracted beam current (SuSI 18 GHz)

Below the optimum Bmin/BECR Above the optimum Bmin/BECR

Xe35+

Xe35+

Periodic oscillations of the beam current

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), 65002.

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)

6

4 3

85

7

7 7

21

1. Injection coil 2. Extraction coil3. PM hexapole4. Plasma chamber5. Waveguides6. Extraction7. Pumping8. 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

6

4 3

8

5

7

7 7

21

B GO

120

mm

PMT(a) (b)

biased di sc

WR62

WR75 (d iagnostic port)

visible l ight diagnostic s port

Experimental setup – diagnosticsFaraday cup 5 m downstream in the beam line

Solenoid

Dipole magnet Steering magnet

Faraday Cup

JYFL 14 GHz ECRIS

20 mm collimator

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 spreadFaraday cup 5 m downstream in the beam line

Solenoid

Dipole magnet Steering magnet

Faraday Cup

JYFL 14 GHz ECRIS

20 mm collimator

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, Eµ, 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

Transition from stable to unstable regime

Beam current oscillations in unstable regime

Instabilities limiting the source perfomance

Solid symbols: stable operating regimeOpen symbols: unstable operating regime

Instabilities limiting the source perfomance

Solid symbols: stable operating regimeOpen 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 O6+ 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. > Ar8+)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 Bmin/BECR linked to instabilities?

Electron heating is a stochastic process.

The energy gain / resonance pass is proportional to

Electric field amplitude at the resonance

… and inversely proportional to the

Parallel component of the magnetic field gradient

at the resonance.

Unstable

Why is Bmin/BECR linked to instabilities?

JYFL 14 GHz ECRIS

Bmin/BECR = 0.66 Bmin/BECR = 0.73 Bmin/BECR = 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