ICIS09, Gatlinburg, H. Koivisto Frequency tuning, space charge compensation and hollow beam structure H. Koivisto, V. Toivanen, O. Steczkiewicz, L. Celona,

ICIS09, Gatlinburg, H. Koivisto

Frequency tuning, space charge compensation and hollow beam structure

H. Koivisto, V. Toivanen, O. Steczkiewicz, L. Celona, O. Tarvainen, T. Ropponen, S. Gammino and G. Ciavola

- Frequency tuning vs. intensity

- Frequency tuning vs. beam structure variations

- Frequency tuning vs. emittance

- Frequency tuning vs. calculated modes

- Space charge compensation measurements

Content:

ICIS09, Gatlinburg, H. Koivisto

Frequency tuning: general

The frequency of the JYFL 14 GHz ECRIS was scanned between14.050 - 14.135 GHz (by Rohde & Schwarz signal generator)

Forward power of the Klystron was kept constant using internal Automatic Level Control (ALC) feature

The first frequency tuning experiments:- with mass analyzed beams,- with emittance information,- with TWTA

ICIS09, Gatlinburg, H. Koivisto

- no variations in beam current

- small variations in drain current

- clear variations in reflected power

Frequency tuning vs. ion beam intensity

Ar6+: 110 W

Measurements with different charge states: general behavior

ICIS09, Gatlinburg, H. Koivisto

- small changes in beam current

- clear changes in reflected power

- local minimum in reflected power => local intensity maxima!

Ar9+

ICIS09, Gatlinburg, H. Koivisto

- Similar variations in reflected power as earlier

- strong variations in ion beam intensity!

Conclusion:Effect of frequency tuningincreases vigorously as afunction of charge state!

Ar12+: 520 W

ICIS09, Gatlinburg, H. Koivisto

What causes the intensity variations!

Is it because the total power in the plasma chamber fluctuates with the reflected power (Ptot = Pforward - Preflected)?

Most probably no:

1) Difference in Ptotal of 3 %(490 W and 505 W) cannot generate the difference of 30 % in Ar12+ intensity (7 µA versus 10 µA) ! Losses not included!

2) Same Ptotal cannot generate stable10 µA at 14.06 GHz and very unstable5 µA at 14.08 GHz

Changes in mode structure/plasma-wave coupling?

Ar12+

ICIS09, Gatlinburg, H. Koivisto

Frequency tuning versus beam structure variations

Ar9+

Intensity variations less than ± 10 % Still clear variations in beam structure,

same beam size (same focusing)Mode structure changes?

14.05 GHz

≈ 110 µA

14.09 GHz≈ 105 µA

14.108 GHz≈ 95 µA

ICIS09, Gatlinburg, H. Koivisto

Frequency tuning + TWTA

In some cases two concentric hollow beams can be seen when doublefrequency heating is used with the frequency tuning

- a clear indication that the hollow beam structure can formed also in the plasma

- a clear indication that the frequency tuning causes changes in plasma

a) b) c)

14.070 GHz + 11.56 GHz 14.100 GHz + 11.56 GHz 14.135 GHz + 11.56 GHz

Ar8+

ICIS09, Gatlinburg, H. Koivisto

Frequency tuning versus emittance

- strong effect on the beam emittance

- beculiar behavior: usually lowest emittance for high charge states (9+ and higher) in the beginning of frequency scan, for lower charge states in the middle of the scan! More measurements have to be perform to understand the behavior

ICIS09, Gatlinburg, H. Koivisto

What is the density of calculated modes (JYFL 14 GHz ECRIS)?

Density of calculated modes and density of intensity fluctuationsare at the same order => the intensity variations come fromchanges in mode excitation!?

ICIS09, Gatlinburg, H. Koivisto

∆f=f0/Q

Q-value estimation with the aid of reflected power

Q ≈ 2200 (similar values have been obtainedearlier by Catania group)

ICIS09, Gatlinburg, H. Koivisto

Q-value estimation with the aid of intensity variations:

Ar9+

Q-value has to be relatively high in order to see any intensity variations!

For example: Q = 10 => ∆f = 1.4 GHz , very wide peak!

Q-value of empty chamber:20 000 -100 000

Indicates that most of the poweris dissipated by the plasma

Distance of neighboring peaks≈ 10 MHz => Q = 1410

ICIS09, Gatlinburg, H. Koivisto

Space charge compensation effect:preliminary unpublished results!

Objective: improvement of space charge compensation

beam viewer

emittance scanner

gas feeding

Gas was fed into the beam line close to the focal points (He, N2, Ar)

Faraday cup

ICIS09, Gatlinburg, H. Koivisto

Ar8+, 10 kV, 120 µA, 6.2E-6 mbar

140 π mm mrad

Ar8+, 10 kV, 143 µA, 1.8E-7 mbar

277 π mm mrad

Space charge compensation effect:preliminary unpublished results!

Beam size decreases!Clear optimum for brightness is found!

Indicates that hollow beam structurecan be formed also in the beam line

ICIS09, Gatlinburg, H. Koivisto

- clear improvement in ion beam transmission efficiency (incl. cyclotron)

- marginal change in accelerated beam intensity

a) intensity decreased due to losses caused by high pressure

b) analysis show that the intensity within the acceptance of cyclotron changed marginally

Better space charge compensation can improve the beam quality drastically. Losses has to be avoided: filament?

ICIS09, Gatlinburg, H. Koivisto

Summary

- frequency tuning can be an efficient tool to:

1) Increase the intensity of highly charged ion beams

2) Affect the beam quality

- Q-value with the plasma absorption is high (≈ 2000)

- better space charge compensation can improve the ion beam quality

Please see interesting posters by O. Tarvainen and T. Ropponen