1. 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
Content:
- Frequency tuning vs. intensity
- Frequency tuning vs. beam structure variations
- Frequency tuning vs. emittance
- Frequency tuning vs. calculated modes
- Space charge compensation measurements

2. Frequency tuning: general
The frequency of the JYFL 14 GHz ECRIS was scanned between
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

3. Frequency tuning vs. ion beam intensity
Measurements with different charge states: general behavior
Ar6+: 110 W
- no variations in beam current
- small variations in drain current
- clear variations in reflected power

4. Ar9+
- small changes in beam current
- clear changes in reflected power
- local minimum in reflected power => local intensity maxima!

5. Ar12+: 520 W
Conclusion:
Effect of frequency tuning
increases vigorously as a
function of charge state!
- Similar variations in reflected power as earlier
- strong variations in ion beam intensity!

6. 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:
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!
Ar12+
2) Same Ptotal cannot generate stable
10 µA at GHz and very unstable
5 µA at GHz
Changes in mode structure/
plasma-wave coupling?

7. Frequency tuning versus beam structure variations
14.05 GHz
≈ 110 µA
Ar9+
14.09 GHz
≈ 105 µA
GHz
≈ 95 µA
Intensity variations less than ± 10 %
Still clear variations in beam structure,
same beam size (same focusing)
Mode structure changes?

8. Frequency tuning + TWTAb) c)
GHz GHz
GHz GHz
GHz GHz
Ar8+
In some cases two concentric hollow beams can be seen when double
frequency 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

9. 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

10. What is the density of calculated modes (JYFL 14 GHz ECRIS)?
Density of calculated modes and density of intensity fluctuations
are at the same order => the intensity variations come from
changes in mode excitation!?

11. ∆f=f0/Q Q-value estimation with the aid of reflected power
Q ≈ 2200 (similar values have been obtained
earlier by Catania group)
∆f=f0/Q

12. Q-value estimation with the aid of intensity variations:
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!
Ar9+
Distance of neighboring peaks
≈ 10 MHz => Q = 1410
Q-value of empty chamber:
Indicates that most of the power
is dissipated by the plasma

13. Space charge compensation effect: preliminary unpublished results!
Objective: improvement of space charge compensation
gas feeding
Gas was fed into the beam line close to the focal points (He, N2, Ar)
emittance scanner
Faraday cup
beam viewer

14. Space charge compensation effect: preliminary unpublished results!
Ar8+, 10 kV, 143 µA, 1.8E-7 mbar
Ar8+, 10 kV, 120 µA, 6.2E-6 mbar
277 π mm mrad
140 π mm mrad
Indicates that hollow beam structure
can be formed also in the beam line
Beam size decreases!
Clear optimum for brightness is found!

15. - clear improvement in ion beam transmission efficiency (incl
- 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?

16. 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