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

Frequency tuning: general The frequency of the JYFL 14 GHz ECRIS was scanned between 14.050 - 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

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

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

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!

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 14.06 GHz and very unstable 5 µA at 14.08 GHz Changes in mode structure/ plasma-wave coupling?

Frequency tuning versus beam structure variations 14.05 GHz ≈ 110 µA Ar9+ 14.09 GHz ≈ 105 µA 14.108 GHz ≈ 95 µA Intensity variations less than ± 10 % Still clear variations in beam structure, same beam size (same focusing) Mode structure changes?

Frequency tuning + TWTA b) c) 14.070 GHz + 11.56 GHz 14.100 GHz + 11.56 GHz 14.135 GHz + 11.56 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

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

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!?

∆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

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: 20 000 -100 000 Indicates that most of the power is dissipated by the plasma

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

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!

- 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?

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