Presentation on theme: "Frequency tuning, space charge compensation and hollow beam structure"— Presentation transcript:
1Frequency tuning, space charge compensation and hollow beam structure H. Koivisto, V. Toivanen, O. Steczkiewicz, L. Celona, O. Tarvainen, T. Ropponen, S. Gammino and G. CiavolaContent:- Frequency tuning vs. intensity- Frequency tuning vs. beam structure variations- Frequency tuning vs. emittance- Frequency tuning vs. calculated modes- Space charge compensation measurements
2Frequency tuning: general The frequency of the JYFL 14 GHz ECRIS was scanned betweenGHz (by Rohde & Schwarz signal generator)Forward power of the Klystron was kept constant using internalAutomatic Level Control (ALC) featureThe first frequency tuning experiments:with mass analyzed beams,with emittance information,with TWTA
3Frequency tuning vs. ion beam intensity Measurements with different charge states: general behaviorAr6+: 110 W- no variations in beam current- small variations in drain current- clear variations in reflected power
4Ar9+- small changes in beam current- clear changes in reflected power- local minimum in reflected power => local intensity maxima!
5Ar12+: 520 WConclusion:Effect of frequency tuningincreases vigorously as afunction of charge state!- Similar variations in reflected power as earlier- strong variations in ion beam intensity!
6What causes the intensity variations! Is it because the total power in the plasma chamber fluctuates with thereflected power (Ptot = Pforward - Preflected)?Most probably no:Difference in Ptotal of 3 %(490 W and 505 W) cannot generatethe difference of 30 % in Ar12+ intensity(7 µA versus 10 µA) ! Losses not included!Ar12+2) Same Ptotal cannot generate stable10 µA at GHz and very unstable5 µA at GHzChanges in mode structure/plasma-wave coupling?
7Frequency tuning versus beam structure variations 14.05 GHz≈ 110 µAAr9+14.09 GHz≈ 105 µAGHz≈ 95 µAIntensity variations less than ± 10 %Still clear variations in beam structure,same beam size (same focusing)Mode structure changes?
8Frequency tuning + TWTA b)c)GHz GHzGHz GHzGHz GHzAr8+In some cases two concentric hollow beams can be seen when doublefrequency heating is used with the frequency tuninga clear indication that the hollow beam structure can formed also inthe plasmaa clear indication that the frequency tuning causes changes in plasma
9Frequency tuning versus emittance - strong effect on the beam emittancebeculiar behavior: usually lowest emittance for high charge states(9+ and higher) in the beginning of frequency scan, for lower chargestates in the middle of the scan! More measurements have to beperform to understand the behavior
10What 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!?
11∆f=f0/Q Q-value estimation with the aid of reflected power Q ≈ 2200 (similar values have been obtainedearlier by Catania group)∆f=f0/Q
12Q-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 = 1410Q-value of empty chamber:Indicates that most of the poweris dissipated by the plasma
13Space charge compensation effect: preliminary unpublished results! Objective: improvement of space charge compensationgas feedingGas was fed into the beam line close to the focal points (He, N2, Ar)emittance scannerFaraday cupbeam viewer
14Space charge compensation effect: preliminary unpublished results! Ar8+, 10 kV, 143 µA, 1.8E-7 mbarAr8+, 10 kV, 120 µA, 6.2E-6 mbar277 π mm mrad140 π mm mradIndicates that hollow beam structurecan be formed also in the beam lineBeam 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 intensitya) intensity decreased due to losses caused by high pressureb) analysis show that the intensity within the acceptance ofcyclotron changed marginallyBetter space charge compensation can improve the beam quality drastically.Losses has to be avoided: filament?
16Summary - frequency tuning can be an efficient tool to: 1) Increase the intensity of highly charged ion beams2) Affect the beam quality- Q-value with the plasma absorption is high (≈ 2000)better space charge compensation can improve the ionbeam qualityPlease see interesting posters by O. Tarvainen and T. Ropponen