ICIS2015,Aug , 2015, New York, USA Further improvement of RIKEN 28GHz SC-ECRIS for production of highly charged U ion beam T. Nakagawa (RIKEN, Nishina center) 1.Introduction RIKEN RIBF 2.Effect of magnetic field distribution B min effect Effect of magnetic mirror 3.Highly charged U ion beam with sputtering method Consumption rate Beam intensity 4.Summary

ICIS2015,Aug , 2015, New York, USA M. Nishida et al, PASJ 2015,FSP003 28GHz SC-ECRIS Liquid He free SC-ECRIS(18GHz) New 18GHz ECRIS( under construction) 18GHz ECRIS RILAC RILAC II AVF cyclotron SRC IRC fRC RRC ~345MeV/u

ICIS2015,Aug , 2015, New York, USA 10.8MeV/u 50MeV/u 345MeV/u 0.6MeV/u 3.2keV/u He gas stripper C stripper Efficiency ~15%Efficiency ~30%Efficiency ~4.5% U beam U,Xe beam O, Ca, Kr beam Due to low transmission efficiency ( ~4.5%), we surely need the intense beam production form the ECRIS Construction of the New SC-ECRIS(2009~ )

ICIS2015,Aug , 2015, New York, USA M. Nishida et al, PASJ 2015,FSP003 28GHz SC-ECRIS (SS chamber) 18GHz ECRIS 28GHz SC-ECRIS (Al chamber) ICIS2013 ICIS2015 RIKEN 28GHz SC-ECRIS RILAC II G. D. Alton and D. N. Smithe, Rev. Sci. Instrum. 65, 775 (1994).

ICIS2015,Aug , 2015, New York, USA M. Nishida et al, PASJ 2015,FSP003 ECRIS-2014, Aug 24-28, Nizhny Novgorod 1. Minimization of the consumption rate we need long term continuous beam production (1~2 montns) for RIBF experiment. To meet this requirement, even if we can install 10gr of metal U, the consumption rate should be lower than ~7 mg/h. 2. Optimization of the magnetic field distribution The optimization of the magnetic field distribution is important task to increase the beam intensity. For this reason, we intensively searched the optimum distribution to maximize the beam intensity. 2015~ Increase the beam intensity for RIBF

ICIS2015,Aug , 2015, New York, USA Magnetic field distribution B inj, B ext, B min, B r Magnetic field distribution B inj, B ext, B min, B r Mirror ratio B inj /B min …… Microwave power absorption B min Mirror ratio B inj /B min …… Microwave power absorption B min Microwave power and frequency Gas pressure Geometrical effect chamber size chamber shape B ecr Key parameters of ECR ion source Key parameter of ECRIS

ICIS2015,Aug , 2015, New York, USA B min effect (B min ) opt

ICIS2015,Aug , 2015, New York, USA B min (microwave absorption) gas pressure effect Gas pressure was tuned to maximize the beam intensity Gas pressure effect ? Ar 11+ ion beam Liquid He free Sc-ECRIS(18GHz) B inj ~1.87T, B r ~1.1T B ext ~1.2T, RF~400W, V ext =10kV When the gas pressure was tuned to maximize the beam intensity at each B min, (B min ) opt was ~0.48 T, which is almost same as the value shown above figure

ICIS2015,Aug , 2015, New York, USA B min ( U 35+ ion beam) 28GHz microwaves RF power effect and X-ray heat load Charge state The beam intensity of U 35+ ions and X-ray heat load as a function of B min at two different RF power (~1.5kW and ~1kW), respectively. B inj, B ext and B r were ~3.1, ~1.75 and ~1.88T, respectively. (B min ) opt was ~0.65T for both RF powers. The X-ray heat load increased with increasing the B min, which is mainly due to the magnetic field gradient at ECR zone. It was saturated at B min ~0.7T, even though the magnetic field gradient become gentler. RF~1.5kW ~1.0kW

ICIS2015,Aug , 2015, New York, USA B min ( U 35+ ion beam)(summary) Figures show the summary of the (B min ) opt for several heavy ions with 18 and 28GHz. In case of 18GHz, (B min ) opt was ~0.5T, which is ~0.8B ecr. For 18 GHz microwave operation, B inj, B ext and B r were fixed to 2.3, 1.19 and 1.2T respectively. On the other hand, (B min ) opt with 28GHz was ~0.65T, which is slightly increased with increasing the charge state. It is correspond to the ~0.65B ecr, which is lower than that with lower frequency of 18GHz in this experiment. 18GHz 28GHz RIKEN 28GHz SC-ECRIS

ICIS2015,Aug , 2015, New York, USA Magnetic mirror (B r /B min, B inj /B min, B ext /B min ) It is obvious that B inj, B ext and B r work as a part of the magnetic mirror to confine the plasma. In the mid 1990s, so-called “High B” mode, which basically gives high magnetic mirror ratio to confine the plasma, was proposed to increase the beam intensity of highly charged heavy ions Many laboratories adopted this empirical formula to design the ECR ion source.Using it, they successfully increased beam intensity of highly charged heavy ions. 28GHz U 35+ B ext ~1.75T ~1.45T

ICIS2015,Aug , 2015, New York, USA B inj /B min B ext /B min V // B inj /B ext Magnetic mirror B inj /B min ~B ext /B min B inj /B min >B ext /B min Particle flow

ICIS2015,Aug , 2015, New York, USA B r effect The beam intensity increases with increasing the B r and saturated at certain B r. The saturation point increases with increasing B ext. Figure shows the bema intensity as a function of B r /B ext. Very roughly speaking, the beam intensity is saturated at B r /B ext ~1.2 (B r ) thresh ~1.2B ext 18GHz B min ~0.5T Xe 20+, 18GHz

ICIS2015,Aug , 2015, New York, USA (B r ) thresh ~1.2B ext B r effect To investigate this effect for 28GHz operation, we measured the U35+ ion beam as a function of Br. To measure it in a wide range, B ext was fixed to 1.42T. The beam intensity increased with increasing Br and saturated at Br~1.2Bext, which is same tendency for 18GHz operation. 28GHz B min ~0.5T

ICIS2015,Aug , 2015, New York, USA Loss cone ( ) BrBr B min Loss cone ( ) B ext B min > Loss cone ( ) BrBr B min Loss cone ( ) B ext B min < B r effect At lower B r /B ext ( 1.2), B ext may govern the plasma confinement, because the loss cone at B r is smaller than that at B ext. For these reasons, the beam intensity is saturated. V //

ICIS2015,Aug , 2015, New York, USA B r effect The magnetic field gradient is strongly dependent on the Br. At higher mirror ratio, the electron energy may not be high enough to produce highly charged Kr ions. Therefore, the beam intensity decreases with increasing the Br. (x10 -4 ) Kr 18+

ICIS2015,Aug , 2015, New York, USA B inj effect B ext ~1.75T B ext ~1.45T (B inj ) thresh The beam intensity increased with increasing B inj and became constant above B inj /B ext ~1.6

ICIS2015,Aug , 2015, New York, USA Plasma chamber Biased disc U-rod Support rod(water cooled) To obtain the consumption rate, we measured the total amount of consumption and total sputtering current for long term at fixed sputtering voltage. We obtained that the consumption rate was ~1mg/h for 1mA of sputtering current at the sputtering voltage of - 2kV. R consumption ~ f ( Ion, V sputter )I sputter U production (sputtering method)

ICIS2015,Aug , 2015, New York, USA U production (sputtering method) To obtain the consumption rate, we performed long-term measurements of the total amount of material consumption and total sputtering current, at fixed sputtering voltage. For example, we obtained the consumption rate of ~1 mg/h for 1 mA of sputtering current (ion current + current due to the secondary electron emission) at the sputtering voltage of -2 kV. Consumption rate strongly depended on the sputtering voltage 3) and was proportional to the sputtering current at fixed sputtering voltage. E b : binding energy E 1 : incident energy of ions Energy of sputtered particle Energy of sputtered particles decrease with decreasing the incident energy of the ions M. W. Thompson, Philos. Mag. 18, 377 (1968).

ICIS2015,Aug , 2015, New York, USA U 35+ ion beam production 201e  A(~2.6kW) B inj ~3.11T, B min ~0.62T B ext ~1.78T B r ~1.87T RF 18+28GHz V ext ~22kV W X-ray ~2.8W

ICIS2015,Aug , 2015, New York, USA Summary euA of U 35+ was produced at the RF power of ~2.6kW and the magnetic field strength lower than the ordinary High-B mode operation for 28GHz. 2. The consumption rate of metal U was dramatically reduced from ~8.6 to ~2.8mg/h with sputtering method to produce intense beam of U 35+ (~150euA). Based on these results, we successfully produced stable intense beam of U 35+ ion in RIBF experiment for long term (more than one month) with low material consumption this year. 3. The beam intensity of highly charged heavy ions was saturated at B inj >1.6B ext and B r >1.2B ext for U 35+ ions in a wide range of B ext. 4. The optimum value of B r to maximize the beam intensity is strongly dependent on B min in a certain condition 5. Optimum value of B min for maximizing the beam intensity was dependent on the microwave frequency and the gas pressure.