DEVELOPMENT OF AN ECR ION SOURCE AT CNS*
Y. Ohshiro, T. Katayama, S. Kubono, S. Watanabe, S. Yamaka,
CNS, Saitama, Japan
Abstract
A 14-GHz ECR ion source (named HyperECR) was developed in early 90’s at univ of Tokyo. The HyperECR was moved to RIKEN and installed in the AVF cyclotron for nuclear astrophysics project at CNS. To switch the RIKEN 10-GHz ECR and the HyperECR, rotatable bending magnet was installed. To reduce the losses of the beams transported from the HyperECR to the cyclotron, the design parameters of this system have optimised by using the emittance matching calculations. Thus far, a maximum transmission of 29% has been achieved. Moreover, the beam extraction method was modified by replacing the ion-decelerator. Preliminary experiments show that an increase of beam intensities of the 14N5+ and 14N6+ was observed. In this paper, the progress of the HyperECR and the injection system are summarized.
introduction
In 1999, a HyperECR was moved from the CNS Tanashi to the RIKEN Wako.The new beam lineand a vertical bending magnetfor switching the 10-GHz ECR and the HyperECR was constructed in February 25, 2002 [1]. Since then, the HyperECR and the 10-GHz ECR have been operated to deliver the beams of gaseous [2] and metallic ions [3] alternatively. The extracted beam from AVF cyclotron is transported to neither a CRIB [4] but the RRC (RIKEN Ring Cyclotron) for credit of the research activity at RIKEN and for education program at CNS. When one source supplies the beam to the AVF cyclotron, the other may use preliminary test forthe scheduled beam experiments and develop a new beam in which the highly charged heavy ions. So far, several kinds of ions such as H+, 12C4+, 13C4+, 14N6+ and 20Ne7+were delivered by the HyperECR and used for nuclear and biological experiments.
beam transport system
The HyperECR wasinstalled in the ion source room of the AVF cyclotron. A schematic drawing of the complete setup is shown in Fig. 1and a list of the elements with their specifications is given in Table 1. The beam intensityextracted from the HyperECR was known to increase almost linearly with the extraction voltage up to about 25 kV with a little saturation above 20kV. Constructed transport system provides maximum B of 30 keV, while the AVF cyclotron accepts the ions with energy of about 10 keVdue to its constant-orbit acceleration.
To reduce the losses of the beams transported from the ion source to the AVF cyclotron, we have done three improvements as follows: 1) The source was set close to the beam analyzer by putting together the extraction point of the source and the object point of the analyzer.
Fig. 1. Top view of the HyperECR and the beam transportline to the rotatable bending magnet. The beam line was set with a 20゜deviation angle from that of the conventional ECR.
Table 1. Summary of Beam Analyzing System
and Beam Transport Elements.
Drift length to 108.5 cmanalyzing magnet
Quadrupole magnets G = 0.24 kG/cm (I = 20 A)
pole length = 10 cm
pole gap = 12.1 cm
Analyzing magnet bending angle = 90゜
radius = 50 cm
gap = 8 cm
Bmax = 2.37 kG (I = 220 A)
edge angle = 29.6゜both
entrance and exit
magnification = 1
Drift length to 108.5 cm
the image slits
Solenoid length = 20 cm
bore radius = 12.4 cm
Bmax = 2.5 kG (I = 100 A)
Bending magnet bending angle = 90゜
radius = 50 cm
gap = 8 cm
Bmax = 1.5 kG (I = 102 A)
edge angle = 29.6゜both
entrance and exit
magnification = 1
2) A pair of quadruple magnet was set between the source and the analysing magnet for focusing of emitted beams from the source. 3)As for the analyzer, the magnetic field uniformity was created at ±0.1% to make emission of the beam in a focal plane small.
The analyzing magnet is C-type magnet so far as attach an exhaust port and a monitor port in a vacuum tub. To make a compensatory magnetic field distribution, the partialness gap of 2mm was prepared in both the magnetic poles of thismagnet, and the SIMM of height 2 mm and width 22 mm was attached in the magnetic pole surface. As a result, the uniformity within ±0.1% has been obtained from the center of beam passing through the inside of a pole gap by the width of ±10 cm. The measured magnetic field distribution is shown in Fig. 2.
Fig. 2. A magnetic field distribution of the bending radius direction of an analyzing magnet.
Moreover, the fringe magnetic field of an entrance and the exit of an analyzing magnet are rectifying with an iron frame, and brought the effective magnetic field boundary (EFB) close to 7 mm.
hyperecr
The HyperECR was developed in early ‘90s and used for atomic physics experiments at CNSTanashi. We succeeded in producing the intense beams of highly charged ion beams (e.g. 300 eA of 14N5+ and 60 eA of 14N6+) from this source.
The schematic drawing of this ion source is shown in Fig. 3together with the mirror field distribution used for 12C5+ and 14N6+ production. In this case, the exiting currents are 450 A and 550 A for two solenoid coils, respectively. The mirror ratio is about 2.7 (Bmax ~11 kG and Bmin ~4 kG). The length of the ECR zone is about 7cm. Electrical power consumption was nearly 40 kW.To confine the plasma radially, we used a sextuple magnet made of Nd-Fe-B permanent magnets. The field strength at the surface of the magnets is about 11 kG.
Ｗe investigated the dependence of the beam intensity on the extraction voltage of the ion source. In case of 14N5+ and 14N6+, the beam intensity extracted by 20 kV was found to increase nearly 3 times as that of 10 kV.
Fig. 3. The schematic drawing of HyperECR together with the mirror field distribution used for ion production such as 12C5+ and 14N6+.
ion-decelerator
The ion beam has been extracted by the electrostatic voltage (to be equivalent to injection voltage)of 5 to 11 kV. For the extraction voltage can enhance up to 20 kV to increase the extraction beam intensity without change of injection that, we replaced an extraction system from conventional type to the ion decelerator. The photograph of thenew one is shown in Fig. 4.
The electrodes goes and is arranged in the order of a negative electrode and a ground one from right-hand side. This unit is set near the anode electrode (see Fig. 3) with the gap of about 45 mm between the anode electrode and the negative one. Beam goes through from right to left side.
Ｗe investigated the voltage dependence of the beam intensity on the negative electrode, while the anode electrode that was 10 kV. In case of 14N6+ and 14N5+, the peak value of beam intensity was obtained on the negative electrode voltage of –5 kV (total 15 kV extraction) for 14N6+, and –7 kV (total 17 kV one) for 14N5+. Both ions, the beam intensity was found to increase nearly 2 times.
Fig. 4. The decelerator unit set in the extraction chamber of the HyperECR.
beam transmission efficiency to the AVF cycrotron
The transmission efficiency from the HyperECR ion source to the AVF cyclotron has been measured. A schematic diagram of the ion transport system ispresented in Fig. 5.
Fig. 5. Schematicdiagram of transport system.
Ion beams from the ion source aretransported horizontally (seeⅠin the figure) and vertically injected through the upper hole of the mainmagnet, via a spiral inflector, into the cyclotron by means of series ofabending magnet, a quadrupole quartet and five solenoidmagnets (seeⅡin the figure) [5]. An RF buncher is placedjust in front of the upper magnetic yoke of the cyclotronto give a flight path length of about 2 m.
We have studied the beam transmission, that is, the ratio of the beam intensity at the extraction part of the AVF cyclotron (i3) to that at the injection point (i1).Some results so far obtained are shown in Table1.As shown in the table, maximumtransmission of about 29% has been achieved.The beam bunching effect of RF buncher is 5 from 3 times.
Table 1. Some results of the beams extracted from the AVF cyclotron.
Ion / Acceleration / Analyzed beam / TransmissionSpecies / Energy[MeV/u] / intensity [eA] / efficiency [%]
H+ / 210 / 56 / 14
12C4+ / 135 / 28 / 19
13C4+ / 70 / 16 / 11
14N5+ / 135 / 33 / 13
14N6+ / 6.4 / 5 / 29
20Ne7+ / 6.3 / 12 / 25
For investigation of beam transmission efficiency, the beam emittance was measured recently. The emittance in 14N6+ acceleration of 29% of beam transmission was 115and89mmmradin thehorizontal plane and thevertical plane respectively.In order to decide a design parameter, calculation was performed by assuming 138 and 91mm mrad in both planes. This value is mostly in agreement with measured value. Recently, a beam energy spread has been improved by installation of a flattop acceleration system [6]. The beam transmission efficiency at the use of flattop acceleration system will be done in the near future.
The design parameters of the beam injection linewereoptimised by using the emittance matching calculations for the beam injection through the axial hole (hole lens) to the inflector entrance. Fig. 6 shows the envelopes of the beam in both transverse planes.
Fig.6. Beam envelopes calculated in horizontal and vertical planes.
This calculation result is a solution with which about 95% beam, which came from the source of ion, reaches inflector of a cyclotron. However, in measurement of transmission, it turns out that the 70% beam has lost near the center of a cyclotron.
Conclusion
We had installed the HyperECR, and its beam transport line in AVF cyclotron at RIKEN. The beam transmission efficiency of the HyperECR line has achieved 29%. It is just going to improve further beam transmission efficiency by the method of making beam emittance small.
references
We had installed the Hyper ECR ion source, and its
References
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[2]Y. Ohshiro et al.: RIKEN Accel. Prog. Rep. 36,279
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[3]T. Teranishi et al.: CNS Ann. Rep. 2001, 7 (2002).
[4] Y. Yanagisawa et al.: RIKEN Accel. Prog. Rep. 34, 183(2001).
[5] N. Nakanishi, et al., RIKEN Accel. Prog. Rep. 20 (1986) 188
[6] S. Watanabe et al.CNS-REP-48, Sept., 2002