Two ECR ion sources are used for the axial injection into the superconducting cyclotron

Two ECR ion sources are used for the axial injection into the superconducting cyclotron.

The first one is the superconducting source SERSE, which has been working at LNS since June 1998, as described in the previous meeting. The second one is the conventional CAPRICE-type source, named CAESAR, that was ordered to PANTECHNIK in May 1998, it was ready at the end of 1998 (according to the schedule presented in 1998 EXCYT meeting) and it has been operational since the spring of 1999.

In fig. 1 the ECRIS area is shown in its final arrangement. In spite of the narrow spaces which are available, each source and the relative equipment are accessible.

Both the sources have already accomplished their design currents for the production of light fully stripped ions, as requested by the EXCYT project, namely 5 to 7 pµA for SERSE and 1 to 3 pµA for CAESAR. A full description of the developments of ECR sources at LNS is presented in [1,2,3].

SERSE

Since November 1998 we have been running the source at 18 GHz with single and double frequency heating and the results are excellent. In tab. 1 the currents for some species are given.

It is to be remarked that the current for fully stripped oxygen is about 7 pµA, which means that the demanding requests of the EXCYT project are yet satisfied.

Fig. 1 - The ECR ion sources (ECRIS) room at LNS.

The theoretical approach (High B mode) on which the source design was based was confirmed and recently short tests at 28 GHz suggested a further extrapolation of this design to 28 GHz.

In fig. 2 and 3 the Argon and Xenon charge state distributions are shown.

Ion	Current (eµA)	Ion	Current (eµA)	Ion	Current (eµA)
O⁶⁺	540	Kr²²⁺	66	Au³⁰⁺	20
O⁷⁺	208	Kr²⁵⁺	35	Au³¹⁺	17
O⁸⁺	55	Kr²⁷⁺	7.8	Au³²⁺	14
Ar¹²⁺	200	Kr²⁹⁺	1.4	Au³³⁺	12
Ar¹⁴⁺	84	Kr³¹⁺	0.2	Au³⁴⁺	8
Ar¹⁶⁺	21	Xe²⁷⁺	78	Au³⁵⁺	5.5
Ar¹⁷⁺	2.6	Xe³⁰⁺	38.5	Au³⁶⁺	2.5
Ar¹⁸⁺	0.4	Xe³¹⁺	23.5	Au³⁸⁺	1.1
Kr¹⁷⁺	160	Xe³³⁺	9.1	Au³⁹⁺	0.7
Kr¹⁸⁺	137	Xe³⁴⁺	5.2	Au⁴⁰⁺	0.5
Kr¹⁹⁺	107	Xe³⁶⁺	2	Au⁴¹⁺	0.35
Kr²⁰⁺	74	Xe³⁸⁺	0.9	Au⁴³⁺	0.03

Tab. 1 – Best reproducible currents (in eµA) from SERSE for highly charged ions of gaseous and metallic elements.

Emittance measurements have been carried out in summer 1999, which confirmed the calculations carried out with the computer code IGUN, giving values in the order of 50 to 70 π mm.mrad for O⁶⁺ and O⁷⁺ (for O⁸⁺ , it is even better).

At the beginning of 1999, the vacuum in the extraction box was significantly improved, by means of a 1000 l/s turbomolecular pump and the background pressure of 1*10^-8 mbar at the extraction was helpful for the production of highly charged heavy ions (it is not so relevant for the EXCYT goals).

Fig. 2 - A typical CSD for Argon.

Fig. 3 - A Xenon CSD optimized on 30⁺.

Metallic ions production has been obtained by means of a high temperature oven. During the last years the most part of the beamtime was produced by metallic sample with a good reliability and long term stability.

CAESAR

The availability of a second ECR ion source for the axial injection allows to optimize the operations of the LNS accelerator complex.

The source ECR2 (called CAESAR in the following), with the following features, was ordered to the French company Pantechnik in 1998:

• a higher magnetic field (up to 1.58 T axial, 1.1 T radial), to operate the source at 14 GHz in HBM and at 18 GHz with B/B_ECR close to 2;

• a modified microwave injection for two frequency heating (14 + 18 GHz) ;

• an aluminum plasma chamber ;

• a three electrodes extraction system;

• a maximum voltage of 30 kV.

In tab. 2 the main features of CAESAR are shown. The source was delivered at the end of 1998 and the installation at LNS was completed in March 1999.

CAESAR has been operating as injector for the cyclotron. In 2000 the beam transmission through the analysis section of the beam line has been optimised by means of a new extraction system.

The results in terms of HCI production are good and they are summarized in tab. 3.

Operating frequency	14 and 18 GHz
Maximum radial field on the wall	1.1 T
Maximum axial field at injection	1.25 T (for 14 GHz) 1.58 T (for 18 GHz)
Maximum axial field at extraction	1.0 T (for 14 GHz) 1.35 T (for 18 GHz)
Minimum axial field	0.4 T (for 14 GHz) 0.5 T (for 18 GHz)
Coils supply	Two 1300 A – 60 V
Hexapole	NdFeB made
Extraction system	Accel-decel, 35 kV/12 kV max
Plasma chamber	St. steel or Al made

Tab. 2 – Design parameters of CAESAR.

Ion	Current (eµA)
N⁶⁺	160
¹⁵N⁷⁺	25
O⁶⁺	720
O⁷⁺	105
Ne⁸⁺	170
Ne⁹⁺	14
Ar¹¹⁺	120
Ar¹⁶⁺	2
Ca¹²⁺	52
Ca¹⁴⁺	6
Ni¹⁷⁺	18
Kr²²⁺	10
Kr²⁸⁺	1
Ta²⁷⁺	10

Tab. 3 – Typical currents (in eµA) produced by CAESAR.

Fig. 4 – A charge state distribution optimized for N⁷⁺.

Fig. 5 – A charge state distribution optimized for O⁷⁺.

An important improvement was obtained by replacing the standard stainless steel chamber with an Al chamber, according to the experience of other laboratories, with an increase of 20 to 40% for the currents of HCI.

Two results are particularly valuable for the EXCYT project: the optimization of Ne⁹⁺ (14 eµA), and of fully stripped nitrogen (25 eµA), led to acceptable currents for the production of intense primary beams (fig. 4, 5).