Thirty years have passed since the modern concept of the production of highly charged ions in dense electron beams was first proposed, and less since the first electron beam ion source (EBIS) was built and tested. Milestones from the EBIS history, and future prospects are described in view of today’s needs in the production and use of highly charged ions. Various versions of EBIS, such as the classic EBIS, the electron beam ion trap—EBIT, the cross-over EBIS (T)-XEBIS/T and others were constructed, studied, and used in different laboratories. The ion-by-ion and “evaporative” cooling techniques are used to increase the time of ion confinement in an electron beam, ion charge states and ion yield. An alternative way to increase the ion yield is an increase of the electron beam current. The different concept of an ion source based on EBIS which uses multiple reflection of primary electrons and formation of a so-called electron string state is studied as well. Stability of electron-ion systems and nonexcitation of internal EBIS structure are the main goals in the present EBIS studies. Fast extraction of ions from EBIS, which can be done during several microseconds makes efficient the single turn injection into ion synchrotrons.

Electron String Ion Source (ESIS) Krion-2 (JINR, Dubna) was used for basic and applied research in various aspects of multiply charged heavy ions production. Energy recuperation mode in ESIS has been proofed first and used for production of highly charged ions 84Kr28+÷84Kr32+, 124Xe40÷124Xe44 and Au51+÷ Au54+. Krion-2 ESIS was mounted on high voltage (HV) platform of LU-20 Linac and used as an injector of highly charged ions during Nuclotron run N° 41. Krion-2 ESIS has produced 3.0·107 124Xe42+ ions per pulse of 7 μs duration. This ion beam was injected into LU-20 and Nuclotron, accelerated up to energy of 186 GeV and the extracted Xe beam was used for physics experiments. Electron String Ion Source Krion-2 demonstrated the high reliability and stability running during 30 days on HV platform. We believe that it is due to an extremely low electron beam power, provided by using the electron string mode of operation: 50 W pulse power and about 10 W average power. Other possible application of ESIS could be its use in injection complexes of synchrotrons and cyclotrons for cancer therapy. Slow and fast extraction of C4+ and C6+ beams from Krion-2 ESIS were preliminary studied towards ESIS optimization for medical accelerators requirements.

The reflex mode of EBIS operation in certain conditions (a special construction of electron gun and electron reflector, a precise axial symmetry of solenoid with a strong magnetic field, a sufficient injected electron current at a certain energy which provide a sufficient density of accumulated electrons in EBIS drift space) leads to formation of the so-called electron string state of a pure one component electron plasma. It was shown experimentally, that electron strings are stable in a broad range of operational conditions to be used for production of highly charged ions in electron string ion sources (ESIS) similarly to streaming electron beams in EBIS. The EBIS “Krion-2” in the string mode of operation, by means of which electron string formation was first observed and majority of experimental information on its features has been obtained, was recently used as the source of highly charged ions on the Nuclotron accelerator facility. During two runs in June 2002 and June 2003 N6+, N7+, Ar16+ and Fe24+ ions of 300, 350, 200, and 150 μA pulse current, respectively, produced by this source were accelerated to relativistic energies and used in several experiments. During the runs the source situated on the high voltage platform of the LINAC preinjector worked mostly in an automatic mode of operation and showed high stability and reliability.

The scheme of an internal injection of Au atoms into the working space of the "Krion-2" electron string ion source (ESIS) was applied and tested. In this scheme Au atoms are evaporated from the thin tungsten wire surface in vicinity of the source electron string. Ion beams with charge states up to Au51+ were produced. Ion-ion cooling with use of C and O coolant ions was studied. It allowed increasing of the Au51+ ion yield by a factor of 2. Ions of Kr up to charge state 28+ were also produced in the source. Electron strings were first formed with injection electron energy up to 6 keV. Methods to increase the ESIS ion output are discussed.

During recent years developments for electron-beam ion sources (EBIS) in the classic mode of operation were continued. Additionally, studies of EBIS in the reflection mode of operation were begun. This mode of operation leads to the formation of a dense electron string, where electron current flows in a radial direction across the magnetic field of a solenoid. This string is used for production of highly charged ions in RefEBIS.