Laboratori Nazionali del Sud

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Accelerators

Ion Sources

ION Sources at LNS

INFN – LNS host two high performance injectors for the K800 Superconducting Cyclotron. They are ion sources based on the ECRIS – Electron Cyclotron Resonance Ion Sources (figure 1) concept. ECRIS are plasma-based ion sources feeding particle accelerators with highly charged ions. The two sources installed at LNS are called SERSE and CAESAR. SERSE represented between 90s and early 2000 (the first plasma was ignited on June 13th, 1998) the state of the art of ECRIS worldwide, and it was among the first ones to be equipped by a fully superconducting magnetic system. It is able to produce more that 100 emA of O7+ ion current, and similar currents of heavier and much more ionized ions such as Xe27+. CAESAR is an hybrid source equipped by a pair of copper coils and a permanent magnet hexapole. It is used for less demanding beam currents and charge states, especially for protons or light ions like O and/or C, N, Ne. Charge states around O6+ can be produced up to several tens of emA.

In ECRIS-type machines, the ions are extracted from a high density and high temperature plasma (ne~1010-1013 cm-3, Te~0.1-100 keV) generated by means of the Electron Cyclotron Resonance heating. The plasma is excited inside a cylindrical metallic chamber by microwaves (2.45-28 GHz) and there confined by a MHD stable B-min configuration (an hexapole superimposed to a simple mirror structure), as depicted in figure 2. The importance of highly performing ECRIS development in nuclear physics is due to the specific energy scaling in Cyclotrons or Syncrotrons, which goes like (q/A)2, being q the ion charge state.

FIGURE 1 (a) Typical layout of an ECRIS. These sources are also usually equipped with an oven (not shown in the figure) for beam production from metallic elements. (b) B module: longitudinal trend.
SERSE ECR ion source at INFN-LNS
CAESAR ECR Ion Source at INFN-LNS

Operating frequency 14 and 18 GHz
Maximum radial field on the wall 1.1T
Maximum radial field on the wall 1.1 T
Maximum axial field (injection) 1.58 T
Maximum axial field (extraction) 1.35 T
Minimum axial field 0.4 T
Hexapole NdFeB made 1.1 T
Extraction Accel-Dec, 30kV/12kV Max
Plasma chamber St. steel or Al made
CAESAR operative parameters

O6+ 540 Kr22+ 66 Au30+ 20
07+ 208 Kr25+ 35 Au31+ 17
O8+ 62 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 Au42+ 0.03
SERSE typical currents at 18GHz operating frequency (1997-2000)

Ion Sources for the TANDEM accelerator

The TANDEM accelerator is fed by two sputtering-type ion sources whose working principle is summarized in the sketches below.

Simplified sketch (on the left) and detailed scheme of the Sputtering-type ion sources used at LNS as injector of the TANDEM accelerator.

Picture illustrating the two TANDEM sputtering-type ion sources and the high-voltage platform (up to 450 kV).

Caesium vapours produced by an oven fill an enclosed volume between the cathode and the heated ionizing surface. Cesium ions (ionized by the hot surface) are accelerated towards the cathode, sputtering it. Some materials will preferentially sputter negative ions; others, neutral or positive particles which pick up electrons as they pass through the condensed caesium, producing negative ions.

The availability of different beams depends in a crucial way not only on the source technology, but also on the material preparation and the technology of the cathode. The techniques developed along the years in synergy between ion beams production service and the target laboratory, have been then routinely adopted at LNS and allowed to create a wide “portfolio” of beams including Mg, Ca, Na.

Other beams include 15 μA of H, 2 μA of D,  around 1 μA of 6,7Li, 2 μA of 11B,  20 μA of 12C, 0.2 μA of 13C, 20 μA of O (the same amount for F), 1 μA of CN, 5 μA of Si, 0.25 μA of MgH and 0.55  μA of MgH3, 20 μA of S, 20 μA of Cu, 5 μA of Au, 1.5 μA of Ge.

Ion species

Current [μA]

H

15

D

2

6,7Li

1

11B

2

12C

20

13C

0.2

O

20

CN

1

Si

5

Ge

5

MgH

0.25

MgH3

0.55

S

20

Cu

20

Au

5

Na

0.15

Ion beams produced by the sputtering-type ion sources of the TANDEM accelerator

Plasma diagnostics developed at LNS for plasma based ion sources

Figure 3.  Scheme of the diagnostic tools needed for the complete characterization of ECRIS plasmas.

One of the limiting factors for the full understanding of Electron Cyclotron Resonance Ion Sources physics consists in the few types of diagnostics tools so far available for such compact machines. The “tailoring” of the electron energy distribution function, needed to optimize the ionisation process, as well as the suppression of plasma turbulence and instabilities will require a detailed – possibly space resolved – knowledge of the plasma properties in terms of density, electron and ion temperature, on-line charge state distribution, local magnetic field values.

A wide set of diagnostics tools spanning across the entire electromagnetic spectrum is mandatory: from microwave interferometry to X-ray spectroscopy. Some of these methods are already going to be implemented in ECRIS laboratories spread over the world, and specifically at INFN-LNS, which has played in the recent past a leading role in the field of plasma diagnostics for compact magnetic traps.

“Volume-integrated” X-ray spectroscopy in low energy domain (2-30 keV, by using SDD – Silicon Drift Detectors, 125 eV energy resolution at 5.6 keV) or high energy regime (>30 keV, by using HpGe – High Purity Germanium detectors with some hundreds of eV in energy resolution) have been developed since 2006, giving valuable results in terms of electron energy distribution function at the higher energy domains as a function of the magnetic field gradients in the confining trap. The explanation of the FTE – Frequency Tuning Effect, discovered at LNS in the early 2000s - has been possible by a strict synergy between modelling and X-ray diagnostics. High resolution and spatially-resolved X-ray spectroscopy made by quasi-optical methods (pin-hole cameras) has been already implemented and the result recently published. Millimetric spatial resolutions, with 145 eV energy resolution per pixel, have been achieved in the energy range 0.5-10 keV. INFN has additionally funded the design and development of an high resolution X-ray spectrometer based on the grating technique, nominally able to reach Dl/l=10-3 @ 565 eV of photon energy, that will be installed within the end of 2016 on the testbench called Flexible Plasma Trap (FPT).

General scheme and detailed design of the VESPRI microwave interferometer installed at INFN-LNS for the measurement of the plasma density in ECR-type ion sources.

Inspection of the plasma structure in a ECR-type ion source: a) optical observation; b) X-ray imaging performed by means of a pin-hole camera method.

Other plasma based ion sources developed at LNS

Tha Advanced Ion Source for Handrontherapy (AISHa) source is based on the latest theoretical developments proposed by the R&D group on ion sources, operating for over twenty years in the field of ECR sources and it presents several technological innovations with respect to the current ECR sources, that will allow better performance in terms of charge state, current and versatility of the ion species to be produced, while maintaining the characteristics necessary for the installation in a hospital environment.

The AISHA source is a new hybrid type ECR ion source, since the magnetic field necessary to confine the plasma is radially generated by a permanent magnets based hexapole, while four NbTi solenoids  ensure the axial magnetic confinement.

AISHA uses a compact cryostat that includes a cryocooler type apparatus to allow the h24 operation of the magnet without the supply of liquid helium from the outside.

The microwave injection system has been designed in order to be able to perform the frequency tuning up to very high power (~2 kW @18 GHz).

For its innovative features the source will open new prospects for R&D to the LNS as it will allow to generate a high charge state beam including Helium, Carbon beams and Li+ Be+ and B+ for biology and radiation studies.

Figure 1.  Layout of the superconducting source named AISHa, recently designed and assembled at INFN-LNS for the production of high intensity beams of light ions to be used in hadrontherapy.

VIS – Versatile Ion Source

The Versatile Ion Source (VIS) is based on permanent magnets producing an off-resonance microwave discharge at 2.45 GHz. It operates up to 75 kV without a bulky high voltage platform, producing several tens of mA of proton beams. The VIS source ensures long time operations without maintenance and high reliability in order to fulfil the requirements of the future accelerators. The construction of this source comes from the experience obtained through the TRIPS project. Compared to this, VIS presents a much more simplified extraction geometry and movable permanent magnets. All the devices for the remote control were placed at ground potential, thus leaving only the plasma chamber and the permanent magnets at high voltage; the compact dimensions have also helped to get a better and easier

Render View of the Versatile Ion Source designed and constructed at LNS.

PS-ESS – Proton Source for the European Spallation Source

The ESS accelerator is deemed to operate at different level of performances with a high reliability and this request has been the main driver of the proton source. 

The ESS energy of 2 GeV requires an average macro-pulse current of 62.5 mA (on target) to obtain a beam power of 5 MW.

The proton source is a compact microwave discharge ion source (MDIS) similar to the TRIPS and VIS sources developed at INFN-LNS Catania, and to the SILHI source of CEA Saclay. The beam from the ion source is transported through a low energy beam transport (LEBT) section to the radio frequency quadrupole (RFQ) for bunching and acceleration.

The source named Proton Source for ESS (PS-ESS)  was designed with a flexible magnetic system and a compact tetrode extraction system with the goal to minimize the emittance and the time needed for the maintenance operations. The ESS injector design has taken advantage of recent theoretical updates together with the new plasma diagnostics tools developed at INFN-LNS. The improved know-how will permit to fulfil the requirements of the ESS normal conducting front-end; the proton beam should be 74 mA which can be obtained with a total beam current of about 90 mA. The beam stability during the normal operations (in terms of current and emittance) shall be within ±3.5% as for pulse to pulse variation and ±2% of the beam current if averaged over a period of 50 us. The pulse duration is 2.86 ms with 14 Hz repetition rate. A reliability better than 95% is requested for the whole accelerator, thus meaning that the source reliability is expected to be greater than 99%.

 

The Flexible Plasma Trap

The experience gained over the last four years at INFN-LNS allowed to understand new mechanisms of plasma production, highlighting the main weaknesses of the previous ECR ion sources modelling. The needs of a more flexible magnetic field led to a different type of plasma trap, named Flexible Plasma Trap operating at INFN-LNS. The investigation of alternative plasma heating methods under different magnetic field configurations can be carried out by means of a plasma trap made of three solenoids which allow the tuning of the magnetic field profile. They will be studied with plasma and RF diagnostics placed parallel and perpendicularly to the B axis. The plasma chamber gives the ability to couple different waveguide inputs, including the possibility to have contemporary input of microwaves in two directions one perpendicular to the other and permit at the same time to host different type of diagnostics, either optical and X-ray diagnostics, microwave interferometry and RF analysers. New injection schemes can be designed and explored by pumping electromagnetic waves under proper injection propertiers, tuning the frequency in the range between 3 and 14 GHz, and using sophisticated modelling and diagnostics tools, like the pinhole camera and CCD.

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