Learn more about our experimental accelerator facilities at the Louisiana Accelerator Center.
The Pelletron Accelerator
The accelerator is of so-called tandem type. Negative ions enter an acceleration tube and are accelerated in an electric field to a high voltage terminal. In this first acceleration step they gain energy eV where V is the voltage on the terminal. In the terminal they pass through a thin layer of material where they lose electrons and become positively charged. The material can be a foil or, as in our case, a gas. Multiple electrons can be lost giving rise to ions Xn+ where X denotes the element and n is the charge state. The ions are then accelerated in a second acceleration tube down to ground potential gaining an energy neV. Then the total kinetic energy of the ions is: E = e([n+1]V + Vinj). Here, Vinj is the accelerating potential of the injector. For ions of heavy elements such a Cl, many electrons can be stripped off and n~6. In this case the maximum accelerated ion energy for 1.7 MV terminal potential is: E = 7*1.7+0.03 = 11.93 MeV.
The term “Pelletron” refers method used to charge the high-voltage terminal. This is similar to a van de Graaff generator but where the rubber belt is replaces with chains comprising metal pellets joined with insulating links. An electric motor drives the chains and charge is sprayed on and removed rom them them from high voltage power supplies (Up- and Down-charge).
The potential on the terminal needs to be precisely controlled to 10 V or so. In order to do this a corona discharge is used. By using a high voltage amplifier the corona current can be controlled on a time scale of ms. This bleeding charge via the corona current using feedback circuit the terminal potential can be precisely controlled.
In order to prevent an electrical breakdowns due to the high voltage the entire accelerator is placed in a pressure tank containing pure SF6. During accelerator operation the gas is passed over activated alumina to remove reactive breakdown products. In addition water-cooled heat exchangers are used to remove heat dissipated by the drive motor operation.
The purpose of analysis/switching magnet is to ensure ion with the desired energy, mass and charge state are directed to the experimental stations. There are six beam outlet ports at ±15, ±30 and ±45 degrees. The analysis is a dipole magnet. The magnetic field B is approximately proportional to the excitation current. The ions that pass through the magnet to a given beam line follow path with a precise bending radius ρ that is different for each outlet port. The bending radius is:
Where, M, E and n are the ion mass, energy and charge state, respectively. It follows that by changing the excitation current ions with different mass, energy and charge state can be selected. To precisely measure the magnetic field B a magnetic resonance probe is used.
The Vacuum System
The ion sources are individually pumped with turbo-molecular pumps. The low-energy and high energy (including switching magnet) beam-lines are also pumped with a turbo-pump. The beam-lines leading to the experiment stations are individually isolated with gate valves. Ionization gauges are used measure the pressure in the different regions of the accelerator and beam lines. Gate valves operated by compressed air are used to isolate the accelerator in the event of power failure.
Low-energy negative ion inflection
The Pelletron Accelerator at the Louisiana Accelerator Center is has three ion sources that provide negative ions for injection into the tandem accelerator. The beam from the injector is selected by means of an inflection magnet. After inflection a Faraday cup and electrostatic beam steering is used for beam optimization and conditioning.
Negative ions can be produced from almost all elements in the periodic table with the exception of Ne, Ar, Kr, Xe because their low electron affinities. Nitrogen does not form negative ions, however it can be accelerated as NH2- ions from a mixture of 5% N2 in H2 using a duoplasmatron or CN- from graphite and GaN using a SNIC source. The molecules are then fragmented by the stripper in the high voltage terminal of the accelerator.
1. RF ion source for negative He ions
A NEC Alfatros ion source is used to produce He- ions. These have a short lifetime (410 µs) which is sufficiently long to allow them to be transported to and accelerated in the tandem accelerator before undergoing electron stripping in the high voltage terminal. The ion source uses a RF ion source to produce He+ ions which then capture two electrons during passage through Rb vapor according to:
He+ + Rb ➛ He0 + Rb+; He0 + Rb ➛ He- + Rb+
RF power is provided by a ~100 MHz class B oscillator that is coupled capacitativly to the He plasma via metal electrodes on the outside of the quartz bottle.
2. Duoplasmatron ion source for gases
A NEC duoplasmatron is used to produce negative ions from gas feedstock such as H2, O2 etc. A plasma is produced by electron impact ionization of the feedstock gas using a heated tungsten filament as a ion source. A axial magnetic field is used to concentrate the plasma close to an aperture. Negative ions are created close to the edge of the plasma by electron capture by neutral atoms. The negative ions that impinge on the aperture hole are extracted from the plasma by electric field.
3. SNICS ion source
For ions from solid elements the SNICS source is used to produce negative ions by sputtering. Cs vapor is ionized on a hot tungsten surface (the ionizer) and the ions accelerated to the sputter target made of the feedstock materials. The energetic ions create a cascade of energetic displaced target atoms when they impinge on the target. Some of these target atoms will be directed backwards and if they have sufficient energy to overcome the surface binding potential they are ejected (sputtered) from the material. The Cs atoms from the vapor will condense on the target surface creating on monolayer or so of Cs atoms giving the surface a low work function. Electrons captured from the Cs by the sputtered atoms will become negatively charged creating negative ions. This ion source can be used to produce ions of most elements that can be produced as a pure solid element (C, Mg, Si, Cu, Ag, Au) as well as elements in solid compounds (H- from TiH, O from Al2O3, F from NaF, Cl from AgCl etc.)