Chapter Experimental Setup 32 _______________________________________________________________________________________________________ __________________________ Chapter __________________________ Experimental Setup 3.1 CIBA Facilities CIBA consists of two laboratories, each housing an accelerator. A single-ended 2.5 MV AN-2500 Van de Graaff accelerator from High Voltage Engineering (HVE) is housed in the auxiliary laboratory, providing ion beams to a single beamline leading to a NEC RC43 analytical endstation which allows for broad-beam conventional RBS and PIXE research. In the main laboratory (Fig. 3.1), three beamlines are attached to a single-ended 3.5 MV SingletronTM accelerator from High Voltage Engineering Europa (HVEE) [33] that provides ion beams which are brighter and more stable than those from the belt-driven Van De Graaff accelerators. At the 10° beamline, a state-of-theart proton beam writer [34] is present, which allows for direct-write micro-machining while a nuclear microscope [35] is attached to the 30° beamline which is used for advanced materials analysis and elemental mapping of biological samples. The 45° beamline leads to the HRBS endstation, which is the focus of this thesis. 32 Chapter Experimental Setup 33 _______________________________________________________________________________________________________ 90°° Analyzer magnet Steerer table Accelerator Aperture Analyzer table Switching magnet 45°° HRBS Endstation Nuclear Microscope Proton Beam Writer Fig. 3.1 Layout of the accelerator and beamlines 3.2 SingletronTM accelerator Fig. 3.2 Schematics of the accelerator system. Source: [33] Chapter Experimental Setup 34 _______________________________________________________________________________________________________ 3.2.1 RF Ion source Fig. 3.3 The RF Ion source. Source: [36] The RF ion source model 173 (Fig. 3.3) is used within the Singletron as a heavy duty profile source for proton, alpha and oxygen beams. The neutral gas is released into the ion source, where an RF voltage is applied. Electrons within the gas atoms are excited into oscillation by the RF field, quickly gaining enough kinetic energy to cause ionization and forming a plasma. Positive ions from the plasma are pushed out of the source by an applied electric field from the probe, and the resultant ion beam is focused by applied voltage at the extraction electrode. A set of permanent magnets apply an axial magnet field to create a toroidal motion of electrons that concentrates the plasma near the extraction electrode, increasing the ion beam current. 3.2.2 High voltage power supply The high voltage power supply (Fig 3.4) provides the terminal voltage which accelerates the ions extracted from the ion source along the accelerator tube. The Chapter Experimental Setup 35 _______________________________________________________________________________________________________ driver electrodes are connected to RF oscillator coils which generate an AC voltage which is transferred to the rectifier stack by means of capacitative coupling. Fig. 3.4 The high voltage power supply. Source: [33] The RC time constant of the stack is much larger than the period of the RF generated by the voltage driver, so that an equilibrium high positive voltage is generated at the top of the stack. Alternate capacitors along the stack are charged in the same electrical orientation during each half-cycle of the AC, with the rectifiers ensuring a net flow of current towards the positive accelerator terminal over the AC cycles. Potential differences across each charged capacitor along the stack eventually add up to the terminal voltage. A generating voltmeter (GVM) measures the potential of the terminal relative to the laboratory ground, while a capacitative pickup unit (CPU) measures the ripple of the terminal voltage and feeds the signal back to the voltage regulator controlling the voltage driver. The voltage driver then adjusts the terminal voltage accordingly to reduce the ripple, improving the energy stability of the beam. In the event of a tank spark, the corona/spark interlock will trigger a warning on the control computer and the voltage regulator will switch off the terminal voltage. Chapter Experimental Setup 36 _______________________________________________________________________________________________________ 3.2.3 High voltage insulation and Electron suppression The accelerator tank is filled with a heavy non-toxic insulating gas SF6 at a pressure of bar. SF6 has a dielectric strength approximately 2.5 to times that of air, and serves to prevent corona buildup and tank sparks. Other forms of electrical insulation are the vacuum and glass insulation within the accelerator tube, the plastic support members in the power supply and a variety of insulating cable covers. Also, electron suppression is employed via small permanent magnets within the accelerator to reduce the level of emitted radiation. 3.3 Beam steering and stablization The beam output from the accelerator is adjusted at the steerer table (Fig. 3.5(a)) by electrostatic steerers along x and y axes along the x-y plane normal to the beam (z) direction and subsequently passes through a beam defining slit. A circular aperture checks the exact beam position and defines the maximum beam diameter just before the 90° analyzing magnet. The desired ion species is then selected by the magnet to pass into the stabilizer slit at the analyzer table (Fig. 3.5(b)). The currents created by the beam hitting on the left and the right of the stabilizer slit are monitored, and the slit assembly is connected via a feedback loop to the voltage regulator in the accelerator high voltage power supply. Any instability of the ion beam will cause changes in the slit currents, which in turn triggers a compensation response from the terminal voltage regulator in the accelerator, creating a stabilizing effect on the beam. Chapter Experimental Setup 37 _______________________________________________________________________________________________________ (a) (b) Fig. 3.5 (a) The steerer table and (b) the 90° analyzing magnet on the analyzer table. Source: [33] On both steerer and analyzer tables, a beam profile monitor (BPM) and a faraday cup measure the beam profile along the x-y plane and the total beam current respectively. This facilitates the proper steering and focusing of the beam before it enters the switching magnet to be steered into one of the beamlines. . table and (b) the 90° analyzing magnet on the analyzer table. Source: [33] On both steerer and analyzer tables, a beam profile monitor (BPM) and a faraday cup measure the beam profile along. approximately 2. 5 to 3 times that of air, and serves to prevent corona buildup and tank sparks. Other forms of electrical insulation are the vacuum and glass insulation within the accelerator tube,. auxiliary laboratory, providing ion beams to a single beamline leading to a NEC RC43 analytical endstation which allows for broad-beam conventional RBS and PIXE research. In the main laboratory