Measurements Of Emittance And Ion Species Fraction Of Low-Energy High-Current Pulsed Ion Beams

A recent hotspot of the ion accelerator development is the radio frequency quadrupole (RFQ) accelerator, which has been widely used in large scientific facilities, such as spallation neutron source, accelerator-driven system for clean nuclear power production and radioactive nuclear beam facility, as well as small accelerator based neutron sources. This paper introduced the international and inland progress on the study of RFQ accelerator, electron cyclotron resonance (ECR) ion source, low energy beam transmission (LEBT) and emittance measurement. The relevant development at the Institute of Heavy Ion Physics (IHIP) of Peking University was also introduced. At present, neutron radiography equipment based on the RFQ accelerator is being developed in IHIP, which requires that ECR deuterium ion source could extract enough beam current at the rated extraction voltage with high ion fraction of D~+ and low emittance, and the LEBT should have high transmission efficiency, too. This dissertation focuses on the establishment of methods for measuring and data processing of emittance and ion fraction with high-current pulsed ion beam. The experimental study on emittance and ion fraction of ECR ion source and LEBT system as the injector of RFQ was carried out, which can provide sufficient basis to design the ECR ion source and LEBT system rationally.Emittance measurement unit (EMU) is the main beam diagnostic equipment. To investigate the emittance of ECR ion source and LEBT system, an Allison emittance scanner was developed at Peking University. This dissertation established the method of emittance measurement especially for high-current pulsed ion beam. In order to avoid the problems introduced by pulse loss and beam instability during the process of sampling, high sampling rate (6kHz) was used and multi-pulses can be sampled at every stepping voltage of electrical scanning, so every data point on the phase diagram was calculated by averaging multi-sets of data. Considering the higher background and obvious beam halo of the high-current ion beam, a method with subtracting background and then setting threshold value is used for data processing, which can gives more accurate emittance results. On these bases, the software of this Allison emittance scanner was completed, which includes system control, emittance measurement and data-processing. The commissioning was carried out, then the vacuum of measurement box was improved and the shielding and grounding problem was solved.The investigation on ECR proton ion source PMECR-II showed that the normalized root-mean-square (rms) emittance increases with beam current, but the emittance is not sensitive to the change of extraction voltage, also there is an optimum gas inlet for minimum emittance, which is about 0.25-0.35Ï€.mm.mrad. Plasma electrode with extraction hole of 5mm diameter gives smaller emittance than that with 7mm diameter. The emittance phase diagrams of ions with different mass were measured after solenoid lens, and it is confirmed that the Allison emittance scanner can avoid beam overlap effectively, which is often introduced by the neighboring slits in multi-slits method.Increasing D~+ fraction has great significance. Higher D~+ ion fraction (or proton fraction) means that the required total beam current can be reduced, and the additional beam load of unwanted ions can also be reduced. The pulse shape differences among different ion species were studied and then the ion fraction measurement method of ECR ion source and LEBT was established. The experiments showed that the proton fraction reached an equilibrium value in several hundred microseconds, which makes the average proton fraction during whole pulse lower than the flat top proton fraction. So it might be worth while using longer pulse and throwing away its initial part, when the beam is injected into a RFQ accelerator.The experiment results indicated, the average proton fraction increased with the increasing microwave power, but when the microwave power increased to a certain level, the proton fraction will reach its saturation value and no longer increase with microwave power. The average proton fraction increased with the increasing pulse width. Under a certain microwave power, there was an optimum gas inlet for higher proton fraction. Materials of quartz, aluminum, stainless steel and ceramic were used as the discharge chamber inner liner to study the influence of different materials on proton fraction, discharge efficiency and beam stability. The experiments showed that ceramic material gave the highest net proton beam current, and the proton fraction with ceramic inner liner could reach 94%.This dissertation also investigated the emittance and O~+ ion fraction for ECR oxygen ion source. Its normalized rms emittance was 0.12?0.16Ï€.mm.mrad, and the beam waist could be shifted from 80mm in front of the RFQ entrance to 80mm behind it. Under appropriate gas inlet and microwave power, the O~+ ion fraction could reach 85%, and the net O~+ ion current could be 5mA.The main innovation and distinctive features of this dissertation are: a) Establishing the method of emittance measuring and data-processing especially for high-current pulsed ion beam. b) Establishing the method of average/flat-top ion fraction measuring and data-processing especially for pulsed ion beam.This dissertation also discussed the existent problems in experiments and the proposals for further study.