This paper describes the ramp up of the beam power for the Spallation Neutron Source by ramping up the pulse length, the repetition rate, and the beam current emerging from the H(-) source. Starting out with low repetition rates (< or = 10 Hz) and short pulse lengths (< or = 0.2 ms), the H(-) source and low-energy beam transport delivered from Lawrence Berkeley National Laboratory exceeded the requirements with almost perfect availability. This paper discusses the modifications that were required to exceed 0.2 ms pulse length and 0.2% duty factor with acceptable availability and performance. Currently, the source is supporting neutron production at 1 MW with 38 mA linac beam current at 60 Hz and 0.9 ms pulse length. The pulse length will be increased to approximately 1.1 ms to meet the requirements for neutron production with a power between 1 and 1.4 MW. A medium-energy beam transport (MEBT) beam current of 46 mA with a 5.4% duty factor has been demonstrated for 32 h. A 56 mA MEBT beam current with a 4.1% duty factor has been demonstrated for 20 min at the conclusion of a 12-day production run. This is close to the 59 mA needed for 3 MW neutron productions. Also notable is the Cs(2)CrO(4) cesium system, which dispenses approximately 10 mg of Cs during the startup of the ion source, sufficient for producing the required 38 mA for 4 weeks without significant degradation.
Abstract:The ion source and Low-Energy Transport (LEBT) system that will provide H -ion beams to the Spallation Neutron Source (SNS)** Front End and the accelerator chain have been developed into a mature unit that fully satisfies the operational requirements through the commissioning and early operating phases of SNS. Compared to the early R&D version, many features of the ion source have been improved, and reliable operation at 6% duty factor has been achieved producing beam currents in the 35-mA range and above. LEBT operation proved that the purely electrostatic focusing principle is well suited to inject the ion beam into the RFQ accelerator, including the steering and pre-chopping functions. This paper will discuss the latest design features of the ion source and LEBT, give performance data for the integrated system, and report on commissioning results obtained with the SNS RFQ and Medium-Energy Beam Transport (MEBT) system. Prospects for further improvements will be outlined in concluding remarks. INTRODUCTIONBerkeley Lab has just completed building the linac injector (Front End, FE) for the Spallation Neutron Source project (SNS), and the commissioning of the entire system is proceeding. The main subsystems are the H -ion-source, the low-energy beam-transport system (LEBT), the 2.5-MeV radio-frequency quadrupole (RFQ) accelerator, and the medium-energy beam-transport system (MEBT). Ion source and LEBT are the subject of this paper; their task is to create a 65-keV, 38-mA ion beam, to match and steer it into the RFQ, and to pre-chop it into mini-pulses of about 600 ns duration. The nominal duty factor is 6%, with 1-ms macro-pulse length and 60-Hz repetition rate.Based upon the main design features of the SSC ion source, 1 an R&D version of the SNS ion source was built first to demonstrate the viability of the chosen approach, utilizing an rf driven discharge inside a multicusp plasma generator with magnetic filter, cesium enhancement, and electron suppression at low energy.2 This source version did not allow implementing cesium enhancement and electron suppression at the same time, but both features were proven to work satisfactorily in separate tests.
The Spallation Neutron Source operates reliably at 1.2 MW and will gradually ramp to 1.4 MW. This paper briefly recalls some of the struggles when the unprecedented project was started and ramped to 1 MW over a 3½ year period. This was challenging, especially for the H ion source and the low-energy beam transport system, which make up the H injector. It took several more years to push the H injector to the 1.4 MW requirements, and even longer to reach close to 100% injector availability. An additional breakthrough was the carefully staged, successful extension of the H source service cycle so that disruptive source changes became rare events. More than 7 A·h of extracted H ions have been demonstrated with a single source without maintenance, more than twice the single-source quantity of ions produced by any other high-current H accelerator facility. Achieving the 1.4 MW requirements with close to 100% availability and record-breaking source service cycles were the basis for the 2017 Brightness Award.
The Spallation Neutron Source H − source on the ion source test stand is being used to study the emittance of the H − -ion beam injected into the SNS radiofrequency quadrupole ͑RFQ͒. The emittance measurements are performed with a LBNL Allison scanner that underwent several modifications. The slit width was optimized to improve the signal-to-noise ratio. In addition, the electric deflector plates were replaced with plates featuring a staircased surface. This modification is shown to suppress over 99% of ghost signals generated by the beam hitting the deflector plates. Both modifications, combined with noise suppression measures and a self-consistent analysis, yield highly accurate results. Measured emittances are presented as a function of the ion-beam current.
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