Plasma-Wakefield Accelerator Group

Abstract:

Phase retrieval problem occurs in a number of areas in physics and is the subject of continuing investigation [1-15]. One dimensional case, for example, an electron bunch temporal profile reconstruction, is particularly challenging. Frequently applied methods, are reliable if the Blaschke phase [10-12] contribution is negligible. This, however, is neither known a priori nor can it be assumed for an arbitrary profile. In this work we present a novel algorithm with additional constraints which gives reproducible, stable solutions for profiles, both artificial and experimental, otherwise unresolved by existing techniques.

Abstract:

The PXIE accelerator [1] is the front-end test stand of the proposed Proton Improvement Plan (PIP-II) [2] initiative: a CW-compatible pulsed H-superconducting RF linac upgrade to Fermilab's injection system. The PXIE Ion Source and Low-Energy Beam Transport (LEBT) section are designed to create and transfer a 1-10 mA H-beam, in either pulsed (0.001-16 ms) or DC mode, from the ion source through to the injection point of the RFQ. This paper discusses the range of diagnostic tools - Allison-type Emittance Scanner, Faraday Cup, Toroid, DCCT, electrically isolated diaphragms - involved in the commissioning of the beam line and preparation of the beam for injection into the RFQ.

Abstract:

A series of Beam Position Monitors (BPMs) will be installed at the Front End Test Stand (FETS) at RAL as part of the 3 MeV Medium Energy Beam Transport (MEBT). The BPMs analyse 2 ms long, 60 mA beam pulses delivered to the MEBT by a 324 MHz Radio Frequency Quadrupole (RFQ). Initial linearity and resolution measurements from the prototype button BPMs are shown. The development of the algorithm for the processing of the BPM signals using a commercial PXI-based FPGA card is discussed and initial measurements of the electronics and signal processing are presented. The test-rig used to characterise each BPM and further develop the processing algorithm is described. The position and phase are measured several times throughout the duration of each pulse, and the measurements are made available via an EPICS server.

Abstract:

A CW-compatible, pulsed H- superconducting RF linac (a.k.a. PIP-II) is envisaged as a possible path for upgrading Fermilab's injection complex [1]. To validate the concept of the front-end of such machine, a test accelerator (a.k.a. PXIE) [2] is under construction. The warm part of this accelerator comprises a 10 mA DC, 30 keV H- ion source, a 2m-long LEBT, a 2.1 MeV CW RFQ, and a MEBT that feeds the first cryomodule. In addition to operating in the nominal CW mode, the LEBT should be able to produce a pulsed beam for both PXIE commissioning and modelling of the front-end nominal operation in the pulsed mode. Concurrently, it needs to provide effective means of inhibiting beam as part of the overall machine protection system. A peculiar feature of the present LEBT design is the capability of using the ∼1m-long section immediately preceding the RFQ in two regimes of beam transport dynamics: neutralized and space charge dominated. This paper introduces the PXIE LEBT, reports on the status of the ion source and LEBT installation, and presents the first beam measurements.

Abstract:

A 4 m-long, 324MHz four-vane RFQ, consisting of four coupled sections, has been designed for the Front End Test Stand (FETS) at RAL in the UK. A novel design method, integrating the CAD and electromagnetic design of the RFQ with beam dynamics simulations, was used to optimise the design of the RFQ. With the design of the RFQ fixed, the focus has been on optimising the transmission of the RFQ at 3MeV and matching the output of the FETS Low Energy Beam Transport (LEBT) to the RFQ acceptance. Extensive simulations have been carried out using General Particle Tracer (GPT) to map out the acceptance of the FETS RFQ for a 65 keV H- input beam. Particular attention has focussed on optimising the simulations to match the optimised output of the FETS Penning-type H- ion source. Results are presented of the transverse phase space limits on the RFQ input acceptance in both the zero current and full space charge regimes.