Understanding Secondary Emission Processes and Beam Matter interactions for Optimization of Diagnostic Wire Grid System in Particle Accelerators

Thin targets, in form of foils, stripes or wires, are widely used in beam instrumentation to measure various beam parameters, such as intensity, position and size. All these monitors can differ in geometry and material. Depending on beam parameters such as intensity, energy, transverse and longitudinal size, the detector can suffer thermomechanical stresses. This can potentially perturb the measurement accuracy and degrade the integrity of the detector. The core of this contribution presents the development of a finite-difference model, developed to simulate the particle-detector interactions and predict the detector material heating and damage. This work has been mainly performed in the context of LINAC4, which is the first accelerator at CERN’s accelerator chain. Some other facilities (CERN PS Booster, CERN SPS, GSI facility) will also be mentioned in this document. However, the majority of the studies and conclusions will refer to the energy range (45 keV - 160 MeV) and detector types available at LINAC4 (SEM grids and wire scanners). To understand the purpose, functionality and limitation of thin target detectors, this document introduces the basic principles of transverse beam dynamics and beam matter interactions, focusing on processes such as: energy deposition, secondary electron emission (SEE), electron backscattering, etc. The thermal model implemented during this work (PyTT) to simulate the thermal evolution of thin target detectors is presented and detailed discussed. To assess the reliability of the simulated results, an experiment performed at LINAC4 is presented. This experiment heavily relied on the theory of thermionic emission to indirectly measure the temperature of the detectors during operation. The performance of the simulation tool is discussed, in terms of results sensitivity to parameter uncertainty. Uncertainties in material parameters, such as the emissivity, yield uncertainties in simulation results. To improve our knowledge of the emissivity values of thin tungsten wires, an experimental setup, based on the calorimetric method, was implemented for this work. The experimental setup, emissivity calculation and results are detailed described. Some examples of how this work has been useful for CERN operations, and other facilities (like GSI) are presented in these pages. This include, beam power limit calculations for the CERN Linac4 and SPS diagnostics, beam intensity and profile measurements at LINAC4, thin foi

Development of a Thermal Response Model for Wire Grid Profile Monitors and Benchmarking to CERN LINAC4 Experiments

The operation of wire grids as beam profile monitors, both in terms of measurement accuracy and wire integrity, can be heavily affected by the thermal response of the wires to the energy deposited by the charged particles. A comprehensive model to describe such interaction has been implemented including beam induced heating, all relevant cooling processes and the various phenomena contributing to the wire signal such as secondary emission and H⁻ electron scattering. The output from this model gives a prediction of the wire signal and temperature evolution under different beam conditions. The model has been applied to the wire grids of the CERN LINAC4 160 MeV H-beam and compared to experimental measurements. This successful benchmarking allowed the model to be used to review the beam power limits for operating wire grids in LINAC4

Experimental Investigation of Gold Coated Tungsten Wires Emissivity for Applications in Particle Accelerators

The operation of wire grids and wire scanners as beam profile monitors can be heavily affected, both in terms of measurement accuracy and wire integrity, by the thermal response of the wires to the energy deposited by the charged particles. Accurate measurements of material emissivity are crucial, as Radiative Cooling represent the most relevant cooling process. In this work, we present a method for emissivity measurements of gold-coated tungsten wires based on calorimetric techniques. The dedicated electrical setup allowed allowed transient and steady state measurements for temperatures up to 2000 K. A theoretical description of the measurement technique will be followed up by the electrical set up description and a detailed discussion about the measured results and uncertainties

Secondary Electron Emission (SEM) Grid for the FAIR Proton Linac

New SEM-Grid has been developed for FAIR Proton Linac, the instrument consists of 2 harps fixed together in an orthogonal way. This SEM-Grid will provide higher resolution and accuracy measurements as each harp consists of 64 tungsten wires 100 micro-meter in diameter and 0.5 mm pitch. Each wire is fixed to a ceramic PCB with an innovative dynamic stretching system, this system assures wire straightness under typical thermal expansion due to beam heat deposition. Electric field distribution has been performed, 3 main parameters have been optimized, wires distribution, quantity of polarization electrodes and distance between electrodes and wires. The design and production of the SEM-Grids have been performed by the company Proactive R&D; that has count on the expertise of ESS-Bilbao to define safe operation limits and signal estimation by means of a code developed specifically for this type of calculations. Preliminary validations of the first prototypes shown good values of electric field behaviour signal. After additional beam test validations to be performed on June 2022, final series of the SEM-Grid will be produced and installed on FAIR proton LINAC

PSB H0-H⁻ Monitor Calibration and Commissioning

During the LHC Long Shutdown 2 (LS2), the H⁻ LINAC4 replaced the proton LINAC2 as Proton Synchrotron Booster (PSB) injector. In each of the four PSB rings, the injection region was upgraded to accommodate the necessary elements for a proper H⁻ charge exchange injection systems. Four internal beam dumps (one per ring), installed downstream the stripping foil, block the unstripped H⁻ particles not injected in the ring. The H0-H⁻ monitors consists in 4 titanium plates placed few centimetres upstream of the dump, intercepting partially stripped H⁰ or not stripped H⁻ ions. They allow a continuous monitoring of the stripping efficiency and, connected to an interlock system, block the injection process in case of heavy degradation or breakage of the foil, which would heavily damage the dumps. The contribution will focus on the commissioning and operation these new systems. This will include the calibration campaigns, performed by comparison to beam current transformers during special periods with low intensity beams and no stripping foils. During normal operation it was already possible to monitor stripping inefficiencies below 1% and compare different beams and stripping foil types

Design and Test of Beam Diagnostics Equipment for the FAIR Proton Linac

A dedicated proton injector Linac (pLinac) for the Facility of Antiproton and Ion Research (FAIR) at GSI, Darmstadt, is currently under construction. It will provide a 68 MeV, up to 70 mA proton beam at a duty cycle of max. 35µs / 2.7 Hz for the SIS18/SIS100 synchrotrons, using the existing UNILAC transfer beamline. After further acceleration in SIS100, the protons are mainly used for antiproton production at the Pbar ANnihilations at DArmstadt (PANDA) experiment. The Linac will operate at 325 MHz and consists of a novel so called ‘Ladder’ RFQ type, followed by a chain of CH-cavities, partially coupled by rf-coupling cells. In this paper we present the beam diagnostics system for the pLinac with special emphasis on the Secondary Electron Emission (SEM) Grids and the Beam Position Monitor (BPM) system. We also describe design and status of our diagnostics testbench for stepwise Linac commissioning, which includes an energy spectrometer with associated optical system. The BPMs and SEM grids have been tested with proton and argon beam during several beamtimes in 2022. The results of these experiments are presented and discussed

Commissioning of the Beam Instrumentation for the Half Sector Test in Linac4 with a 160 MeV H⁻ Beam

In the framework of the LHC Injector Upgrade (LIU) project, the Proton Synchrotron Booster (PSB) will be extensively modified during the Long Shutdown 2 (LS2, 2019-2020) at CERN [1]. This includes a new injector, Linac4, which will provide a 160 MeV H⁻ beam and a complete new injection section for the PSB composed essentially of a chicane and a stripping foil system. The equivalent of half of this new injection chicane, so-called Half-Sector Test (HST), was tempo-rarily installed in the Linac4 transfer line to evaluate the performance of the novel beam instrumentation, such as, stripping foils, monitoring screens, beam cur-rent transformers, H⁰/H⁻ monitor and dump, beam loss monitors, and beam position monitors. The results of the instrumentation commissioning of the HST are presented in this paper

Commissioning of the Stripping Foil Units for the Upgrade of the PSB H⁻ Injection System

The PSB will be extensively upgraded during the next long shutdown of the CERN accelerator complex, to double the brightness of the stored beams. The existing multi-turn injection will be replaced by a charge exchange system designed for the 160 MeV hydrogen ions provided by Linac4. Part of the injection equipment has been temporarily installed along the Linac4-to-PSB transfer line and tested with beam. This allowed to gain experience with the system, test the related diagnostics and benchmark calculations with measurements. An additional permanent stripping foil test stand is also installed right after the Linac and will be used to characterise new foils for possible future applications. The main outcomes, issues and applied or planned mitigations are presented for both installations

Commissioning the New CERN Beam Instrumentation Following the Upgrade of the LHC Injector Chain

The LHC injectors Upgrade (LIU) program has been fully implemented during the second long shutdown (LS2), which took place in 2019-20. In this context, new or upgraded beam instrumentation was developed to cope with H⁻ beam in LINAC4 and the new Proton Synchrotron Booster (PSB) injection systems which would provide high brightness proton beams in the rest of the injector complex. After a short overview of the newly installed diagnostics, the main focus of this paper will move to the instruments already commissioned with the beam. This will include LINAC4 diagnostics, the PSB H⁰/H⁻ monitor, the PSB Trajectory Measurement System, and the PS beam gas ionization monitor. In addition, particular emphasis will be given to the first operational experience with the new generation of fast wire scanners installed in all injector synchronous

Beam Instrumentation for the CERN LINAC4 and PSB Half Sector Test

The construction, installation and initial commissioning of CERN's LINAC4 was completed in 2016 with H⁻ ions successfully accelerated to its top energy of 160 MeV. The accelerator is equipped with a large number of beam diagnostic systems that are essential to monitor, control and optimize the beam parameters. A general overview of the installed systems and their functional specifications will be followed by a summary of the most relevant results. This includes transverse profile monitors (wire scanners, wire grids and a laser profile monitor), beam position and phase monitors (whose ToF measurements were essential for adjusting RF cavity parameters), beam loss monitors, beam current transformers and longitudinal beam shape monitors. This contribution will also cover the beam instrumentation for the so-called PSB Half Sector Test, which has been temporarily installed in the LINAC4 transfer line to study H⁻ stripping efficiency. At this facility it was possible to test the new H⁰/H⁻ beam current monitor, designed to monitor the stripping efficiency and an essential element of the beam interlock system when the LINAC4 is connected to the PSB in 2019