Cherenkov light-based beam profiling for ultrarelativistic electron beams

Abstract

We describe a beam profile monitor design based on Cherenkov light emitted from a charged particle beam in an air gap. The main components of the profile monitor are silicon wafers used to reflect Cherenkov light onto a camera lens system. The design allows for measuring large beam sizes, with large photon yield per beam charge and excellent signal linearity with beam charge. The profile monitor signal is independent of the particle energy for ultrarelativistic particles. Different design and parameter considerations are discussed. A Cherenkov light-based profile monitor has been installed at the FACET User Facility at SLAC. We report on the measured performance of this profile monitor.

Introduction

For a number of accelerator facilities there is a need for precise diagnostic methods for measuring the transverse beam profile of ultrarelativistic electron beams with large transverse sizes, for example beams with large energy spread in a spectrometer line. These types of spectrometers may be required in various branches of advanced accelerator experiments like plasma wake field acceleration [1], laser plasma acceleration [2], and two-beam acceleration machines where drive beams are heavily decelerated with large energy spread, such as CLIC [3].

We present here a beam profile diagnostic method based on Cherenkov light generated by an electron beam traveling through air. The profile monitor works equally well for electrons and positrons. This type of diagnostic was first used for plasma experiments at the SLAC Final Focus Test Beam Facility [4], [5] and has been further developed for the spectrometer at the FACET User Facility [1], [6], as described in this paper.

The use of Cherenkov radiation for profile monitoring has the significant advantage over the more established use of optical transition radiation (OTR), in that the light yield per beam electron may be much larger. Cherenkov radiation in air generates on the order of 30 photons per electron per meter, in the optical range (shown below). In comparison the OTR energy spectrum, ${\mathit{dW}}_{\mathrm{otr}}/d\omega$, from relativistic electrons is approximately given by [7]

$$\frac{{\mathit{dW}}_{\mathrm{otr}}}{d\omega }\approx 4.9\times {10}^{-37}\ln \gamma$$

where γ is the Lorentz factor, which yields about 0.05 OTR photons per electron per surface unit for ultrarelativistic beams, in the optical range. By using enough path length to generate Cherenkov light, one may easily get a factor 100 stronger signal with a detector based on Cherenkov light compared to an OTR-based setup. For beams with large energy spread, the Cherenkov light has the added advantage over OTR that the light yield is independent of the particle energy. High light yield is important in advanced accelerator experiments where small charge signals may be of great experimental interest, as illustrated for example by the low charge accelerated tail described in [4]. On the other hand, beams in the FACET experimental area may have very high charge densities, reaching on the order of 1000 nC/mm². The Cherenkov profile monitor has the advantage that the light yield is linear in charge over the full charge density range. Scintillating materials used for beam profiling, for example Kodak Lanex, also provide high light yield. These type of screens, however, may cease to be linear in charge at certain charge densities. In [8] several types of Lanex are reported to have a saturated signal at densities of less than 100 pC/mm², four orders of magnitude lower than the FACET peak charge density. Large charge density may also damage scintillating materials, while our setup based on Cherenkov radiation has proven resistant to damage, as reported later in this paper. Another advantage of the Cherenkov profile monitor is that upstream photons, originating from upstream beam interactions with beam line elements or, in our case, the plasma, will not generate Cherenkov radiation. The Cherenkov profile monitor can therefore separate the incoming electrons from incoming photons. Finally, as we will discuss, our profile monitor design provides the possibility of large field of views, and is a relatively robust and simple system to set up as well as a cost-effective solution for a profile monitor.

We first discuss general principles and parameter considerations. We then describe the design of the Cherenkov light-based profile monitor for the spectrometer for the FACET User Facility and report on its performance.