LANSCE 1L Harp Data Acquisition System Upgrade: First Results

Efforts applied toward the upgrade of the LANSCE 1L harp beam diagnostic data acquisition system have completed with the system’s successful deployment in late December 2014. Leveraging the principle of secondary electron emission, the data acquisition system measures the particle beam-induced, negative charge-loss response of a statically-located, harp-style, beam diagnostic sensor. The harp's sense wires span two orthogonal planes, transversely oriented with the beam's direction of travel resulting in two orthogonal profiles. The profile data provided by this beam diagnostic system allows LANSCE operators to measure the particle beam's transverse properties prior to reaching its final destination: the 1L target. Details will be provided with respect to the system's final hardware architecture, the system's theoretical beam response model, and the system's measured beam response. INTRODUCTION Shown in Fig. 1, the 1L harp is a fixed-position beam diagnostic sensor for measurement of the beam’s transverse profiles immediately prior to impingement on the 1L target. The sensor is composed of three planes of silicon-carbide (SiC) fibers; two sense planes for measuring horizontal and vertical beam profiles, and a bias plane for absorption of secondary electrons. Each sense plane is composed of seventeen sense fibers spaced at 6 mm intervals. All fibers connect to individual 10 pF capacitors at one end and a cable plant at the other end for signal transmission to the data acquisition system [1]. Figure1: 1L harp sensor. PRINCIPLE OF OPERATION The 1L harp operates on the principle of secondary electron emission resulting from the interaction of the particle beam with the harp’s sense fibers. As the highenergy (800 MeV) H+ particle beam passes through the fiber, electrons from the material are forcefully removed from the fiber’s surface into free space, leaving a positive charge gain within the fiber. The positive voltage resulting from the fiber’s loss of electrons attracts a flow of electron current from the signal conditioning circuit to the fiber, neutralizing the charge difference. This current and its associated net charge are transformed by the signal conditioning circuitry into a voltage signal proportionally related to the charge. Since the particle beam’s transverse particle density is generally Gaussian, each fiber in the plane receives a different amount of beam flux. This beam flux translates into secondary electron emission differences resulting in charge differences at the signal conditioning circuitry and finally a voltage difference at the analog-to-digital converter (ADC) dedicated to each fiber. Plotting the resulting voltages as a function of the fiber’s relative position creates a Gaussian profile corresponding to the beam’s transverse particle density. General beam and 1L harp parameters are listed in Table 1. Table 1: 1L Harp and Beam Properties