Title
Marya D. Pearson
Office of Science, Science Undergraduate Laboratory Internship Program
Stanford University
Stanford Linear Accelerator Center
Menlo Park, California
August 15, 2008
Prepared in partial fulfillment of the requirement of the Office of Science, Department of
Energy’s Science Undergraduate Laboratory Internship under the direction of Ernest L.
Williams Jr. in the Controls division at Stanford Linear Accelerator Center.
Participant:
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Signature
Research Advisor:
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ABSTRACT
MARYA PEARSON (Norfolk State University, Norfolk, VA 23504) ERNEST
WILLIAMS JR. (Stanford Linear Accelerator Center, Menlo Park, CA 94025).
INTRODUCTION
Feedback is a concept of communication, and in this aspect it is communication
between many diagnostic devices that require feedback to maintain control of the Linac
Coherent Light Source (LCLS) beam line. High energy electrons travel the two-mile
linear accelerator (linac) for physics experiments. For quality experimental values, the
electron beam must maintain its position at the origin of the transverse plain. In order to
allow the beam to consistently travel in the correct position, a network is needed to
facilitate communication between the various control and actuator devices.
The network implemented in the linac must satisfy some constraints outlined by
LCLS physicists in the Physics Requirement Document (PRD). From this document, the
fast feedback network should aim to function at a rate of 120 Hz, and provide an
architecture that is scaleable, flexible, deterministic and reliable. Some network
connections exist between the beam position monitor (BPM), a private BPM switch, and
designated input/output control (IOC) computers. It is the connection from the IOC, a
master CPU, and the actuators that is the focus of this project. In short, the IOC receives
BPM data in ≤ 1ms, and then passes the information to a master CPU which creates
algorithms with the data in Matlab to produce instructions for the actuators. The
actuators, either magnets or klystrons, receive this data in ≤ 1ms and respond to the
instructions in 6ms. Thereby, in approximately 8ms, the entire feedback loop is
complete. According to the 8.3 ms time budget, this time estimate satisfies the outlined
constraint, however, it hardly provides a cushion. By choosing an appropriate network
technology and architecture, the 1ms between IOC, CPU, and actuator can be reduced
and deliver the fast feedback for LCLS.
METHODS
While there are many network technologies to choose from, the most suitable
options for this project are reflective memory, point-to-point Ethernet, and multicast. Of
the many technologies, these three feature similar and unique characteristics. Cost,
latency, speed, error, and data transfer, along with the mentioned physics requirements,
are the decisive factors for comparing these technologies. Reflective memory (RM) is a
memory bus technology in which data is replicated and stored in local memory until it
reaches its destination [ ]. For example, if the IOC, CPU, and actuator are connected
using RM, the data from the IOC would be copied and stored at each node. Though this
function can be done speedily, the disadvantage is that every node connected with RM
will receive the same data whether it needs the data or not; in this case the actuators
would receive unnecessary BPM data. Another consideration for RM is the system
topology. Reflective memory can be configured in either a star topology or a ring
topology. The two differ in the hardware required for a connection. A star topology is
connected using RM cabling with transmit, receive ends, a RM hub that allows up to 256
nodes to connect, and a VMIPMC card that houses the actual RM technology (Refer to
Appendix). Then, ring topology simply connects each device directly to the next using
RM cabling. For example, a ring configuration between the IOC, CPU, and actuator
would attach the IOC to CPU, CPU to actuators, and actuators to IOC. Each node, or
device, in both configurations, must have a RM card to participate in the feedback loop.
Considering the fifteen feedback loops listed in the PRD, RM can be very costly.
Point-to-point Ethernet works well as a local area network technology, but as for
the LCLS rigid needs point to point may not be able to deliver. The name is a quick
description of the technology involved in this network architecture. To set up point-topoint Ethernet, simply connect each device to the next using Ethernet cables. For
instance, a point-to-point connection between the IOC, CPU, and actuator would connect
the IOC to CPU and CPU to actuator. The concern here is that the latency would
increase exponentially with the number of nodes, according to the equation for latency in
this case:
15n2 + 85n + 3
Using Ethernet as the network backbone, and multicast to satisfy broad, fast
communication requirements is probably the best fit. Multicast is a form of selectively
communicating data to many consumer devices. On the physical layer, Ethernet cabling
connects all devices, then a switch, programmed to support multicast, directs the data
traffic. Further, the switch directs the traffic to a multicast group, or a set of users
authorized to receive the data. Several advantages apparent in this technology is the
flexibility of the configuration. If multiple CPUs need information from an IOC,
multicast allows for all to receive the information, given they subscribe to the multicast
group.