June 18, 2004  




Richard Helm, Early Beam Dynamicist, and a Bit of SLAC History

By Gregory Loew

When Richard Helm died in Palo Alto on May 2, SLAC lost the first member of its staff who made beam dynamics his full time occupation. Helm studied at Stanford University and earned his Ph.D. in 1956 as part of Robert Hofstadter’s team engaged
in the famous electron scattering experiments which measured the cross-sections of many nuclei. In 1958 he returned to HEPL after two years at Los Alamos and soon became an expert in the behavior of electron beams, a field of work which he pursued for the rest of his career.

Dick Helm, a SLAC pioneer, was noted for his beam dynamics work. (Courtesy of Teresa Mize)

Today, beam dynamics is a recognized specialty but in the early days of Project M and SLAC, such work was carried out only as a sideline by many of us until Helm entered the field. I had the privilege of working with him quite often. He was incredibly smart and also incredibly modest, a master of understatement, and a man of few words. One of Helm’s characteristics was that he wrote most of his beam dynamics equations by making all constants equal to 1, which meant that a common mortal like me had to painfully fill in the gaps. But Helm never lost track of these gaps when the time came to give numerical estimates! When I would tell him he leftout the velocity of light, he would just give me a little smile, and say “Oh well.”

My first technical contact with Helm took place when he wrote a mimeographed (!) note warning us that the couplers that feed the microwave power to the accelerator sections would, in their uncompensated design, create field asymmetries that detrimentally kick the beam sideways. At first, he discovered that there was an amplitude asymmetry (which we fixed as a result of his warning), but then, when Sector 1 was built, additional asymmetry was discovered, which Helm identified as a phase asymmetry. That effect was then fixed by alternating the waveguide feeds of the remaining 29 sectors of the linac in a pattern designated as BABA-ABAB. Meanwhile, Helm had singlehandedly designed the entire quadrupole and steering dipole array for the machine.

When the linac was turned on in 1966, beam breakup (which we affectionately called BBU) was discovered and caused a lot of excitement and anxiety. The basic cause of BBU was soon recognized as cumulative growth of transverse electromagnetic wakefield forces which limited the linac current to 15 mA (one third of the original spec). To remove this limit, Helm calculated how to redeploy the quadrupole array and to dimple the accelerator sections in situ (which others then accomplished with a manual dimpling tool). Computers in those days had limited power, but in 1969 Helm figured out, almost by hand, what sections had to be dimpled by how much and where. Eventually the linac current rose to 80mA.

It is worthwhile mentioning that these coupler and beam breakup problems, which Helm successfully addressed, still play a major role in the design of the NLC.

Many of you, who in subsequent years worked with Helm on all the other SLAC machines, must have appreciated him as a scientist and as a human being. I am sure we will all miss him.


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Wednesday June 16, 2004 by Emily Ball