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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.
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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.
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