10 Million Million Voltsor Bust…

Tim KoethApril 29, 2008

Inspiration and Determination:A Historical Path to Higher Energies

The history of accelerator physics has been one of adventure, bravado, and sometimes shear luck. Some say accelerator physics is just a discipline of engineering, but in fact advancement in the field has always been from the finger tips of physicists, well mostly…

Accelerator physics has been a staircase evolution: Inspiration followed by determination over and over again.

I am going to give a brief over view of the past 80 years.

In 1929 the motto was “10 Million Volts or Bust”; in 2008 it is 10 Million Million Volts or bust.

A Livingston Plot

Fermilab

LHC

“Start the Ball Rolling”1927: Lord Rutherford requested a “copious supply” of

projectiles more energetic than natural alpha and beta particles. At the opening of their High Tension Laboratory, Rutherford went on to reiterate the goal:

What we require is an apparatus to give us a potential of the order of 10 million volts which can be safely accommodated in a reasonably sized room and operated by a few kilowatts of power. We require too an exhausted tube capable of withstanding this voltage… I see no reason why such a requirement cannot be made practical.1

MANY FAILED ATTEMPTS1928: Curt Urban, Arno Brasch, and Fritz Lange successfully achieved 15 MV by harnessing lightning in the Italian Alps !2

The two who survived the experiment went on to design an accelerator tube capable of withstanding that voltage.

Just one example:

Cockcroft & Walton’s Voltage Multiplier:

Cockcroft WaltonRutherford

The multiplier worked, but is generally limited to 750kV. A C-W stack begins the Fermilab chain of accelerators

C&W 1951 Nobel PrizeAttributed with being the first to artificially disintegrate nuclei.

Wideroe Linac1929: Rolf Wideroe

R. Wideroe proposed an accelerator by using an alternating voltage across several accelerating “gaps.”

It was not without a myriad of problems - Focusing of the beam- Vacuum leaks- Oscillating high voltages- Length- Imagination

His professor refused any further work because it was “sure to fail.”

Never the less, thankfully Wideroe still published his idea in Archiv fur Electrotechnic

Wideroe in the 1960’s having the last laugh…

Linear Accelerators

SLAC – Stanford California2 miles long50 GeV

The proposed International Linear Collider33 km long

Wideroe won the Wilson Accelerator Prize in 1992

Inspiration: Ernest Orlando LawrenceIn April 1929, UC Berkley’s youngest Physics professor happened across Archiv fur Electrotechnic.

Not able to read German he just looked at the diagrams and pictures of the journal.

Immediately after seeing Wideroe’s schematic, Ernest fully comprehended it’s implications.1939 Nobel Prize

1929: The Cyclotron

rmV

rF2 qVBFB

qB

mVr

“R cancels R !”

r

Vf 2

m

qBf

2

Lawrence quickly jotted down:

and equated with and solved for r:

and substituted V in terms of

The Cyclotron Frequency

Determination: The Grad Student M. Stanley Livingston

M. Stanley Livingston (GS) Ernest Lawrence

High Voltage DEE

Dummy DEE

The First Operational Cyclotron

Determination: Weak Focusing

Intentionally introduce radial B-field component at the cost of an vertical gradient: to be coined weak focusing.

- Although the cyclotron worked, the beam intensity was very weak. Lawrence - Lawrence: wire grids and iron shims

Livingston removed the grids while Lawrence was out of town beam intensity shot up.

Livingston took this remarkable finding to Lawrence. To which Lawrence responded: “It’s obvious what happening…”

Seemed to be No Limit With Focusing …

Lawrence believed the only limit on energy was the size of the magnet. - 27-inch ( 5MeV ) cyclotron - construction- 60-inch (16 MeV) cyclotron - design- 184-inch (100 MeV ) cyclotron - fantasy

Hans Bethe disagreed ! practical only to 20 MeV for protons.

Characteristically, Ernest Lawrence was not dissuaded & proceeded full steam… “there is always more than one way to skin a cat.” – EOL

m

qBf

2

Weak focusing could only go so far..

The greater the energy, the larger the radius, but the gap (AKA aperture) had to correspondingly grow to produce the needed gradient. Thus the magnets were getting impractically large.

For a sense of scale: I am sitting in the magnetic gap of Enrico Fermi’s Cyclotron shortly before it was dismantled.

We have to pause for World War II…

In Parallel: Robert J. Van de Graff1931-4

Van de Graff (VDG) achieved 1.5 MV in 1931, by charge exchange onto metal spheres. The “Van de Graff” worked, but progress towards higher voltages was slow…

He went on to propose two 20 foot spheres on 20 foot towers capable of 10 MeV.

The resulting awesome VDG installation at MIT stood 43 feet about the ground and the spheres were 15 feet in diameter.

It promised 10 MV, but was not realized until after WWII.

Simple construction: many labs could easily obtain a VDG VDG generators are still used today - they can provide very mono-energetic beam - only mAmps of beam current - the biggest are limited to about 25MeV

Inspiration: Phase Stability beats Relativity

Edwin McMillan of UC Berkley, and the Russian V.I. Veksler independently discovered Phase stability in 1945.

Simply stated the principle of Phase Stability is:

-The synchronous particle arrives at each successive accelerating gap at the same phase, incurring the same incremental acceleration.- Slow traveling ions arrive at the next gap “late” & receive more push-Fast traveling ions arrive at the next gap “early” & receive less pushThus, a “band” of ions continuously oscillate about and follow the phase of “stability” during acceleration.

The stability was robust enough to allow adiabatic changes in the accelerating frequency: enabling the oscillating voltage to change with the relativistic mass increase.

The cat was skinned ! McMillan proposed the synchrotron in a letter to Phys Rev in 1945 & won the Nobel Prize in 1951 (for chemistry)

Post WWII: Return to circular accelerators:

The Synchrotron:-Constant radius:

- Inject a low energy beam and accelerate up- Ramp B-field & modulate accelerating frequency- Extract and send beam to target- repeat

- Beam has pulsed structure as a result

Berkeley BevatronBrobeck, Lawrence, McMillan, and Cooksey sitting in the large aperture of the Bevatron10,000 turns: Still need transverse focusing

Grad Student Bob Wilson was fired several times by Lawrence, the final time it was for leaving a 2x4 in the vacuum chamber of the Bevatron.

Inspiration: The Era of Strong Focusing

-1953: E. D. Courant and H. S. Snyder of BNL discovered that rotating one of their weak focusing cosmotron magnets would create a strong focus in one plane. -Further investigation showed that periodically alternating the “weak focusing” gradient magnets had net focusing effect that was much stronger. Hence, the magnet’s aperture could be greatly reduced.

Analogous to series of converging and diverging lenses to produce a net focusing effect.

(Courant won the Wilson Accelerator Prize in 1987)

A Slight Embarrassment …During a 1953 visit to the US, shortly after Courant, Livingston & Snyder published their results in Phys. Rev., the Greek owner of an elevator repair company, Nicholas Christofilos, read their article at a Brooklyn library.

Christofilos marched out to BNL, and pointed out to them that he not only sent this idea to them in a 1950 letter, but that he held a US patent on the principle.

Christofilos was immediately offered a position at BNL !

Ultimately a settlement was made for the rights to strong focusing. Nicholas Christofilos

Strong Focusing & Synchrotrons- strong focusing + synchrotrons realized the goal of “unlimited” size thus

unlimited achievable energy. Many accelerators types have benefitted from strong focusing, but since the 1950’s

the synchrotron has been at the energy frontier being the workhorse HEP community.

The first operational strong focusing synchrotron was Cornell’s 1.2 GeV. The machine, already under construction under Bob Wilson, was retrofitted with poletips to be a strong focusing machine.

400 GeV synchrotron dipole magnet. (Quads not shown)

400 MeV synchrocyclotron

Fermilab: Inspiration and Determination

Robert R. Wilson

In just a few years (1968 to 1972) Bob Wilson directed the construction of the “National Accelerator Lab” (now known as Fermilab) to accelerate protons to 400 GeV.

Rutgers participated in Fermilabs first experiment:-Tom Devlin-Felix Sannes-Richard Plano

Fermilab ~ 1972

Inspiration: Colliders

P. Panofsky (SLAC) G. I. Budker (Novosibirsk)

2

EEcol

For a low energy proton, hitting stationary proton, the available collision energy is:

Relativity complicates issues even more, when E >> M then the available collision energy is:

22 2 E

EMcEcol

Collider Benefit:Two particles of equal mass traveling head on, after collision, has a total combined KE of zero, thus the entire energy of both particles is available as collision energy.

The challenge:Luminosity….

I am omitting a deep history-Princeton/Stanford double ring-VEP I, II, III, IV (Novosibirsk)-SPEAR (SLAC) & DORIS (DESY)- ISR C. Rubbia & Van de meer-SPbarPS (CERN)-SLC(SLAC) & LEP (CERN)

Always a friendly competition !

As the remainder go towards the KE of the system after collision.

Fermilab: Inspiration and DeterminationIn the 1980’s Rich Orr, Helen Edwards, Richard Lundy, Alvin Tollestrup directed the Tevatron effort: a supplement to the 400 GeV synchrotron with a superconducting synchrotron to reach 1 TeV.

Present day Fermilab site Installation of the Tevatron below the existing Main Ring accelerator

RU is a CDF collaborator

anti-proton ring anti-proton source

Main InjectorThe Tevatron has been at the energy frontier for over 25 years !

The LHC at CERN: 7 TeV (14 TeVcol)- US participation- 27 km circumference (old LEP tunnel)- ~ 100m below surface- crosses Swiss/French border- First beam expected in 2009 - 360 MJ stored energy in the beam-

RU is a CMS collaborator

SPS: sister to Fermilab’s main ring

Best Light Sources Around: Accelerators serve many disciplines

(e.g. condensed matter, Biology, Astrophysics)

• 1st Generation: “parasitic” synchrotron light• 2nd Gen: dedicated low emittance synchrotrons: NSLS at BNL• 3rd Gen: 2nd Gen with Insertion devices• 4th Gen: FEL’s, XFEL: very short time structure for high times

resolution

Challenges of the present:• Push for higher energy

– Cost reduction• RF superconductivity• Superconducting magnets• Energy recovery

– Size constraint• Higher accelerating gradients• Stronger magnets

– Material limitation• High Tc Superconducting materials

– Intensity

The A0 Photoinjector at Fermilab is an example of an Advanced Accelerator R&D facility.

Interesting programs- Plasma Wakefield acceleration- Laser induced acceleration- Dielectric acceleration- Energy recovery- Muon colliders- Int’l Linear collider- FEL/ERL (light sources)- etc…

Students, I ask: What’s Next ?• Accelerator physics has been a history of innovation followed

determination. We find ourselves as were in 1927, as we now need a source of inspiration to develop higher gradients in order to make the next generation of accelerators feasible.

In 80 years from now, will we be able to quote from 2008:

“What we require is an apparatus to give us a potential of the order of 10 million million million volts which can be safely accommodated in a reasonably sized building and operated by a few megawatts of power… I see no reason why such a requirement cannot be made practical.”