The Central High School Cyclotron

Version 1.00 (15-Nov-15)

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Samuel M. Goldwasser
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    Author and Copyright

    Author: Samuel M. Goldwasser

    For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

    Copyright © 1994-2015
    All Rights Reserved

    Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

    1. This notice is included in its entirety at the beginning.
    2. There is no charge except to cover the costs of copying.

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    This document summarize my experiences in high school many years ago with an atom smasher, sort of. :) The material below used to be buried somewhere in Sam's Laser FAQ but the other sections there became unhappy that a cyclotron had little to do with lasers. So it's now here. ;-)

    First, the major players (complete names are not included since I am unable to contact them for permission to make this public):

    Now, Central High School (CHS) wasn't a technical high school but was and still is to some extent, one of the principle academic high schools in Philadelphia, PA, USA. At that time, it was also an all boys school, with Girls' High, a long block away. Now, it is co-ed.

    There were the usual debating, rock climbing, sports, cycling, and all the other associations, clubs, societies, whatever, typical of any high school.

    There were also the technical ones. We had an amateur radio society and photo society, of course. There were also probably organization for chemistry and biology but I don't really recall as I wasn't enthusiastic about chemistry (despite having acquired a cabinet full of beakers, flasks, and such) and biology was even lower on my list of fun activities. But I do recall a field trip to the Pine Barrens in New Jersey to check out the trees. :)

    I also managed to get involved in the Advanced Physics Lab (APL) where I did inherit a ruby laser based on a mid-60s Popular Science design (that never worked or maybe I was too chicken to turn up the capacitor charging circuit to reach threshold). (See Modern Mechanix - Build Your Own Laser for the original article if you're curious.) Mostly, what was done in the APL is that Fred and I abused burned out light bulbs by among other things, driving a 110 VAC to 2.5 kV transformer on 220 VAC and I would swear that for several seconds, achieved a plasma jet through the bulb in open air. But it also looked good on my academic record!

    There was also the... Cyclotron Society. Myself, Doug, and Fred were the principle members. There was also a guy named Gary but he used the place more for political reasons and always seemed to find excuses NOT to do anything technical. Fred and I eventually got him thrown out of the Cyclotron Society on the basis of some infraction which to this day I don't know was real or not. Douglas was in some ways similar and the room (to be described below) DID make a good book repository and hangout! He, at least, didn't interfere with progress.

    Now, lest you think this is the sort of accelerator you have seen on documentaries or even bad Sci-Fi movies, you would be disappointed. It was small - the maximum energy was theoretically if everything was optimal, maybe 1 million electron volts (1 MEV). The diameter of the magnet pole pieces were 7 inches! However, everything was there - magnets, RF source, vacuum system, gauges, the works.

    What is a Cyclotron?

    The cyclotron is the earliest type of atom smasher which uses the combination of a magnetic field to confine the (usually) protons and an RF field to accelerate them. It was invented by Ernest O. Lawrence in 1930. In a cyclotron, both the magnetic field and RF frequency are constant. A charged particle traveling perpendicular to the magnetic field lines will follow a circular trajectory with the diameter determined by its energy (speed). For velocities much less than the speed of light, the period of each orbit and thus the frequency of rotation is a constant. So, if a little boost if given to the particles as they circle in the magnetic field, their energy will increase along with the orbit diameter.

    Cyclotrons can be of a wide range of sizes. Lawrence's original invention was only about 5 inches so ours at least was larger than that! The upper limit on size is imposed by relativity. Once the energy of the particles approaches their rest mass (from E=mc2 or equivalently, their speed approaches the speed of light, the constant magnetic field/RF frequency behavior doesn't work anymore and they would lose synchronism with the RF. I've seen a cyclotron with magnet pole pieces about 2 meters in diameter but even this is probably pushing things. There were atom smashers called "syncro-cyclotrons" which were physically similar but varied the RF frequency as the particles spiraled outward to maintain synchronism. Nowadays, the modern versions are mostly "synchrotrons" which do away with all but the outer ring and may be miles in diameter producing energies of 100s of Giga electron-volts (GEV). The "Super Conducting Super Collider" was supposed to be over 20 miles in diameter (perhaps more, I forget) but that died when BIG science fell out of favor in Congress. The largest one was at Betavia, IL for awhile but that has probably been surpassed by something at CERN in Europe by now.

    The CHS Cyclotron Facility

    I wish I could have been involved in the original construction of this thing, but my tenure at Central had more to do with what might be termed a major rework and upgrade. The details of how a public high school ended up with an atom smasher may forever remain a mystery but what is known is that the original version was cobbled together by students in the years prior to my high school days. The main mover and shaker was probably Donald, who if I recall correctly wrote a paper of sorts on "The effects of High Energy Protons on Semiconductors" - he stuck transistors with their covers removed inside the cyclotron and measured some combination of parameters and how they changed with exposure to high energy protons. Now, this was most likely totally bogus for reasons that will be come clear below, but I think it did win him a National Merit Scholarship.

    The Cyclotron Room was on the main corridor in the school basement just around the corner from the Bookstore and opposite one of the storage areas for those Nuclear fallout rations the Government was so fond of stockpiling around the country (I think the mice benefitted mostly). This was sort of fitting and I suppose there were some teachers who wouldn't rule out a nuclear accident from our activities! We shared the approximately 15 x 20 foot room with one of two huge water chillers for the school. (I never did quite figure out if these were for the fountains or something else.) However, the 5 gallon container of refrigeration oil left by HVAC techs next to the chiller came in handy from time-to-time.

    The power supplies, RF source, and instrumentation were mounted in a 6 foot and 3 foot rack. The magnet with the vacuum chamber was behind it with the vacuum system underneath. The "instrumentation" was a microamp meter for measuring beam current later upgraded to one with a tube based preamp. We had a collection of probably non-functional radiation meters most likely "liberated" from that storage room across the hall. There may have also been an actual working Geiger counter as well. Our test equipment consisted of a very abused Triplet 1K ohm/volt VOM with a bent needle, having been unwound from the right-hand stop more than once.

    The magnet:

    This was a HUGE 7 inch resistive (hey, no one knew about superconducting magnets in those days) magnet wound with a lot of #20 wire. (Once, we found the coil with an open connection - possibly cut by a saboteur! Maybe that Gary fellow I mentioned - really never found out. Or, it may have just been a natural failure. Needless to say, there was a minor panic until the break was found!). The magnet was on a 99 year lease for $1 from some company, maybe GE, like they'd ever want it back! I don't know how they got it into the place, in pieces I guess.

    The magnet was horizontal. To install or remove the vacuum chamber required unscrewing some huge bolts and then prying apart the yoke. Now, keep in mind that this thing weighed in at about 6 tons even though it is a rather small magnet. So, replacing anything inside the vacuum chamber was always an interesting exercise. Underneath was a very fragile glass diffusion pump that somehow survived.

    Originally, power for the magnet came from a 200 VDC or so power supply. We eventually concluded that this was grossly underpowered (another reason to suspect the cyclotron never really worked until the prior management) so we changed it to the 2,400 VDC electric utility pole transformer based power supply that was originally used for the RF source (mercury vapor rectifiers and all that). It was controlled by a BIG Variac. With this improvement, it probably was running near the 20,000 (2 Tesla) limit of an iron core magnet. It was quite impossible to extract any ferrous/magnetic objects from between the pole pieces with the magnet fully energized.

    The vacuum system:

    We had a Sargent-Welch rotary vane mechanical pump, maybe a 1405 but could have been the "economy version", and an all-glass (except for the tower and heater) oil diffusion pump. The Welch usually needed to be started by hand as do many of these older pumps. Belt guard, what's a belt guard? :) There must have been a box fan or something to cool the diffusion pump since it did not use tap water. The diffusion pump was about 3 inches in diameter with a glass O-ring flange-flange reducer for the 1 inch coupling to the vacuum chamber squashed between the magnet pole-piece right above it. There were no baffles, cold traps, or dryers.

    An ion gauge provided our only real indication of vacuum level other than that wonderful clacking sound the mechanical pump made when it has drawn down below 10 microns or so. We had the obligatory hand-held Tesla ('Oudin') coil for leak testing though it was used much more often to chase the unwanted visitors from the cyclotron room. :) Anyhow, once the whole affair was drenched in Red Glyptal, leaks really weren't an issue!

    I seem to recall a scavenged thermocouple gauge as well but that may have been for the "Linear Time-of-Flight Resonance Mass Spectrometer" I started to build. That at least got me a free cruise on a Navy destroyer escort and my first plane flight to the Newport Navy Base as a consolation price at the local science fair since it was not completed and never really worked - but the charts and front panel were impressive! I still have the Science Fair posters. Oops, that's another story.....

    The vacuum system would do at least 10-6 Torr when it was cooperating. Usually, it didn't take long to get there but since we really didn't trust the ion gauge all that much, we usually left the thing running overnight. (The actual pump-down time was probably measured in a very small number of minutes.) But, the ion gauge tube was never damaged from loss of vacuum. :) The controller might have been a Varian RG-31X, a semi-antique even at that time.

    The vacuum chamber:

    For our huge 7 inch magnet, we needed a vacuum chamber large enough for the Ds (the electrodes that actually accelerate the protons - shaped like the letter 'D') and a little clearance. In all, it was about 12 inches in diameter and two inches thick. The 'D's (actually only one, see below) was mounted on a metal stud fixed in Epoxy passing through a glass insulator.

    Most cyclotrons have a pair of 'D's and drive them with a balanced RF source. Someone decided that this wasn't necessary, so ours had a single brass 'D' made of sheet metal brazed at the edges with the RF between it and the ground of the vacuum chamber. In principle, this would work though I suspect there would be less of a focusing effect. What did we know? I seem to recall later replacing the single 'D' with a pair made from machined aluminum.

    The RF source:

    Originally, a 200 W amplifier driven by a 50 W exciter generated the RF (about 20 MHz if I recall correctly) for the 'Ds'. This was based on a pair of 811 tubes (remember those?). I later built a 1 kW linear amp (from the AARL Handbook) using an EIMAC 3-400Z or something like that. The original power supply (866 mercury rectifier tube based) was moved to the magnet and a new solid state HV power supply was built for the amp.

    True R and D

    I consider the years I spent working on that machine to be more closely akin to true scientific research than anything since - designing giga instruction per second high performance 3-D visualization/graphics accelerators or microchip lasers just doesn't have the same feel!

    I had to learn - on my own - about high vacuum technology, high power (well, relatively speaking) RF, instrumentation, at least a little E/M and high energy physics, and much more. Keep in mind that no one else - including any of the teachers at Central High - had a clue about ANY of this! I'm sure they thought we were building a bomb...

    I also learned a lot about locksmithing (including all about making master keys using solder-fills and lock picking) - one has to be resourceful to succeed in these endeavors. The teachers were aware of this and kind of accepted it (as well as benefitting at times), realizing the limitations of an environment where advanced science was more along the lines of dissecting a worm. :)

    To be continued as I think of more tid-bits...

    Other Home-Built Cyclotrons

    I don't know how many of these were successfully constructed prior to the CHS Cyclotron in the early 1960s. However, there have been several, if not many, built since then both by individuals and as projects in various high schools and colleges. Check out the following links:

    Some brief references to our cyclotron may even be found by a determined Web search (at least one in the bio of a person who recalls having been involved). I welcome any info or links to the CHS Cyclotron or any other home-built cycltrons that have existed.

    The First Generation Central High School Cyclotron

    I recently heard from one of the original builders of the Central High School Cyclotron, Lawrence H. Zuckerman, Pleasanton CA, (Email: (, who was in the 218th class. Here is a paraphrase of our email thread:

    (From: Larry.)

    I graduated with the 218th class of Central High School. I was a member of the Cyclotron Society and helped to build this accelerator when I was a freshman in the '58-'59 school year. When we completed construction and performed a brief test, our work was publicized by the Heart Association, and a newspaper article with pictures of us appeared all over the country.

    I remember guys who were much older than me -- had just graduated in the 210th class and started college, such as Nathaniel Ostroff and Joshua Horowitz. I may think of other names later.

    Our faculty advisor was an MIT-educated physicist and later School District of Philadelphia science curriculum executive, was Fred Hofkin.

    (From: Sam.)


    Hey, wow! You're the first person from the original construction team I've ever heard from. I have seen a few Web references to other "personal" cyclotron projects, but nothing about Central until what I wrote.

    We were involved from '66 to '69 or '70.

    Anything you can add or correct would be most appreciated, with whatever attribution you'd like!

    (From: Larry.)

    It was Hofkin who told us that the cyclotron really only needed one Dee. I did not know any physics then and never thought about it all these years. Now that I am thinking about it, the single Dee architecture worries me.

    You mentioned that the magnet assembly is there on a 99 year lease. I vaguely remember that fact, and also that the "lessor" is the University of Pennsylvania, where I went to study Physics starting in 9/62.

    One of my tasks, assigned to me by Hofkin, was to determine the number of turns on the magnet (so that, using the current value, the field density and deflection could be calculated). I recollect (maybe incorrectly) that there were four coils, two slightly dis-similar pairs. He said I could use any method I wished, except for unwinding it. ;-) I measured the wire gauge to determine ohms/thousand feet. I measured the resistance of each coil. The total of 72,800 turns rings a bell, (but that was 50 years ago!).

    My other task was to build the control panel, which was nothing more than a relay rack panel with switches, chassis, and 120 Volt AC connectors. I did that OK, except giving the older boys a laugh by using bell wire; so Nat re-wired it with #18 zip/lamp cord, using both conductors in parallel.

    The cyclotron calculations (which I certainly did not do at the time) said that the frequency needed for the Dee was about 7 MHz. You mentioned a small ham rig for that, which could have been a DX-20. I don't remember the power amplifier.

    We fired up the equipment, opened it, and found a mark on the side in a plausible location. Was it from a proton beam? I think it was too far from the Dee to be an RF arc.

    The main (second stage oil diffusion) vacuum pump was donated to us by EIMAC (Eitel-McCullough), a fact mentioned in the AP/UPI released newspaper article that had the picture of the six of us standing around the cyclotron. I think this happened late in the school year.

    I shall check with the CHS Associated Alumni (of which I am a member) to get more information. If the cyclotron is still there, I would like to see it the next time I visit Philadelphia, which is once or twice a year. I have also been looking for information about Fred Hofkin.

    As a bit of irony, for the past fifteen years, I have been living about 10 miles from the (E. O.) Lawrence Livermore Labs. I am an RF design engineer at NATIONAL SEMICONDUCTOR.

    If I think of anything else or obtain more documentation, I'll send it to you. Nice work on your web site!

    (From: Sam.)

    Thanks for the info.

    Using the same basic calculations as you did, we concluded that the magnet was way underpowered by the original power supply and moved it over to the rectified pig-pole transformer supply.

    Hmmmm, let's see. Assuming a mean diameter of 2 feet and #20 AWG wire, your 72,800 turns is about 450,000 feet with 10 ohms/thousand feet for #20 AWG wire, that would be roughly 4,500 ohms. I seem to recall we measured 6,000 ohms, so not too far off. :)

    I built a 1 kW linear amp from the AARL handbook that was then driven by the DX-20 exciter. It replaced what was a home-built RF amp, supposedly about 200 W.

    That calculation of 7 MHz may have been correct for the weaker magnet, though I think it was even weaker than that. I seem to recall we eventually ran it on 1 or 2 kV, rather than the 200 V it was on originally. I'm not sure what the RF frequency we were running at, perhaps 20 MHz for a (20 kgauss) saturated iron magnet. But our only means of measuring field strength was by the student and hammer method - how many average CHS students it took to pull our standard hammer from the core when the magnet was energized! :)

    The single Dee thing always bothered me also, but I seem to recall other references to it, so perhaps it's not totally bogus, though probably less efficient.

    As far as whether it ever worked, I rather doubt any real proton beam from this machine would have had enough power to make a mark but who knows. We finally got it to expose a piece of dental X-ray film, and that was marginal.

    The most dodgy thing was probably the ion source due to its simplicity, had to be operated on the border between high vacuum and a pressure too high to sustain a beam.

    And I'm still amazed that the glass diffusion pump survived! :)

    As far as to whether the thing is still there, I have no idea. But who would want to move a 6 ton magnet!

    Anything else you come up with will be welcome.

    (From: Larry.)

    It is great that you guys resumed work on the cyclotron after my days at CHS and did so much; I knew nothing about this. Following my freshman year, I built much amateur radio transmitting and receiving equipment while in high school, but I have no recollection that anyone ever went back into the cyclotron room.

    I imagine you are correct that any proton beam from this machine could not make a visible mark.

    If the cyclotron was ever cleared out, it was probably to make room for a girls' lavatory!

    (From: Sam.)

    Or to add another water chiller. :)

    I think we were actually the third generation. Between your team and ours, there were Donald and several others.

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