Cyclotron The cyclotron was one of the earliest types of particle accelerators, and is still used as the first stage of some large multi-stage particle accelerators. It makes use of the magnetic force on a moving charge to bend moving charges into a semicircular path between accelerations by an applied electric field. The applied electric field accelerates electrons between the "Dees" of the magnetic field region. The field is reversed at the cyclotron frequency to accelerate the electrons back across the gap. When the cyclotron principle is used to accelerate electrons, it has been historically called a betatron. The cyclotron principle as applied to electrons is illustrated below.

CYCLOTRON FOR BEAM THERAPY APPLICATION

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Abstract

The proton beam for radiation therapy application in Russia for the first time [1] was created in 1967 on the base of Phasotron (Laboratory of Nuclear Problems JI). Now an energy of extracted proton beam is Ер=680 MeV, intensity Ip=3 mkA [2].

A six–cabin medical facility has been developed and put into operation on this beam [3]. Now in practice of treatment on medical beam LNP JINR the most frequently used beam has the energy 170 MeV

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PARAMETER	С – 235 IBA	C-250 ACCEL	C-190(H-) JINR LNP	C-200p JINR LNP
Energy of protons (MeV)	235	250	70-190	~200
Average magnetic field (T) At center At extraction radii	1.739 2.165	~4 ~4	0.77 0.92	1.33 1.64
Extraction radius (m)	1.08	~0.9	~2.1	1.4
Magnetic field at extraction radius (T) hill valley	3.09 0.985	4.0 1.6	0.6 1.1	2.65 0.95
Gap (mm) valley hill	600 96-9	-	380 140	400 50
Number of sectors	4	4	4	4
Main coil ampere turn (kA)	525	-	150	340
Power consumption (kW)	190	40(cooling)	120	170

The magnetic system consist of sectors (1), poles (2), ring top and bottom horizontal yokes (3), coils (4) and vertical yoke (5) (see figs. 1). The required configuration of the magnetic field is formed using a spiraled and angular extent of sector shims depending on radius.

The complete angular extent of one sector on a pole composes 55º, thus there is an opportunity to place two 42º resonators in valley.

Beam Dynamics

In figs. 4 -7 the dynamic characteristics of beam in the magnetic field are given. The betatron frequencies of axial and radial motion (fig. 4) are in allowable limits

Figure 2: Computer model of the magnetic system of C200p (bottom part of the magnet, hole for coaxial line of RF system can be seen)

Working point diagram along the acceleration in C200р is presented in figure 5. The point to point distance is 10 MeV. The most dangerous resonance Q_r-Q_z=1 is crossed two times at energies 130 and 170 MeV. Modeling of particle dynamics showed that no axial amplitude increase observed after the resonance (see below) if no skew harmonics presented in magnetic field map. Further computations have to define permissible limits of such harmonics.

Figure 3: Magnetic field map computed by the RADIA code

Figure 4: Free betatron frequencies along radius

1,001,051,101,151,201,250,00,10,20,30,40,50,60,7Qr-2Qz=12Qr-Qz=2210 MeV10 MeVQr+2Qz=2Point distance=10 MeVQr-Qz=1  Working point diagram of C200p

Figure 6: Phase motion of central particle

7: Axial motion of one particle

Phase motion of central particle computed along the acceleration shows good accuracy of a isochronous field. Particle resonance orbital frequency is 20.4545 MHz. Axial particle motion along acceleration in magnetic field with no skew harmonics is shown in fig. 8. Amplitude of particle radial oscillation was 5 mm during this computations. Changing of axial oscillations amplitude corresponds to the dependence of axial betatron frequency on the radius.

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SCINTIST WORKING ON CYCLOTRON

If you walk from the Berkeley campus uphill along Cyclotron Road to the Lawrence Berkeley Laboratory (LBL), past the security gates and into a eucalyptus grove, you will find yourself at the 88" Cyclotron. The “FAQ” sheet in the lobby anticipates the visitor’s question, “Have [scientists] ever discovered anything really cool at this cyclotron?” and answers, “Well, no discoveries honored by a Nobel Prize originated here. Not yet.” Now time is running out for the cyclotron’s Nobel aspirations. The nuclear science facility, scheduled to close in November of this year, has already reduced its operating schedule from seven days a week to four and a half.

The 88" Cyclotron’s very name places it in a gradually disappearing scientific niche. The goal of a cyclotron is to accelerate ions—atoms missing one or more electrons—to high energies and then to collide them with stationary atoms. This cyclotron’s 88 inches measure the diameter of its magnet, a number that determines the maximum energy to which it can accelerate ions. The name is anachronistic in an era when science is largely driven by the push to higher energies and bigger facilities, since the 88" was not the largest cyclotron—by more than two times—even in 1961, when it was built. To be sure, the 88" cyclotron has at various times been the nations or the world’s best, by one measure or another. But as Stuart Freedman, professor of physics at UC Berkeley and senior scientist at the cyclotron, points out, with an annual operating budget of five to six million dollars, “this isn’t big science; this is a small operation.”

Nuclear science in the United States has set its sights on construction of the next big operation, the Rare Isotope Accelerator (RIA). With the field’s federal funding stagnant, nuclear science can barely afford research and development for RIA, much less RIA’s projected cost of $1 billion. In November of 2001, the US Department of Energy (DOE), which provides 85% of the funds for nuclear science in the United States, recommended closure of the 88" Cyclotron if budgets were to become tight.

The committee that prepared the DOE report did not want the 88" Cyclotron closed and called the possible closure “a significant loss to the nuclear physics community.” Freedman points out that“these reports are meant to be sed. One way to use it is for getting money: to say, ‘look at all the good research that will be lost if the nuclear science budget is cut.’” The nuclear science budget did, indeed, increase in 2002, but the strategy failed in 2003: In February the DOE announced that push had indeed come to shove, and that the 88" cyclotron would have to close by November.


Putting new ions in an old accelerator
nuclear scientists like those at the 88", their title notwithstanding, do not design nuclear bombs or nuclear reactors. They leave those tasks to engineers and a few “applied” nuclear scientists, while they explore the fundamental properties of atomic nuclei. Their wares are the protons and neutrons (collectively “nucleons”) that make up the nucleus, the quarks that compose nucleons themselves, and the particles that carry forces between them. From these particles nuclear scientists learn about the laws that govern nature, on scales from subatomic to stellar.

Freedman’s own research at the 88", in collaboration with staff scientist Paul Vetter, uses low- energy ions accelerated by the cyclotron to study the so-called weak interaction. One of the four “fundamental” forces of nature and a linchpin of modern particle physics, the weak interaction helps to regulate nuclear reactions in stars, and creates the nearly-undetectable neutrinos that make up between 10––20% of the universe’s mass. Freedman collides the accelerated ions with atoms to create radioactive nuclei, which then stream directly into his “atom trap.” By monitoring the radioactive decay of the nuclei held in this laser-beam trap, Freedman and Vetter hope to discover new information about the weak interaction.

Research at the 88" represents a branch of nuclear physics, in which relatively low energies are desirable—though the energies are still high enough to require a cyclotron. At these energies, nuclear interactions lead to forms of “collective” order, in which groups of nucleons take on strange new properties. The nucleons can form pairs that become insensitive to the interactions around them, staying put while the unpaired portion of the nucleus rotates, or the whole nucleus can assume a “super-deformed” state, elongated like a football. Low-energy ions may also be collided with target materials in hopes of creating new elements—an enterprise at which earlier Berkeley accelerators excelled, beginning with the 1940 discovery of plutonium by Edwin McMillan, Philip Abelson, Glenn Seaborg, and Emilio Segrè using a 37" cyclotron on the Cal campus. The 88" Cyclotron is now the only US facility that searches for such heavy elements.

None of the research in this branch of nuclear science requires that the cyclotron accelerate ions to especially high energies. Instead, nuclear scientists seek to accelerate ions of ever-heavier elements to about the same modest energy per nucleon. Because a cyclotron uses electric fields to accelerate ions, the force it can exert depends on the ion’s charge––that is, on the number of electrons removed. Heavier ions, being more sluggish, must have more electrns removed than lighter ones in order to reach the same energies.

When the 88" Cyclotron first opened, available ion sources only allowed it to accelerate ions containing a few nucleons, like bare protons or the nuclei of helium atoms. In the late 1960s, scientists at Berkeley and elsewhere invented what were then considered “heavy-ion” sources: devices that could remove up to six electrons from elements as heavy as neon, with 20 nucleons.