Charles M. Ankenbrandt

Gaseous Hydrogen and Muon Accelerators

Ionization cooling, a method for shrinking the size of a particle beam, is an essential technique for future particle accelerators that use muons. In this technique, muons lose energy in all three directions by passing through an absorber while only the longitudinal energy is regenerated by RF cavities. Thus the beam phase space area decreases down to the limit of multiple scattering in the energy absorber. Hydrogen is the material of choice for ionization cooling because of its long radiation length relative to its energy loss. In the application discussed here, dense gaseous hydrogen also suppresses RF breakdown by virtue of the Paschen effect, thereby allowing higher accelerating gradients and a shorter and less‐expensive cooling channel. As described in this paper, a channel of RF cavities pressurized with about 3 tons of cold hydrogen gas could provide transverse muon cooling for a Muon Collider or Neutrino Factory. The present status of this research effort and several issues related to the use of h...

Longitudinal Motion of the Beam in the Fermilab Booster

When the Fermilab Booster Accelerator is operated at or above 1.5 × 1012 protons per pulse (extraction current about 155 mA) large amplitude coupled bunch longitudinal dipole oscillations occur between transition and extraction times. The oscillations do not contribute to beam loss in the booster but, because the beam is transferred synchronously into preexisting buckets in the Main Ring, the oscillations contribute to a deterioration of beam quality in the main ring. Two mode numbers have been established for the instabilities and the primary source frequencies have been isolated, although the offending objects have not. Operation of one of the eighteen accelerating cavities at a harmonic number one unit lower than the operating value (83 instead of 84) effectively damps the motion by the introduction bunch to bunch synchrotron tune spread.

STUDIES OF BREAKDOWN IN A PRESSURIZED RF CAVITY

Microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 805 MHz RF cavity experiments1 have been used to investigate the mechanism of RF breakdown.2 The images show evidence for melting and boiling in small regions of ~10 micron diameter on tungsten, molybdenum, and beryllium electrode surfaces. In these experiments, the dense hydrogen gas in the cavity prevents electrons or ions from being accelerated to high enough energy to participate in the breakdown process so that the only important variables are the fields and the metallic surfaces. The distributions of breakdown remnants on the electrode surfaces are compared to the maximum surface gradient E predicted by an ANSYS model of the cavity. The local surface density of spark remnants, proportional to the probability of breakdown, shows a strong exponential dependence on the maximum gradient, which is reminiscent of Fowler-Nordheim behavior of electron emission from a cold cathode. New simulation results have shown good agreement with the breakdown behavior of the hydrogen gas in the Paschen region and have suggested improved behavior with the addition of trace dopants such as SF6.3 Present efforts are to extend the computer model to include electrode breakdown phenomena and to use scanning tunneling microscopy to search for work function differences between the conditioned and unconditioned parts of the electrodes.

Muon bunch coalescing

The idea of coalescing multiple muon bunches at high energy to enhance the luminosity of a muon collider provides many advantages. It circumvents space-charge, beam loading, and wakefield problems of intense low- energy bunches while restoring the synergy between muon colliders and neutrino factories based on muon storage rings. A sampling of initial conceptual design work for a coalescing ring is presented here.

Recent Innovations in Muon Beam Cooling

Eight new ideas are being developed under SBIR/STTR grants to cool muon beams for colliders, neutrino factories, and muon experiments. Analytical and simulation studies have confirmed that a six-dimensional (6D) cooling channel based on helical magnets surrounding RF cavities filled with dense hydrogen gas can provide effective beam cooling. This helical cooling channel (HCC) has solenoidal, helical dipole, helical quadrupole, and helical sextupole magnetic fields to generate emittance exchange and achieve 6D emittance reduction of over 3 orders of magnitude in a 100 m segment. Four such sequential HCC segments, where the RF frequencies are increased and transverse physical dimensions reduced as the beams become cooler, implies a 6D emittance reduction of almost five orders of magnitude. Two new cooling ideas, Parametric-resonance Ionization Cooling and Reverse Emittance Exchange, then can be employed to reduce transverse emittances to a few mm-mr, which allows high luminosity with fewer muons than previously imagined. We describe these new ideas as well as a new precooling idea based on a HCC with z dependent fields that can be used as MANX, an exceptional 6D cooling demonstration experiment.

Suppression of Transverse Instabilities by Fast Feedback in the Fermilab Booster

Systems to damp radial and vertical instabilities of individual rf bunches in the Fermilab Booster are being implemented. The positions of individual bunches are derived from stripline pickups. The position information is transmitted over a variable delay, amplified, and applied to deflectors after one almost complete revolution, 6.25 horizontal and 6.75 vertical betatron wavelengths downstream of the pickup. Motivation, system concepts, design considerations, and initial operating experience are described here. Accompanying papers describe the electronics for signal processing and transmission.

Use of Helical Transport Channels for Bunch Recombination

Cooling scenarios for a high-luminosity Muon Collider require bunch recombination for optimal luminosity. In this report we note that the tunable chronicity property of a helical transport channel (HTC) makes it a desirable component of a bunch recombiner. A large chronicity HTC is desirable for the bunch recombining transport, while more isochronous transport may be preferred for rf manipulations. Scenarios for bunch recombination are presented, with initial 1-D simulations, in order to set the stage for future 3-D simulation and optimization. HTC transports may enable a very compact bunch recombiner.

Reducing the longitudinal emittance of the 8-GeV beam via the rf manipulation in a booster cycle

Bunch rotation will cause the longitudinal emittance growth whenever there are far more A rf stations than B rf stations, or vice versa. An alternate method via optimizing the RFSUM curve in a Booster cycle has been investigated using the ESME simulation. Since the rf manipulation at transition crossing can reduce the longitudinal emittance 31% and the momentum spread 17%, eventually, the rms momentum spread of 2.98 MeV and the longitudinal emittance of 0.061 eV {center_dot} sec with 95% of the beam can be achieved at 8-GeV.

Experimental estimate of beam loading and minimum rf voltage for acceleration of high intensity beam in the Fermilab Booster

The difference between the rf voltage seen by the beam and the accelerating voltage required to match the rate of change of the Booster magnetic field is used to estimate the energy loss per beam turn. Because the rf voltage (RFSUM) and the synchronous phase can be experimentally measured, they can be used to calculate the effective accelerating voltage. Also an RFSUM reduction technique has been applied to measure experimentally the RFSUM limit at which the beam loss starts. With information on beam energy loss, the running conditions, especially for the high intensity beam, can be optimized in order to achieve a higher intensity beam from the Fermilab Booster.

Reducing the extraction loss via laser notching the H- beam at the Booster injection revolution frequency

With the requirement for more protons per hour from Booster, the radiation is a limiting factor. Laser notching the H{sup -} beam at the Booster injection revolution frequency and properly aligning those notches on top of each other at the injection and relative to the trigger of firing extraction kickers can remove most of the extraction loss caused by the slow rise time of the kicker field.