Patent Application
20170071054
The present invention relates to linear accelerator (linac) accelerating modules and more particularly to a method to suppress back-acceleration of field-emitted particles in RF accelerators.
So-called electron loading in radio-frequency (RF) accelerating cavities is the primary cause for cavity performance limitations today. Electron loading can limit the desired energy gain, add cryogenic heat load, damage accelerator components and increase accelerator downtime depending on the induced trip rates. Trip rates are of particular concern for next generation facilities such as Accelerator Driven Subcritical Reactors or Energy Recovery Linacs for Free Electron Lasers.
Electron loading can be attributed to mainly three phenomena, i.e. field emission (FE), multiple impact electron amplification (short: multipacting) and RF electrical breakdown. In all cases, electrons are involved either being released from the enclosing RF surfaces or generated directly within the RF volume by ionization processes with the rest gas (even in ultra-high vacuum), e.g. due to cosmic radiation. The free electrons can absorb a considerable amount of the RF energy provided by external power sources thereby constraining the achievable field level and/or causing operational failures.
Field emission has been a prevalent issue, particularly in superconducting RF (SRF) cavities, whereas RF electrical breakdown and multipacting can be controllable within limits by adequate design choices. Though SRF cavities may readily exceed accelerating fields (Eacc) of 20 MV/m, the onset of parasitic electron activities may start at field levels as low as a few MV/m. Field emission becomes a major concern when the electrons emitted are captured by the accelerating RF field and directed close to the beam axis through a series of cavities or cryomodules.
The free electrons can then accumulate a comparable amount of energy as the main beam would over the same distance. This can present a considerable ‘dark current’ with damaging risks (e.g. when hitting undulator magnets). The electrons can be directed either down- or upstream the accelerator depending on the site and time of origin.
The concern with field emission stems from its exponential increase with Eacc (the acceleration gradient), which is well verified experimentally. Note that FE is a quantum-mechanical process that can be described by the (simplified) Fowler-Nordheim (FN) equation:
J denotes the peak current density (in A/m2) (current I over effective emission area Aeff), Epeak the local surface electrical field (in V/m), Φ the local material work function (in eV), and a and b, which are the 1st and 2nd FN-constants, respectively (a≈1.541434·106 A·eV·V−2 and b≈6.83089−109 eV−3/2·V/m). Field emission requires surface fields in the order of GV/m. Peak fields in SRF cavities however only reach up to a few ten MV/m. Therefore a local field enhancement factor βenh is introduced, which in SRF cavities requires βenh>50 to produce meaningful emission currents. In fact, such large enhancement factors and higher are often encountered depending on the nature of the field emitter.
Emitted electrons eventually hit surfaces internal or external to cavity cryomodules depending on the site and time of origin, which determines trajectories and energies. Upon impact, electrons not only can create additional heating, but also can induce secondary particle showers and gamma rays via bremsstrahlung. This in turn can cause radio-activation of accelerator components once electrons accumulate energies above the threshold for neutron production, which is in the order of 10 MeV for the metals employed. For instance, very high radiation levels and radio-activation due to FE has been a concern in CEBAF upgrade cryomodules. The primary process for neutron production by electrons is the absorption of bremsstrahlung photons, i.e. via photonuclear reactions. The threshold energy can thus be obtained within a few cavity cells depending on field levels.
Maintaining extremely clean environments throughout cavity fabrication, post-processing and assembly is of major importance to mitigate particulates that may create FE sites. However, the existence of field emitters cannot be excluded even when obeying strict protocols following industrial standards. Based on today's experience a large fraction of SRF cavities remain plagued by FE.
A first object of the invention is to provide a method for suppressing upstream field emission in RF accelerators.
A second object of the invention is to reduce electron loading to improve the performance of radio-frequency (RF) accelerating cavities.
A further object is to reduce the electron loading in order to improve the energy gain, reduce the cryogenic heat load, lessen the damage accelerator components, and reduce accelerator downtime depending on the induced trip rates.
These and other objects and advantages of the present invention will be better understood by reading the following description along with reference to the drawings.
The present invention is a method for suppressing of upstream-directed field emission in RF accelerators. The method is not restricted to a certain number of cavity cells, but ideally requests similar operating field levels in all cavities to efficiently annihilate the once accumulated energy. Such a field balance is desirable to minimize dynamic RF losses, but not necessarily achievable in reality depending on individual cavity performance (e.g. early Q0-drop or quench field). Yet, even with some discrepancy in operating fields, the method of the present invention can achieve a significant energy reduction for upstream-directed electrons within a relatively short distance. Electrons will then impact surfaces at rather low energies. With the dark current being reduced, so are issues with heating and damage of accelerator components as well as radiation levels including neutron generation and thus radio-activation.
The present invention provides a practical method for suppressing FE in accelerating structures even in presence of field-emitting sites. Though important for SRF cavity cryomodules, the method applies generally to any type of RF accelerator. The benefit is a significant reduction of energy accumulation of upstream traveling field-emitted electrons, which mitigates dark current directed to the injector. The method is deemed most efficient for speed-of-light (β=1) structures accounting for the fact that the electrons are swiftly accelerated to relativistic energies once captured by the RF field such that the travel distance per RF period is nearly equal to that of the main beam. The method is advantageous in that it does not require an alteration of the cavity design. The method includes adjusting the beam tube length (Ltube) between cavities to obey:
Herein Lcell is the cavity cell length (˜βλ/2, λ=wavelength of accelerating mode) and N is an integer number. Ltube is often chosen to be 3·Lcell in SRF cavity cryomodules. This implies that RF fields in cavities oscillate synchronously at all times. The main beam accelerated in one cavity will then experience the same accelerating field after passage to the next cavity without phase adjustment (theoretically and assuming constant velocity). However, the RF phase can be technically tuned for each cavity depending on the tube length. The cavity interconnecting tube length cannot be chosen arbitrarily small, since it has to accommodate space for fundamental power couplers, pick-up probes for RF feedback control as well as HOM dampers and bellows depending on design requirements.
When applying the method, one also has to take into account isolation requirements between couplers of neighbouring cavities to avoid cross-talk effects that impede the low level RF control. This for instance concerns crosstalk between a power coupler of one cavity and the pick-up probe of the adjacent cavity or two power couplers facing each other. When using stainless steel bellows between cavities, the thermal losses in the bellows favour to place cavity flanges further away from the cavity cells. All the aforementioned considerations usually make N=0 and 1 impractical in SRF cryomodules. For N=2 (Ltube=2.5·Lcell) however one obtains a reasonably long section for practical and thermal requirements, while saving cryomodule length and thus costs compared to 3·Lcell. Otherwise N=3 should be chosen.
Field-emitted electrons moving downstream would be accelerated in the same way once efficiently captured by the RF assuming no significant phase slippage occurs. Electrons directed upstream will have to start when the field peaks in the opposite direction (−1) corresponding to a 180° phase shift to the accelerating field in the same cell. Assuming this to be the time when field-emitted electrons arrive in the mid of the 1st cell in the downstream cavity (leftmost unfilled dot), these will reach the end cell of the upstream cavity when the field peaks again for further acceleration upstream (−1 at 2nd unfilled dot). Consequently in this case (Ltube=N·Lcell), electrons may accumulate the same energy gain whether directed up- or downstream.
Referring to the bottom plot of
Note that in reality field-emitted electrons are emitted during a finite phase range. This causes differing trajectories and energy spread among particles. Perfect energy annihilation cannot be achieved for all possible trajectories.
Trajectories also depend on the specific cavity shape. The proposed method however provides a significant reduction of upstream energies in all conceivable cases when obeying equation (2).
Although the description above contains many specific descriptions, materials, and dimensions, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.
This application claims the benefit of Provisional U.S. Patent Application Ser. No. 62/215,870 filed Sep. 9, 2015.
The United States Government may have certain rights to this invention under Management and Operating Contract No. DE-AC05-06OR23177 from the Department of Energy.
| Number | Date | Country | |
|---|---|---|---|
| 62215870 | Sep 2015 | US |
