Advanced Accelerator Concepts 2008 Euclid Techlabs LLC CVD Diamond Dielectric Accelerating...

Advanced Accelerator Concepts 2008Euclid Techlabs LLC

CVD Diamond Dielectric Accelerating Structures *

P. Schoessow, A. Kanareykin (Euclid Techlabs), R. Gat (CTS Inc.), J. Butler

(NRL)

*Work supported by US DOE


Simplicity of fabrication: The device is simply a tube of dielectric surrounded by a conducting cylinder. This is a great advantage for high frequency (~30 GHz) structures compared to conventional structures where extremely tight fabrication tolerances are required. The relatively small diameter of these devices also facilitates placement of quadrupole lenses around the structures.

Dielectrics can potentially exhibit high breakdown thresholds relative to copper, and high shunt impedance.

Reduced sensitivity to the single bunch beam break-up (BBU) instability: The frequency of the lowest lying HEM11 deflecting mode is almost always lower than that of the TM01 accelerating mode.

Easy parasitic mode damping. Potential challenges of using dielectric materials in a high power RF environment are breakdown and thermal heating, although problems with dielectric charging are easily mitigated by using a dielectric with a small dc conductivity.

Dielectric Structures at High Frequencies


Development of a cylindrical diamond-based dielectric loaded accelerator (DLA) structure. The electrical and mechanical properties of diamond make it an ideal candidate material for use in dielectric accelerating structures:

permittivity=5.7 high RF breakdown level (GV/m), extremely low dielectric losses (tan δ<10-4) highest thermoconductive coefficient available (2×103 Wm−1 K−1) .

The method we plan to use for fabrication of the diamond tubes is based on CVD (Chemical Vapor Deposition). A sustained accelerating gradient larger than 600 MV/m, in excess of the limits experimentally observed for conventional copper cavities should be attainable.

Overview


CVD diamond is made when a dilute mixture of methane (CH4) in hydrogen is chemically excited to produce atomic hydrogen and hydrocarbon radicals.

Diamond bond (sp3) slightly more stable under hydrogen bombardment than the graphitic (sp2).

In most commercial systems excitation is performed using microwave radiation; hot filaments also used Microwaves partly ionize and cause intense heating of the gas mixture up to 4000°C. The diamond film forms on a surface held at about 900°C in proximity to the excited gas. Typical pressures are sub-atmospheric (100 Torr), film growth rates ~15 pm/hr depending on reactor design and power. Turnkey microwave reactors capable of unattended diamond deposition over areas of up to 12” in diameter are commercially available The present cost for relatively low quality CVD diamond is ~few hundred $/carat. (A carat of diamond is 200 mg or approximately a piece of size 10×10×0.6 mm3.)

CVD Diamond Manufacture


CVD Diamond Manufacture


Commercial PECVD reactor


Diamond Structure Prototypes



Segmented Structures

Highest diamond quality achieved with this process


Negative electron affinity (NEA) results in extremely high SEE (secondary electron emission) coefficients. Hydrogenated CVD diamond films possess a strong NEA with a high coefficient of secondary electron emission (up to ~60). Dehydrogenated or oxidized diamond surfaces show a positive electron affinity and demonstrate SEE coefficients ~1 and consequently can be used as a dielectric loading material for the high gradient DLA structures with reduced or suppressed multipacting performance. Diamond surfaces may be oxidized in a number of ways (e.g. oxidizing acid or oxygen plasma treatments), resulting in PEA. CVD diamond bulk and films can be fabricated with a secondary electron emission yield ~1.

Requires that desorption of hydrogen from the diamond surface or oxidation of the surface.Use of nitrogen or other CVD diamond dopants are being considered. Study secondary electron emission surface properties by SEE spectroscopy methods planned at NRL.

CVD Diamond Surface and Multipacting Performance


CVD Diamond Surface and Multipacting Performance: Effects of Annealing

Secondary electron yields for hydrogenated single-crystal diamond surface before and after annealing at 950 °C. Sharp decrease of the SEE yields (~1) after the annealing resulting in dehydrogenation of the diamond surface.

Changes in the secondary-electron yield of CVD diamond after sample heating.

Before heating

5 min at 500 °C

5 min at 500 °C

15 min at 1000 °C


n o v a c u u m g a p In n e r D ia m e te r 1 .5 m m 3 m m

R o u te r , c m 0 .1 9 0 .2 4 P a t tn , d B /m -4 .7 -4 .5

, M Ω msr 1 5 2 8 5

. .z d ie l z a cce lE E 1 1

. .r d ie l z a cce lE E 0 .3 7 0 .6 8

. .r m e ta l z a cce lE E 0 .1 7 0 .2 5

2 0 0 0 m v a c u u m g a p In n e r D ia m e te r

1 .5 m m 3 m m R o u te r , c m 0 .1 9 0 .2 4 P a t tn , d B /m -0 .2 2 -0 .2 4

, M Ω msr 2 6 2 1 3 2

. .z d ie l z a cc e lE E 1 1

. .r d ie l z a cc e lE E 0 .9 0 .9

. .r m e ta l z a cc e lE E 0 .4 9 0 .7 9

34 GHz Structure Prototypes: Effect of Radial Vacuum Gap

Diamond-based cylindrical DLA structure parameters in case of: (a) no vacuum gap, (b) 2 mm vacuum gap between the outer diamond surface and the copper wall. Note the high shunt impedance of 262 M/m for the vacuum gap case, and 152 M/m for the “no-gap” design. Surface field ratio Emetal/Eaccelerating > 0.17 for a 1.5 mm beam channel aperture.


Axial (red) and transverse (blue) electric field profiles along the structure radius, normalized to the acceleration gradient Eaccel. Diamond tube inner diameter 2a = 1.5 mm, outer diameter 2b = 3.79 mm, wall thickness is 1.15 mm; (A) No gap; (B) 2mm vacuum gap width.

34 GHz Structure Prototypes: Effect of Vacuum Gap

A B


We plan to develop and demonstrate a cylindrical Dielectric Loaded Accelerating (DLA) structure based on a diamond waveguide. Use of CVD (Chemical Vapor Deposition) diamond as a loading material will allow high accelerating gradients up to 0.5-1.0 GV/m provided that the diamond surface can sustain a 1-2GV/m breakdown rf field as expected. CVD process technology is rapidly developing; the CVD diamond fabrication process is becoming fast and inexpensive. Multipacting performance of the CVD diamond can be dramatically suppressed by diamond surface dehydrogenation through annealing or chemical treatment.

Summary

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Advanced Accelerator Concepts 2008 Euclid Techlabs LLC CVD Diamond Dielectric Accelerating...

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