# Presentation: 108000

## CVD Diamond Dielectric Accelerating Structures * 

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

- *Work supported by US DOE

## Dielectric Structures at High Frequencies

- 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**

## Overview

-  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 Manufacture

-  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

**Commercial PECVD reactor**

**Notes:**
Schematic drawing of a CVD reactor similar to that being used by CTS for PECVD production of diamond tubes.

## Diamond Structure Prototypes

**Diamond Structure Prototypes**

**Notes:**
llustration of the technology for the diamond tube CVD fabrication process, 2.4 cm inner diameter diamond tube developed by CTS, Inc. 

## Slide 8

**Notes:**
End view of the completed 0.5 mm wall thickness diamond tube. 

## Segmented Structures

**Segmented Structures**

- Highest diamond quality achieved with this process

## CVD Diamond Surface and Multipacting Performance 

-  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

**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*

**Notes:**
I.L. Krainsky, V.M. Asnin and A.G. Petukhov. Report NASA/TP—1999-208692, (1999). 
A.Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, and R. Abrams. J. Appl Phys. 82, 1860-1867 (1997)

## 34 GHz Structure Prototypes: Effect of Radial Vacuum Gap

**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.

## 34 GHz Structure Prototypes: Effect of Vacuum Gap

- Axial (red) and transverse (blue) electric field profiles along the structure radius, normalized to the acceleration gradient *E*accel. Diamond tube inner diameter 2a = 1.5 mm, outer diameter 2*b* = 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

**Notes:**
Longitudinal (red) and transverse (blue) electric field profiles normalized to the acceleration gradient Eaccel  for the structure parameters: (a) - inner diameter 2a = 1.5 mm, outer diameter 2b = 3.79 mm, diamond thickness is 1.15 mm;  (b) -  vacuum gap width is of 2 mm, inner diameter 2a = 1.5 mm, outer diameter 2b = 3.79 mm, diamond thickness is 1.15 mm. 

## Summary

-  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**